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  • 1. C ONTENTSFOREWORD iiiUNIT I : GEOGRAPHY AS A DISCIPLINE 1-12 1. Geography as a Discipline 2UNIT II : THE EARTH 13-38 2. The Origin and Evolution of the Earth 14 3. Interior of the Earth 21 4. Distribution of Oceans and Continents 30UNIT III : LANDFORMS 39-74 5. Minerals and Rocks 40 6. Geomorphic Processes 45 7. Landforms and their Evolution 58UNIT IV : CLIMATE 75-110 8. Composition and Structure of Atmosphere 76 9. Solar Radiation, Heat Balance and Temperature 79 10. Atmospheric Circulation and Weather Systems 88 11. Water in the Atmosphere 98 12. World Climate and Climate Change 103UNIT V : WATER (OCEANS) 111-125 13. Water (Oceans) 112 14. Movements of Ocean Water 120UNIT VI : LIFE ON THE EARTH 126-140 15. Life on the Earth 127 16. Biodiversity and Conservation 135 GLOSSARY 141-144
  • 2. UNIT I GEOGRAPHY AS A DISCIPLINEThis unit deals with• Geography as an integrating discipline; as a science of spatial attributes• Branches of geography; importance of physical geography
  • 3. CHAPTER GEOGRAPHY AS A DISCIPLINEY ou have studied geography as one of the of the earth’s surface. The understanding and components of your social studies course the skills obtained in modern scientific upto the secondary stage. You are techniques such as GIS and computeralready aware of some of the phenomena of cartography equip you to meaningfullygeographical nature in the world and its contribute to the national endeavour fordifferent parts. Now, you will study ‘Geography’ an independent subject and learn about the Now the next question which you may likephysical environment of the earth, human to ask is — What is geography? You know thatactivities and their interactive relationships. earth is our home. It is also the home of manyTherefore, a pertinent question you can ask at other creatures, big and small, which live onthis stage is — Why should we study the earth and sustain. The earth’s surface isgeography? We live on the surface of the earth. not uniform. It has variations in its physicalOur lives are affected by our surroundings in features. There are mountains, hills, valleys,many ways. We depend on the resources to plains, plateaus, oceans, lakes, deserts andsustain ourselves in the surrounding areas. wilderness. There are variations in its socialPrimitive societies subsisted on ‘natural means and cultural features too. There are villages,of subsistence’, i.e. edible plants and animals. cities, roads, railways, ports, markets andWith the passage of time, we developed many other elements created by human beingstechnologies and started producing our food across the entire period of their culturalusing natural resources such as land, soil and development.water. We adjusted our food habits and This variation provides a clue to theclothing according to the prevailing weather understanding of the relationship between theconditions. There are variations in the natural physical environment and social/culturalresource base, technological development, features. The physical environment hasadaptation with and modification of physical provided the stage, on which human societiesenvironment, social organisations and cultural enacted the drama of their creative skills withdevelopment. As a student of geography, you the tools and techniques which they inventedshould be curious to know about all the and evolved in the process of their culturalphenomena which vary over space. You learn development. Now, you should be able toabout the diverse lands and people. You attempt the answer of the question posedshould also be interested in understanding the earlier as to “What is geography”? In verychanges which have taken place over time. simple words, it can be said that geographyGeography equips you to appreciate diversity is the description of the earth. The termand investigate into the causes responsible for geography was first coined by Eratosthenese,creating such variations over time and space. a Greek scholar (276-194 BC.). The word hasYou will develop skills to understand the globe been derived from two roots from Greekconverted into maps and have a visual sense language geo (earth) and graphos (description).
  • 4. GEOGRAPHY AS A DISCIPLINE 3Put together, they mean description of the earth. changing earth and untiring and ever-activeThe earth has always been seen as the abode human beings. Primitive human societies wereof human beings and thus, scholars defined directly dependent on their immediategeography as, “the description of the earth as environment. Geography, thus, is concernedthe abode of human beings”. You are aware of with the study of Nature and Humanthe fact that reality is always multifaceted and interactions as an integrated whole. ‘Human’the ‘earth’ is also multi-dimensional, that is is an integral part of ‘nature’ and ‘nature’ haswhy many disciplines from natural sciences the imprints of ‘human’. ‘Nature’ has influencedsuch as geology, pedology, oceanography, different aspects of human life. Its imprints canbotany, zoology and meteorology and a be noticed on food, clothing, shelter andnumber of sister disciplines in social sciences occupation. Human beings have come to termssuch as economics, history, sociology, political with nature through adaptation andscience, anthropology, etc. study different modification. As you already know, the presentaspects of the earth’s surface. Geography is society has passed the stage of primitivedifferent from other sciences in its subject societies, which were directly dependent onmatter and methodology but at the same time, their immediate physical environment forit is closely related to other disciplines. sustenance. Present societies have modifiedGeography derives its data base from all the their natural environment by inventing andnatural and social sciences and attempts their using technology and thus, have expanded thesynthesis. horizon of their operation by appropriating and We have noted that there exist variations utilising the resources provided by nature. Withover the surface of the earth in its physical as the gradual development of technology, humanwell as cultural environment. A number of beings were able to loosen the shackles of theirphenomena are similar and many are dissimilar. physical environment. Technology helped inIt was, therefore, logical to perceive geography reducing the harshness of labour, increasedas the study of areal differentiation. Thus, labour efficiency and provided leisure togeography was perceived to study all those human beings to attend to the higher needs ofphenomena which vary over space. life. It also increased the scale of productionGeographers do not study only the variations and the mobility of the phenomena over the earth’s surface The interaction between the physical(space) but also study the associations with environment and human beings has been verythe other factors which cause these variations. succinctly described by a poet in the followingFor example, cropping patterns differ from dialogue between ‘human’ and ‘nature’ (God).region to region but this variation in cropping You created the soil, I created the cup, youpattern, as a phenomenon, is related to created night, I created the lamp. You createdvariations in soils, climates, demands in the wilderness, hilly terrains and deserts; Imarket, capacity of the farmer to invest and created flower beds and gardens. Humantechnological inputs available to her/him. beings have claimed their contribution usingThus, the concern of geography is to find out natural resources. With the help of technology,the causal relationship between any two human beings moved from the stage ofphenomena or between more than one necessity to a stage of freedom. They have putphenomenon. their imprints everywhere and created new A geographer explains the phenomena in possibilities in collaboration with nature. Thus,a frame of cause and effect relationship, as it we now find humanised nature anddoes not only help in interpretation but also naturalised human beings and geographyforesees the phenomena in future. studies this interactive relationship. The space The geographical phenomena, both the got organised with the help of the means ofphysical and human, are not static but highly transportation and communication network.dynamic. They change over time as a result of The links (routes) and nodes (settlements of allthe interactive processes between ever types and hierarchies) integrated the space and
  • 5. 4 FUNDAMENTALS OF PHYSICAL GEOGRAPHYgradually, it got organised. As a social science present world is being perceived as a globaldiscipline, geography studies the ‘spatial village. The distances have been reduced byorganisation’ and ‘spatial integration’. better means of transportation increasing Geography as a discipline is concerned with accessibility. The audio-visual media andthree sets of questions: information technology have enriched the data (i) Some questions are related to the base. Technology has provided better chances identification of the patterns of natural of monitoring natural phenomena as well as and cultural features as found over the the economic and social parameters. surface of the earth. These are the Geography as an integrating discipline has questions about what? interface with numerous natural and social (ii) Some questions are related to the sciences. All the sciences, whether natural or distribution of the natural and human/ social, have one basic objective, of cultural features over the surface of the understanding the reality. Geography earth. These are the questions about attempts to comprehend the associations of where? phenomena as related in sections of reality. Taken together, both these questions take Figure 1.1 shows the relationship of geographycare of distributional and locational aspects of with other sciences. Every discipline, concernedthe natural and cultural features. These with scientific knowledge is linked withquestions provided inventorised information of geography as many of their elements vary overwhat features and where located. It was a very space. Geography helps in understanding thepopular approach during the colonial period. reality in totality in its spatial perspective.These two questions did not make geography Geography, thus, not only takes note of thea scientific discipline till the third question was differences in the phenomena from place toadded. The third question is related to the place but integrates them holistically whichexplanation or the causal relationships may be different at other places. A geographerbetween features and the processes and is required to have a broad understanding ofphenomena. This aspect of geography is related all the related fields, to be able to logically integrate them. This integration can beto the question, why? understood with some examples. Geography Geography as a discipline is related to influences historical events. Spatial distancespace and takes note of spatial characteristics itself has been a very potent factor to alter theand attributes. It studies the patterns of course of history of the world. Spatial depthdistribution, location and concentration of provided defence to many countries,phenomena over space and interprets them particularly in the last century. In traditionalproviding explanations for these patterns. It warfare, countries with large size in area, gaintakes note of the associations and inter - time at the cost of space. The defence providedrelationships between the phenomena over by oceanic expanse around the countries ofspace and interprets them providing the new world has protected them from warsexplanations for these patterns. It also takes being imposed on their soil. If we look at thenote of the associations and inter-relationships historical events world over, each one of thembetween the phenomena resulting from the can be interpreted geographically.dynamic interaction between human beings In India, Himalayas have acted as greatand their physical environment. barriers and provided protection but the passes provided routes to the migrants andGEOGRAPHY AS AN INTEGRATING DISCIPLINE invaders from Central Asia. The sea coast hasGeography is a discipline of synthesis. It encouraged contact with people from East andattempts spatial synthesis, and history Southeast Asia, Europe and Africa. Navigationattempts temporal synthesis. Its approach is technology helped European countries toholistic in nature. It recognises the fact that colonise a number of countries of Asia andthe world is a system of interdependencies. The Africa, including India as they got accessibility
  • 6. GEOGRAPHY AS A DISCIPLINE 5through oceans. The geographical factors have econometrics. Maps are prepared throughmodified the course of history in different parts artistic imagination. Making sketches, mentalof the world. maps and cartographic work require Every geographical phenomenon undergoes proficiency in arts.change through time and can be explainedtemporally. The changes in landforms, climate, Geography and Social Sciencesvegetation, economic activities occupations andcultural developments have followed a definite Each social science sketched in Figure 1.1 hashistorical course. Many geographical features interface with one branch of geography. Theresult from the decision making process by relationships between geography and historydifferent institutions at a particular point of have already been outlined in detail. Everytime. It is possible to convert time in terms of discipline has a philosophy which is the raisonspace and space in terms of time. For example, d’etre for that discipline. Philosophy providesit can be said that place A is 1,500 km from roots to a discipline and in the process of itsplace B or alternately, it can also be said that evolution, it also experiences distinct historicalplace A is two hours away (if one travels by processes. Thus, the history of geographicalplane) or seventeen hours away (if one travels thought as mother branch of geography isby a fast moving train). It is for this reason, included universally in its curricula. All thetime is an integral part of geographical studies social science disciplines, viz. sociology,as the fourth dimension. Please mention other political science, economics and demographythree dimensions? study different aspects of social reality. The Figure1.1 amply depicts the linkages of branches of geography, viz. social, political,geography with different natural and social economic and population and settlements aresciences. This linkage can be put under two closely linked with these disciplines as eachsegments. one of them has spatial attributes. The core concern of political science is territory, peoplePhysical Geography and Natural Sciences and sovereignty while political geography isAll the branches of physical geography, as also interested in the study of the state as ashown in Figure 1.1, have interface with natural spatial unit as well as people and their politicalsciences. The traditional physical geography behaviour. Economics deals with basicis linked with geology, meteorology, hydrology attributes of the economy such as production,and pedology, and thus, geomorphology, distribution, exchange and consumption. Eachclimatology, oceanography and soil geography of these attributes also has spatial aspects andrespectively have very close link with the here comes the role of economic geography tonatural sciences as these derive their data from study the spatial aspects of production,these sciences. Bio-Geography is closely related distribution, exchange and botany, zoology as well as ecology as human Likewise, population geography is closelybeings are located in different locational niche. linked with the discipline of demography. A geographer should have some proficiency The above discussion shows thatin mathematics and art, particularly in drawing geography has strong interface with naturalmaps. Geography is very much linked with the and social sciences. It follows its ownstudy of astronomical locations and deals withlatitudes and longitudes. The shape of the earth methodology of study which makes it distinctis Geoid but the basic tool of a geographer is a from others. It has osmotic relationship withmap which is two dimensional representation other disciplines. While all the disciplines haveof the earth. The problem of converting geoids their own individual scope, this individualityinto two dimensions can be tackled by does not obstruct the flow of information as inprojections constructed graphically or case of all cells in the body that have individualmathematically. The cartographic and identity separated by membranes but the flowquantitative techniques require sufficient of blood is not obstructed. Geographers useproficiency in mathematics, statistics and data obtained from sister disciplines and
  • 7. 6Figure 1.1 : Geography and its relation with other subjects FUNDAMENTALS OF PHYSICAL GEOGRAPHY
  • 8. GEOGRAPHY AS A DISCIPLINE 7attempt synthesis over space. Maps are very BRANCHES OF GEOGRAPHY (BASED ONeffective tools of geographers in which the SYSTEMATIC APPROACH)tabular data is converted into visual form tobring out the spatial pattern. 1. Physical GeographyBRANCHES OF GEOGRAPHY (i) Geomorphology is devoted to the study of landforms, their evolution and relatedPlease study Figure 1.1 for recapitulation. It has processes.very clearly brought out that geography is an (ii) Climatology encompasses the study ofinterdisciplinary subject of study. The study of structure of atmosphere and elementsevery subject is done according to some of weather and climates and climaticapproach. The major approaches to study types and regions.geography have been (i) Systematic and (iii) Hydrology studies the realm of water(ii) Regional. The systematic geography approach over the surface of the earth includingis the same as that of general geography. This oceans, lakes, rivers and other waterapproach was introduced by Alexander Von bodies and its effect on different lifeHumboldt, a German geographer (1769-1859) forms including human life and theirwhile regional geography approach was activities.developed by another German geographer and a (iv) Soil Geography is devoted to study thecontemporary of Humboldt, Karl Ritter processes of soil formation, soil types,(1779-1859). their fertility status, distribution and In systematic approach (Figure 1.2), a use.phenomenon is studied world over as a whole, 2. Human Geographyand then the identification of typologies orspatial patterns is done. For example, if one is (i) Social/Cultural Geography encom-interested in studying natural vegetation, the passes the study of society and itsstudy will be done at the world level as a first spatial dynamics as well as the culturalstep. The typologies such as equatorial rain elements contributed by the society.forests or softwood conical forests or monsoon (ii) Population and Settlement Geographyforests, etc. will be identified, discussed and (Rural and Urban). It studies populationdelimited. In the regional approach, the world growth, distribution, density, sex ratio,is divided into regions at different hierarchical migration and occupational structurelevels and then all the geographical phenomena etc. Settlement geography studies thein a particular region are studied. These characteristics of rural and urbanregions may be natural, political or designated settlements. (iii) Economic Geography studies economicregion. The phenomena in a region are studied activities of the people includingin a holistic manner searching for unity in agriculture, industry, tourism, trade,diversity. and transport, infrastructure and Dualism is one of the main characteristics services, etc.of geography which got introduced from the (iv) Historical Geography studies thevery beginning. This dualism depended on the historical processes through which theaspect emphasised in the study. Earlier scholars space gets organised. Every region haslaid emphasis on physical geography. But undergone some historical experienceshuman beings are an integral part of the earth’s before attaining the present day status.surface. They are part and parcel of nature. They The geographical features alsoalso have contributed through their cultural experience temporal changes and thesedevelopment. Thus developed human form the concerns of historicalgeography with emphasis on human activities. geography.
  • 9. 8 FUNDAMENTALS OF PHYSICAL GEOGRAPHY Figure 1.2 : Branches of geography based on systematic approach (v) Political Geography looks at the space 3. Biogeography from the angle of political events and The interface between physical geography studies boundaries, space relations and human geography has lead to the between neighbouring political units, development of Biogeography which delimitation of constituencies, election includes: scenario and develops theoretical (i) Plant Geography which studies the framework to understand the political spatial pattern of natural vegetation in behaviour of the population. their habitats.
  • 10. GEOGRAPHY AS A DISCIPLINE 9 (ii) Zoo Geography which studies the (c) Field Survey Methods spatial patterns and geographic (d) Geo-informatics comprising characteristics of animals and their techniques such as Remote habitats. Sensing, GIS, GPS, etc. (iii) Ecology /Ecosystem deals with the The above classification gives a scientific study of the habitats comprehensive format of the branches of characteristic of species. geography. Generally geography curricula is (iv) Environmental Geography concerns taught and learnt in this format but this world over leading to the realisation of format is not static. Any discipline is bound environmental problems such as land gradation, pollution and concerns for to grow with new ideas, problems, methods conservation has resulted in the and techniques. For example, what was once introduction of this new branch in manual cartography has now been geography. transformed into computer cartography. Technology has enabled scholars to handleBRANCHES OF GEOGRAPHY BASED ON REGIONAL large quantum of data. The internet providesAPPROACH (FIGURE1.3) extensive information. Thus, the capacity to1. Regional Studies/Area Studies attempt analysis has increased tremendously. Comprising Macro, Meso and Micro GIS has further opened vistas of knowledge. Regional Studies GPS has become a handy tool to find out exact2. Regional Planning locations. Technologies have enhanced the Comprising Country/Rural and Town/ capacity of attempting synthesis with sound Urban Planning theoretical understanding.3. Regional Development You will learn some preliminary aspects of4. Regional Analysis these techniques in your book, Practical work There are two aspects which are common in Geography – Part I (NCERT, 2006). You will to every discipline, these are: continue to improve upon your skills and (i) Philosophy learn about their application. (a) Geographical Thought (b) Land and Human Interaction/ PHYSICAL GEOGRAPHY AND ITS IMPORTANCE Human Ecology (ii) Methods and Techniques This chapter appears in the book entitled (a) Cartography including Computer Fundamentals of Physical Geography. The Cartography contents of the book clearly reflect its scope. (b) Quantitative Techniques/Statistical It is therefore, appropriate to know the Techniques importance of this branch of geography. Figure 1.3 : Branches of geography based on regional approach
  • 11. 10 FUNDAMENTALS OF PHYSICAL GEOGRAPHYPhysical geography includes the study of sea-food, oceans are rich in mineral resources.lithosphere (landforms, drainage, relief and India has developed the technology forphysiography), atmosphere (its composition, collecting manganese nodules from oceanicstructure, elements and controls of weather bed. Soils are renewable resources, whichand climate; temperature, pressure, winds, influence a number of economic activities suchprecipitation, climatic types, etc.), hydrosphere as agriculture. The fertility of the soil is both(oceans, seas, lakes and associated features naturally determined and culturally induced.with water realm) and biosphere ( life forms Soils also provide the basis for the biosphereincluding human being and macro-organism accommodating plants, animals and microand their sustaining mechanism, viz. food organisms.chain, ecological parameters and ecologicalbalance). Soils are formed through the process What is Geography?of pedogenesis and depend upon the parentrocks, climate, biological activity and time. Geography is concerned with the description and explanation of the areal differentiation ofTime provides maturity to soils and helps in the earth’s surface.the development of soil profiles. Each element Richard Hartshorneis important for human beings. Landformsprovide the base on which human activities are Geography studies the differences oflocated. The plains are utilised for agriculture. phenomena usually related in different parts of the earth’s surface.Plateaus provide forests and minerals. HettnerMountains provide pastures, forests, touristspots and are sources of rivers providing waterto lowlands. Climate influences our house The study of physical geography istypes, clothing and food habits. The climate emerging as a discipline of evaluating andhas a profound effect on vegetation, cropping managing natural resources. In order topattern, livestock farming and some achieve this objective, it is essential toindustries, etc. Human beings have developed understand the intricate relationship betweentechnologies which modify climatic elements physical environment and human a restricted space such as air conditioners Physical environment provides resources, andand coolers. Temperature and precipitation human beings utilise these resources andensure the density of forests and quality of ensure their economic and culturalgrassland. In India, monsoonal rainfall sets the development. Accelerated pace of resourceagriculture rhythm in motion. Precipitation utilisation with the help of modern technologyrecharges the ground water aquifers which has created ecological imbalance in the world.later provides water for agriculture and Hence, a better understanding of physicaldomestic use. We study oceans which are the environment is absolutely essential forstore house of resources. Besides fish and other sustainable development. EXERCISES 1. Multiple choice questions. (i) Which one of the following scholars coined the term ‘Geography’? (a) Herodotus (c) Galileo (b) Erathosthenese (d) Aristotle (ii) Which one of the following features can be termed as ‘physical feature’? (a) Port (c) Plain (b) Road (d) Water park
  • 12. GEOGRAPHY AS A DISCIPLINE 11 (iii) Make correct pairs from the following two columns and mark the correct option. 1. Meteorology A. Population Geography 2. Demography B. Soil Geography 3. Sociology C. Climatology 4. Pedology D. Social Geography (a) 1B,2C,3A,4D (c) 1D,2B,3C,4A (b) 1A,2D,3B,4C (d) 1C,2A,3D,4B (iv) Which one of the following questions is related to cause-effect relationship? (a) Why (c) What (b) Where (d) When (v) Which one of the following disciplines attempts temporal synthesis? (a) Sociology (c) Anthropology (b) Geography (d) History 2. Answer the following questions in about 30 words. (i) What important cultural features do you observe while going to school? Are they similar or dissimilar? Should they be included in the study of geography or not? If yes, why? (ii) You have seen a tennis ball, a cricket ball, an orange and a pumpkin. Which one amongst these resembles the shape of the earth? Why have you chosen this particular item to describe the shape of the earth? (iii) Do you celebrate Van Mahotsava in your school? Why do we plant so many trees? How do the trees maintain ecological balance? (iv) You have seen elephants, deer, earthworms, trees and grasses. Where do they live or grow? What is the name given to this sphere? Can you describe some of the important features of this sphere? (v) How much time do you take to reach your school from your house? Had the school been located across the road from your house, how much time would you have taken to reach school? What is the effect of the distance between your residence and the school on the time taken in commuting? Can you convert time into space and vice versa? 3. Answer the following questions in about 150 words. (i) You observe every day in your surroundings that there is variation in natural as well as cultural phenomena. All the trees are not of the same variety. All the birds and animals you see, are different. All these different elements are found on the earth. Can you now argue that geography is the study of “areal differentiation”? (ii) You have already studied geography, history, civics and economics as parts of social studies. Attempt an integration of these disciplines highlighting their interface.
  • 13. 12 FUNDAMENTALS OF PHYSICAL GEOGRAPHY Project Work Select forest as a natural resource. (i) Prepare a map of India showing the distribution of different types of forests. (ii) Write about the economic importance of forests for the country. (iii) Prepare a historical account of conservation of forests in India with focus on Chipko movements in Rajasthan and Uttaranchal.
  • 14. UNIT II THE EARTHThis unit deals with• Origin and evolution of the earth; Interior of the earth; Wegener’s continental drift theory and plate tectonics; earthquakes and volcanoes
  • 15. CHAPTER THE ORIGIN AND EVOLUTION OF THE EARTHD o you remember the nursery rhyme argument. At a later date, the arguments “…Twinkle, Twinkle little star…”? considered of a companion to the sun to have been coexisting. These arguments are called Starry nights have always attracted us since binary theories. In 1950, Otto Schmidt inthe childhood. You may also have thought of Russia and Carl Weizascar in Germanythese stars and had numerous questions in somewhat revised the ‘nebular hypothesis’,your mind. Questions such as how many stars though differing in details. They considered thatare there in the sky? How did they come into the sun was surrounded by solar nebulaexistence? Can one reach the end of the sky? containing mostly the hydrogen and heliumMay be many more such questions are still along with what may be termed as dust. Thethere in your mind. In this chapter, you will friction and collision of particles led tolearn how these “twinkling little stars” were formation of a disk-shaped cloud and theformed. With that you will eventually also read planets were formed through the process ofthe story of origin and evolution of the earth. accretion.ORIGIN OF THE EARTH Modern Theories However, scientists in later period took up theEarly Theories problems of origin of universe rather than thatA large number of hypotheses were put forth of just the earth or the planets. The mostby different philosophers and scientists popular argument regarding the origin of theregarding the origin of the earth. One of the universe is the Big Bang Theory. It is also calledearlier and popular arguments was by German expanding universe hypothesis. Edwinphilosopher Immanuel Kant. Mathematician Hubble, in 1920, provided evidence that theLaplace revised it in 1796. It is known as universe is expanding. As time passes, galaxiesNebular Hypothesis. The hypothesis considered move further and further apart. You canthat the planets were formed out of a cloud of experiment and find what does the expandingmaterial associated with a youthful sun, which universe mean. Take a balloon and mark somewas slowly rotating. Later in 1900, Chamberlain points on it to represent the galaxies. Now, ifand Moulton considered that a wandering star you start inflating the balloon, the pointsapproached the sun. As a result, a cigar-shaped marked on the balloon will appear to be movingextension of material was separated from the away from each other as the balloon surface. As the passing star moved away, Similarly, the distance between the galaxies isthe material separated from the solar surface also found to be increasing and thereby, thecontinued to revolve around the sun and it universe is considered to be expanding.slowly condensed into planets. Sir James Jeans However, you will find that besides the increaseand later Sir Harold Jeffrey supported this in the distances between the points on the
  • 16. THE ORIGIN AND EVOLUTION OF THE EARTH 15balloon, the points themselves are expanding. The expansion of universe means increaseThis is not in accordance with the fact. in space between the galaxies. An alternativeScientists believe that though the space to this was Hoyle’s concept of steady state. Itbetween the galaxies is increasing, observations considered the universe to be roughly the samedo not support the expansion of galaxies. So, at any point of time. However, with greaterthe balloon example is only partially correct. evidence becoming available about the expanding universe, scientific community at present favours argument of expanding universe. The Star Formation The distribution of matter and energy was not even in the early universe. These initial density differences gave rise to differences in gravitational forces and it caused the matter to get drawn together. These formed the bases for development of galaxies. A galaxy contains a large number of stars. Galaxies spread over vast distances that are measured in thousands of light-years. The diameters of individual galaxies range from 80,000-150,000 light years. A galaxy starts to form by accumulation Figure 2.1 : The Big Bang of hydrogen gas in the form of a very large The Big Bang Theory considers the cloud called nebula. Eventually, growingfollowing stages in the development of the nebula develops localised clumps of gas. Theseuniverse. clumps continue to grow into even denser gaseous bodies, giving rise to formation of (i) In the beginning, all matter forming the stars. The formation of stars is believed to have universe existed in one place in the form taken place some 5-6 billion years ago. of a “tiny ball” (singular atom) with an unimaginably small volume, infinite A light year is a measure of distance and temperature and infinite density. not of time. Light travels at a speed of (ii) At the Big Bang the “tiny ball” exploded 300,000 km/second. Considering this, violently. This led to a huge expansion. the distances the light will travel in one It is now generally accepted that the year is taken to be one light year. This equals to 9.461×10 12 km. The mean event of big bang took place 13.7 billion distance between the sun and the earth years before the present. The expansion is 149,598,000 km. In terms of light continues even to the present day. As it years, it is 8.311 minutes of a year. grew, some energy was converted into matter. There was particularly rapid Formation of Planets expansion within fractions of a second after the bang. Thereafter, the The following are considered to be the stages expansion has slowed down. Within first in the development of planets : three minutes from the Big Bang event, (i) The stars are localised lumps of gas the first atom began to form. within a nebula. The gravitational force (iii) Within 300,000 years from the Big within the lumps leads to the formation Bang, temperature dropped to 4,500 K of a core to the gas cloud and a huge and gave rise to atomic matter. The rotating disc of gas and dust develops universe became transparent. around the gas core.
  • 17. 16 FUNDAMENTALS OF PHYSICAL GEOGRAPHY (ii) In the next stage, the gas cloud starts of them are much larger than the terrestrial getting condensed and the matter planets and have thick atmosphere, mostly of around the core develops into small- helium and hydrogen. All the planets were formed rounded objects. These small-rounded in the same period sometime about 4.6 billion objects by the process of cohesion develop years ago. Some data regarding our solar system into what is called planetesimals. are given in the box below. Larger bodies start forming by collision, and gravitational attraction causes the Why are the inner planets rocky while material to stick together. Planetesimals others are mostly in gaseous form? are a large number of smaller bodies. (iii) In the final stage, these large number The difference between terrestrial and jovian of small planetesimals accrete to form planets can be attributed to the following a fewer large bodies in the form of conditions: planets. (i) The terrestrial planets were formed in the close vicinity of the parent starOUR SOLAR SYSTEM where it was too warm for gases to condense to solid particles. JovianOur Solar system consists of nine planets. The planets were formed at quite a distanttenth planet 2003 UB313 has also been recently location.sighted. The nebula from which our Solar (ii) The solar wind was most intense nearersystem is supposed to have been formed, the sun; so, it blew off lots of gas andstarted its collapse and core formation some dust from the terrestrial planets. Thetime 5-5.6 billion years ago and the planets solar winds were not all that intense towere formed about 4.6 billion years ago. Our cause similar removal of gases from thesolar system consists of the sun (the star), 9 Jovian planets.planets, 63 moons, millions of smaller bodies (iii) The terrestrial planets are smaller andlike asteroids and comets and huge quantity their lower gravity could not hold theof dust-grains and gases. escaping gases. Out of the nine planets, mercury, venus,earth and mars are called as the inner planets The Moonas they lie between the sun and the belt ofasteroids the other five planets are called the outer The moon is the only natural satellite of theplanets. Alternatively, the first four are called earth. Like the origin of the earth, there haveTerrestrial, meaning earth-like as they are made been attempts to explain how the moon wasup of rock and metals, and have relatively high formed. In 1838, Sir George Darwin suggesteddensities. The rest five are called Jovian or Gas that initially, the earth and the moon formed aGiant planets. Jovian means jupiter-like. Most single rapidly rotating body. The whole mass The Solar System Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto Distance* 0.387 0.723 1.000 1.524 5.203 9.539 19.182 30.058 39.785 Density@ 5.44 5.245 5.517 3.945 1.33 0.70 1.17 1.66 0.5-0.9 Radius# 0.383 0.949 1.000 0.533 11.19 9.460 4.11 3.88 -0.3 Satellites 0 0 1 2 16 about 18 about 17 8 1 * Distance from the sun in astronomical unit i.e. average mean distance of the earth is 149,598,000 km = 1 @ Density in gm/cm3 # Radius: Equatorial radius 6378.137 km = 1
  • 18. THE ORIGIN AND EVOLUTION OF THE EARTH 17became a dumb-bell-shaped body and started getting separated depending on theireventually it broke. It was also suggested that densities. This allowed heavier materials (likethe material forming the moon was separated iron) to sink towards the centre of the earthfrom what we have at present the depression and the lighter ones to move towards theoccupied by the Pacific Ocean. surface. With passage of time it cooled further However, the present scientists do not and solidified and condensed into a smaller size.accept either of the explanations. It is now This later led to the development of the outergenerally believed that the formation of moon, surface in the form of a crust. During theas a satellite of the earth, is an outcome of ‘giant formation of the moon, due to the giant impact,impact’ or what is described as “the big splat”. the earth was further heated up. It is throughA body of the size of one to three times that of the process of differentiation that the earthmars collided into the earth sometime shortly forming material got separated into differentafter the earth was formed. It blasted a large layers. Starting from the surface to the centralpart of the earth into space. This portion of parts, we have layers like the crust, mantle,blasted material then continued to orbit the outer core and inner core. From the crust to theearth and eventually formed into the present core, the density of the material increases. Wemoon about 4.44 billion years ago. shall discuss in detail the properties of each of this layer in the next chapter.EVOLUTION OF THE EARTHDo you know that the planet earth initially was Evolution of Atmosphere and Hydrospherea barren, rocky and hot object with a thin The present composition of earth’s atmosphereatmosphere of hydrogen and helium. This is is chiefly contributed by nitrogen and oxygen.far from the present day picture of the earth. You will be dealing with the composition andHence, there must have been some events– structure of the earth’s atmosphere in Chapter 8.processes, which may have caused this change There are three stages in the evolution offrom rocky, barren and hot earth to a beautiful the present atmosphere. The first stage isplanet with ample amount of water and marked by the loss of primordial atmosphere.conducive atmosphere favouring the existence In the second stage, the hot interior of the earthof life. In the following section, you will find contributed to the evolution of the atmosphere.out how the period, between the 4,600 million Finally, the composition of the atmosphere wasyears and the present, led to the evolution of modified by the living world through thelife on the surface of the planet. process of photosynthesis. The earth has a layered structure. From The early atmosphere, with hydrogen andthe outermost end of the atmosphere to the helium, is supposed to have been stripped offcentre of the earth, the material that exists is as a result of the solar winds. This happenednot uniform. The atmospheric matter has the not only in case of the earth, but also in all theleast density. From the surface to deeper terrestrial planets, which were supposed todepths, the earth’s interior has different zones have lost their primordial atmosphere throughand each of these contains materials with the impact of solar winds.different characteristics. During the cooling of the earth, gases and How was the layered structure of the water vapour were released from the interior earth developed? solid earth. This started the evolution of the present atmosphere. The early atmosphere largely contained water vapour, nitrogen,Development of Lithosphere carbon dioxide, methane, ammonia and veryThe earth was mostly in a volatile state during little of free oxygen. The process through whichits primordial stage. Due to gradual increase the gases were outpoured from the interior isin density the temperature inside has called degassing. Continuous volcanicincreased. As a result the material inside eruptions contributed water vapour and gases
  • 19. 18 FUNDAMENTALS OF PHYSICAL GEOGRAPHY Geological Time Scale Eons Era Period Epoch Age/ Years Life/ Major Events Before Present Quaternary Holocene 0 - 10,000 Modern Man Pleistocene 10,000 - 2 million Homo Sapiens Cainozoic Tertiary Pliocene 2 - 5 million Early Human Ancestor (From 65 Miocene 5 - 24 million Ape: Flowering Plants million years and Trees to the Oligocene 24 - 37 Ma Anthropoid Ape present Eocene 37 - 58 Million Rabbits and Hare times) Palaeocene 57 - 65 Million Small Mammals : Rats – Mice Mesozoic Cretaceous 65 - 144 Million Extinction of Dinosaurs 65 - 245 Jurassic 144 - 208 Million Age of Dinosaurs Million Mammals Triassic 208 - 245 Million Frogs and turtles Permian 245 - 286 Million Reptile dominate-replace amphibians Carboniferous 286 - 360 Million First Reptiles: Palaeozoic Vertebrates: Coal beds 245 - 570 Devonian 360 - 408 Million Amphibians Million Silurian 408 - 438 Million First trace of life on land: Plants Ordovician 438 - 505 Million First Fish Cambrian 505 - 570 Million No terrestrial Life : Marine Invertebrate Proterozoic 570 - 2,500 Million Soft-bodied arthropods Archean 2,500 - 3,800 Million Blue green Algae: Pre- Unicellular bacteria Cambrian Hadean 3,800 - 4,800 Million Oceans and Continents 570 Million - 4,800 form – Ocean and Million Atmosphere are rich in Carbon dioxide Origin of 5,000 Million Origin of the sun Stars 5,000 - 13,700 Supernova Million 12,000 Million Origin of the universe Big Bang 13,700 Millionto the atmosphere. As the earth cooled, the that the oceans are as old as 4,000 millionwater vapour released started getting years. Sometime around 3,800 million yearscondensed. The carbon dioxide in the ago, life began to evolve. However, around 2,500-3,000 million years before the present,atmosphere got dissolved in rainwater and the the process of photosynthesis got evolved. Lifetemperature further decreased causing more was confined to the oceans for a long time.condensation and more rains. The rainwater Oceans began to have the contribution offalling onto the surface got collected in the oxygen through the process of photosynthesis.depressions to give rise to oceans. The earth’s Eventually, oceans were saturated with oxygen,oceans were formed within 500 million years and 2,000 million years ago, oxygen began tofrom the formation of the earth. This tells us flood the atmosphere.
  • 20. THE ORIGIN AND EVOLUTION OF THE EARTH 19Origin of Life living substance. The record of life that existed on this planet in different periods is found inThe last phase in the evolution of the earth rocks in the form of fossils. The microscopicrelates to the origin and evolution of life. It is structures closely related to the present formundoubtedly clear that the initial or even the of blue algae have been found in geologicalatmosphere of the earth was not conducive for formations that are much older than these werethe development of life. Modern scientists refer some 3,000 million years ago. It can beto the origin of life as a kind of chemical assumed that life began to evolve sometimereaction, which first generated complex organic 3,800 million years ago. The summary ofmolecules and assembled them. This evolution of life from unicellular bacteria to theassemblage was such that they could duplicate modern man is given in the Geological Timethemselves converting inanimate matter into Scale on page 18. EXERCISES 1. Multiple choice questions. (i) Which one of the following figures represents the age of the earth? (a) 4.6 million years (c) 4.6 billion years (b) 13.7 billion years (d) 13.7 trillion years (ii) Which one of the following has the longest duration? (a) Eons (c) Era (b) Period (d) Epoch (iii) Which one of the following is not related to the formation or modification of the present atmosphere? (a) Solar winds (c) Degassing (b) Differentiation (d) Photosynthesis (iv) Which one of the following represents the inner planets? (a) Planets between the sun and the earth (b) Planets between the sun and the belt of asteroids (c) Planets in gaseous state (d) Planets without satellite(s) (v) Life on the earth appeared around how many years before the present? (a) 13.7 billion (c) 4.6 billion (b) 3.8 million (d) 3.8 billion 2. Answer the following questions in about 30 words. (i) Why are the terrestrial planets rocky? (ii) What is the basic difference in the arguments related to the origin of the earth given by : (a) Kant and Laplace (b) Chamberlain and Moulton
  • 21. 20 FUNDAMENTALS OF PHYSICAL GEOGRAPHY (iii) What is meant by the process of differentiation? (iv) What was the nature of the earth surface initially? (v) What were the gases which initially formed the earth’s atmosphere? 3. Answer the following questions in about 150 words. (i) Write an explanatory note on the ‘Big Bang Theory’. (ii) List the stages in the evolution of the earth and explain each stage in brief. Project Work Collect information about the project “Stardust” (website: and along the following lines. (i) Which is the agency that has launched this project? (ii) Why are scientists interested in collecting Stardust? (iii) Where from has the Stardust been collected?
  • 22. CHAPTERINTERIOR OF THE EARTHW hat do you imagine about the nature SOURCES OF INFORMATION ABOUT THE INTERIOR of the earth? Do you imagine it to be The earth’s radius is 6,370 km. No one can a solid ball like cricket ball or a reach the centre of the earth and makehollow ball with a thick cover of rocks i.e. observations or collect samples of the material.lithosphere? Have you ever seen photographs Under such conditions, you may wonder howor images of a volcanic eruption on the scientists tell us about the earth’s interior andtelevision screen? Can you recollect the the type of materials that exist at such depths.emergence of hot molten lava, dust, smoke, fire Most of our knowledge about the interior ofand magma flowing out of the volcanic crater? the earth is largely based on estimates andThe interior of the earth can be understood only inferences. Yet, a part of the information isby indirect evidences as neither any one has nor obtained through direct observations and analysis of materials.any one can reach the interior of the earth. The configuration of the surface of the earth Direct Sourcesis largely a product of the processes operating The most easily available solid earth materialin the interior of the earth. Exogenic as well as is surface rock or the rocks we get from miningendogenic processes are constantly shaping areas. Gold mines in South Africa are as deepthe landscape. A proper understanding of the as 3 - 4 km. Going beyond this depth is notphysiographic character of a region remains possible as it is very hot at this depth. Besides mining, scientists have taken up a number ofincomplete if the effects of endogenic processes projects to penetrate deeper depths to exploreare ignored. Human life is largely influenced the conditions in the crustal portions. Scientistsby the physiography of the region. Therefore, world over are working on two major projectsit is necessary that one gets acquainted with such as “Deep Ocean Drilling Project” andthe forces that influence landscape “Integrated Ocean Drilling Project”. Thedevelopment. To understand why the earth deepest drill at Kola, in Arctic Ocean, has soshakes or how a tsunami wave is generated, it far reached a depth of 12 km. This and manyis necessary that we know certain details of the deep drilling projects have provided largeinterior of the earth. In the previous chapter, volume of information through the analysis ofyou have noted that the earth-forming materials collected at different depths.materials have been distributed in the form of Volcanic eruption forms another source of obtaining direct information. As and when thelayers from the crust to the core. It is interesting molten material (magma) is thrown onto theto know how scientists have gathered surface of the earth, during volcanic eruptioninformation about these layers and what are it becomes available for laboratory analysis.the characteristics of each of these layers. This However, it is difficult to ascertain the depth ofis exactly what this chapter deals with. the source of such magma.
  • 23. 22 FUNDAMENTALS OF PHYSICAL GEOGRAPHYIndirect Sources information about the interior of the earth. Hence, we shall discuss it in some detail.Analysis of properties of matter indirectlyprovides information about the interior. We Earthquakeknow through the mining activity thattemperature and pressure increase with the The study of seismic waves provides a completeincreasing distance from the surface towards picture of the layered interior. An earthquakethe interior in deeper depths. Moreover, it is in simple words is shaking of the earth. It is aalso known that the density of the material also natural event. It is caused due to release ofincreases with depth. It is possible to find the energy, which generates waves that travel inrate of change of these characteristics. Knowing all directions.the total thickness of the earth, scientists haveestimated the values of temperature, pressure Why does the earth shake?and the density of materials at different depths. The release of energy occurs along a fault. AThe details of these characteristics with fault is a sharp break in the crustal rocks.reference to each layer of the interior are Rocks along a fault tend to move in oppositediscussed later in this chapter. directions. As the overlying rock strata press Another source of information are the them, the friction locks them together. However,meteors that at times reach the earth. However, their tendency to move apart at some point ofit may be noted that the material that becomes time overcomes the friction. As a result, theavailable for analysis from meteors, is not from blocks get deformed and eventually, they slidethe interior of the earth. The material and the past one another abruptly. This causes astructure observed in the meteors are similar release of energy, and the energy waves travelto that of the earth. They are solid bodies in all directions. The point where the energy isdeveloped out of materials same as, or similar released is called the focus of an earthquake,to, our planet. Hence, this becomes yet another alternatively, it is called the hypocentre. Thesource of information about the interior of the energy waves travelling in different directionsearth. reach the surface. The point on the surface, The other indirect sources include nearest to the focus, is called epicentre. It isgravitation, magnetic field, and seismic activity. the first one to experience the waves. It is a pointThe gravitation force (g) is not the same at directly above the focus.different latitudes on the surface. It is greaternear the poles and less at the equator. This is Earthquake Wavesbecause of the distance from the centre at theequator being greater than that at the poles. All natural earthquakes take place in theThe gravity values also differ according to the lithosphere. You will learn about differentmass of material. The uneven distribution of layers of the earth later in this chapter. It ismass of material within the earth influences sufficient to note here that the lithosphere refersthis value. The reading of the gravity at different to the portion of depth up to 200 km from theplaces is influenced by many other factors. surface of the earth. An instrument calledThese readings differ from the expected values. ‘seismograph’ records the waves reaching theSuch a difference is called gravity anomaly. surface. A curve of earthquake waves recordedGravity anomalies give us information about on the seismograph is given in Figure 3.1. Notethe distribution of mass of the material in the that the curve shows three distinct sectionscrust of the earth. Magnetic surveys also each representing different types of waveprovide information about the distribution of patterns. Earthquake waves are basically of twomagnetic materials in the crustal portion, and types — body waves and surface waves. Bodythus, provide information about the waves are generated due to the release of energydistribution of materials in this part. Seismic at the focus and move in all directions travellingactivity is one of the most important sources of through the body of the earth. Hence, the name
  • 24. INTERIOR OF THE EARTH 23body waves. The body waves interact with the propagation. As a result, it creates densitysurface rocks and generate new set of waves differences in the material leading to stretchingcalled surface waves. These waves move along and squeezing of the material. Other threethe surface. The velocity of waves changes as waves vibrate perpendicular to the direction ofthey travel through materials with different propagation. The direction of vibrations ofdensities. The denser the material, the higher S-waves is perpendicular to the wave directionis the velocity. Their direction also changes as in the vertical plane. Hence, they create troughsthey reflect or refract when coming across and crests in the material through which theymaterials with different densities. pass. Surface waves are considered to be the most damaging waves. Emergence of Shadow Zone Earthquake waves get recorded in seismo- graphs located at far off locations. However, there exist some specific areas where the waves are not reported. Such a zone is called the ‘shadow zone’. The study of different events reveals that for each earthquake, there exists Figure 3.1 : Earthquake Waves an altogether different shadow zone. Figure 3.2 There are two types of body waves. They (a) and (b) show the shadow zones of P andare called P and S-waves. P-waves move faster S-waves. It was observed that seismographsand are the first to arrive at the surface. These located at any distance within 105° from theare also called ‘primary waves’. The P-waves epicentre, recorded the arrival of both P andare similar to sound waves. They travel S-waves. However, the seismographs locatedthrough gaseous, liquid and solid materials. beyond 145° from epicentre, record the arrivalS-waves arrive at the surface with some time of P-waves, but not that of S-waves. Thus, alag. These are called secondary waves. An zone between 105° and 145° from epicentre wasimportant fact about S-waves is that they can identified as the shadow zone for both the typestravel only through solid materials. This of waves. The entire zone beyond 105° does notcharacteristic of the S-waves is quite receive S-waves. The shadow zone of S-wave isimportant. It has helped scientists to much larger than that of the P-waves. Theunderstand the structure of the interior of the shadow zone of P-waves appears as a bandearth. Reflection causes waves to rebound around the earth between 105° and 145° awaywhereas refraction makes waves move in from the epicentre. The shadow zone of S-wavesdifferent directions. The variations in the is not only larger in extent but it is also a littledirection of waves are inferred with the help of over 40 per cent of the earth surface. You cantheir record on seismograph. The surface draw the shadow zone for any earthquakewaves are the last to report on seismograph. provided you know the location of the epicentre.These waves are more destructive. They cause (See the activity box on page 28 to know how todisplacement of rocks, and hence, the collapse locate the epicentre of a quake event).of structures occurs. Types of EarthquakesPropagation of Earthquake Waves (i) The most common ones are the tectonicDifferent types of earthquake waves travel in earthquakes. These are generated due todifferent manners. As they move or propagate, sliding of rocks along a fault plane.they cause vibration in the body of the rocks (ii) A special class of tectonic earthquake isthrough which they pass. P-waves vibrate sometimes recognised as volcanicparallel to the direction of the wave. This exerts earthquake. However, these are confinedpressure on the material in the direction of the to areas of active volcanoes.
  • 25. 24 FUNDAMENTALS OF PHYSICAL GEOGRAPHY (v) The earthquakes that occur in the areas of large reservoirs are referred to as reservoir induced earthquakes. Measuring Earthquakes The earthquake events are scaled either according to the magnitude or intensity of the shock. The magnitude scale is known as the Richter scale. The magnitude relates to the energy released during the quake. The magnitude is expressed in absolute numbers, 0-10. The intensity scale is named after Mercalli, an Italian seismologist. The intensity scale takes into account the visible damage caused by the event. The range of intensity scale is from 1-12. EFFECTS OF EARTHQUAKE Earthquake is a natural hazard. The following are the immediate hazardous effects of earthquake: (i) Ground Shaking (ii) Differential ground settlement (iii) Land and mud slides (iv) Soil liquefaction (v) Ground lurching (vi) Avalanches (vii) Ground displacement (viii) Floods from dam and levee failures (ix) Fires (x) Structural collapse (xi) Falling objects (xii) Tsunami The first six listed above have some bearings upon landforms, while others may be considered the effects causing immediate Figure 3.2 (a) and (b) : Earthquake Shadow Zones concern to the life and properties of people in the region. The effect of tsunami would occur(iii) In the areas of intense mining activity, only if the epicentre of the tremor is below sometimes the roofs of underground oceanic waters and the magnitude is mines collapse causing minor tremors. sufficiently high. Tsunamis are waves These are called collapse earthquakes. generated by the tremors and not an(iv) Ground shaking may also occur due to earthquake in itself. Though the actual quake the explosion of chemical or nuclear activity lasts for a few seconds, its effects are devices. Such tremors are called explosion devastating provided the magnitude of the earthquakes. quake is more than 5 on the Richter scale.
  • 26. INTERIOR OF THE EARTH 25Frequency of Earthquake Occurrences STRUCTURE OF THE EARTHThe earthquake is a natural hazard. If a tremor The Crustof high magnitude takes place, it can causeheavy damage to the life and property of It is the outermost solid part of the earth. It ispeople. However, not all the parts of the globe brittle in nature. The thickness of the crustnecessarily experience major shocks. We shall varies under the oceanic and continental discussing the distribution of earthquakes Oceanic crust is thinner as compared to theand volcanoes with some details in the next continental crust. The mean thickness of oceanic crust is 5 km whereas that of the continental is around 30 km. The continental crust is thicker in the areas of major mountain systems. It is as much as 70 km thick in the Himalayan region. It is made up of heavier rocks having density of 3 g/cm3. This type of rock found in the oceanic crust is basalt. The mean density of material in oceanic crust is 2.7 g/cm3. The Mantle The portion of the interior beyond the crust is A view of the damaged Aman Setu at the LOC called the mantle. The mantle extends from in Uri, due to an earthquake Moho’s discontinuity to a depth of 2,900 km.chapter. Note that the quakes of high The upper portion of the mantle is calledmagnitude, i.e. 8+ are quite rare; they occur asthenosphere. The word astheno meansonce in 1-2 years whereas those of ‘tiny’ types weak. It is considered to be extending upto 400occur almost every minute. km. It is the main source of magma that finds
  • 27. 26 FUNDAMENTALS OF PHYSICAL GEOGRAPHY been released out in the recent past. The layer below the solid crust is mantle. It has higher density than that of the crust. The mantle contains a weaker zone called asthenosphere. It is from this that the molten rock materials find their way to the surface. The material in the upper mantle portion is called magma. Once it starts moving towards the crust or it reaches the surface, it is referred to as lava. The material that reaches the ground includes lava flows, pyroclastic debris, volcanic bombs, ash and dust and gases such as nitrogen compounds, sulphur compounds and minor amounts of chlorene, hydrogen and argon. Volcanoes Volcanoes are classified on the basis of nature of eruption and the form developed at the Figure 3.4 : The interior of the earth surface. Major types of volcanoes are as follows:its way to the surface during volcanic Shield Volcanoeseruptions. It has a density higher than the Barring the basalt flows, the shield volcanoescrust’s (3.4 g/cm 3 ). The crust and the are the largest of all the volcanoes on the earth.uppermost part of the mantle are called The Hawaiian volcanoes are the most famouslithosphere. Its thickness ranges from 10-200 km.The lower mantle extends beyond theasthenosphere. It is in solid state.The CoreAs indicated earlier, the earthquake wavevelocities helped in understanding theexistence of the core of the earth. The core-mantle boundary is located at the depth of2,900 km. The outer core is in liquid state whilethe inner core is in solid state. The density ofmaterial at the mantle core boundary is around5 g/cm3 and at the centre of the earth at 6,300 Shield Volcanokm, the density value is around 13g/cm3. Thecore is made up of very heavy material mostlyconstituted by nickel and iron. It is sometimesreferred to as the nife layer.VOLCANOES AND VOLCANIC LANDFORMSYou may have seen photographs or pictures ofvolcanoes on a number of occasions. A volcanois a place where gases, ashes and/or moltenrock material – lava – escape to the ground. Avolcano is called an active volcano if thematerials mentioned are being released or have Cinder Cone
  • 28. INTERIOR OF THE EARTH 27examples. These volcanoes are mostly made more than 50 m. Individual flows may extendup of basalt, a type of lava that is very fluid for hundreds of km. The Deccan Traps fromwhen erupted. For this reason, these volcanoes India, presently covering most of theare not steep. They become explosive if Maharashtra plateau, are a much larger floodsomehow water gets into the vent; otherwise, basalt province. It is believed that initially thethey are characterised by low-explosivity. The trap formations covered a much larger areaupcoming lava moves in the form of a fountain than the present.and throws out the cone at the top of the ventand develops into cinder cone. Mid-Ocean Ridge VolcanoesComposite Volcanoes These volcanoes occur in the oceanic areas. There is a system of mid-ocean ridges moreThese volcanoes are characterised by than 70,000 km long that stretches througheruptions of cooler and more viscous lavas all the ocean basins. The central portion of thisthan basalt. These volcanoes often result in ridge experiences frequent eruptions. We shallexplosive eruptions. Along with lava, large be discussing this in detail in the next chapter.quantities of pyroclastic material and ashesfind their way to the ground. This material VOLCANIC LANDFORMSaccumulates in the vicinity of the vent openingsleading to formation of layers, and this makes Intrusive Formsthe mounts appear as composite volcanoes. The lava that is released during volcanic eruptions on cooling develops into igneous rocks. The cooling may take place either on reaching the surface or also while the lava is still in the crustal portion. Depending on the location of the cooling of the lava, igneous rocks are classified as volcanic rocks (cooling at the surface) and plutonic rocks (cooling in the crust). The lava that cools within the crustal portions assumes different forms. These forms are called intrusive forms. Some of the forms Composite Volcano are shown in Figure 3.5.CalderaThese are the most explosive of the earth’svolcanoes. They are usually so explosive thatwhen they erupt they tend to collapse onthemselves rather than building any tallstructure. The collapsed depressions are calledcalderas. Their explosiveness indicates thatthe magma chamber supplying the lava is notonly huge but is also in close vicinity.Flood Basalt ProvincesThese volcanoes outpour highly fluid lava thatflows for long distances. Some parts of theworld are covered by thousands of sq. km ofthick basalt lava flows. There can be a series offlows with some flows attaining thickness of Figure 3.5 : Volcanic Landforms
  • 29. 28 FUNDAMENTALS OF PHYSICAL GEOGRAPHYBatholiths conduit from below. It resembles the surfaceA large body of magmatic material that cools volcanic domes of composite volcano, onlyin the deeper depth of the crust develops in the these are located at deeper depths. It can be regarded as the localised source of lava thatform of large domes. They appear on the surface finds its way to the surface. The Karnatakaonly after the denudational processes remove plateau is spotted with domal hills of granitethe overlying materials. They cover large areas, rocks. Most of these, now exfoliated, areand at times, assume depth that may be several examples of lacoliths or These are granitic bodies. Batholiths arethe cooled portion of magma chambers. Lapolith, Phacolith and SillsLacoliths As and when the lava moves upwards, aThese are large dome-shaped intrusive bodies portion of the same may tend to move in awith a level base and connected by a pipe-like horizontal direction wherever it finds a weak Activity : Locating an Epicentre For this you will need Data from 3 seismograph stations about the time of arrival of P-waves, S-waves. Procedure 1. Find the time of arrival of P and S-waves of the given quake for the three stations for which you have the data. 2. Compute the time lag between the arrival of P and S-waves for each station; it is called time lag. (Note that it is directly related to the distance of the seismograph from the focus.) A. Basic rule : For every second of time lag, the earthquake is roughly 8 km away from you. 3. Using the rule quoted above, convert the time lag into distance ( # seconds of time lag * 8) for each station. 4. On a map locate the seismograph stations. 5. Draw circles, taking the seismograph stations as the centre, with the radius equal to the distance you have calculated in the previous step. (Do not forget to convert distance as per the map scale.) 6. These circles will intersect each other in a point. This point is the location of the epicentre. In normal practice, the epicentres are located using computer models. They take into account the structure of the earth’s crust. The locations with accuracy within a few hundred metres can be achieved. The procedure outlined here is a much simplified version of what is normally done, although the principle is the same. In the following diagram, the epicentre is located using this procedure. It also contains a table giving necessary data. Why don’t you try for yourself ? Data Arrival time of Station P-waves S-waves Hour Min. Sec. Hour Min. Sec. S1 03 23 20 03 24 45 S2 03 22 17 03 23 57 S3 03 22 00 03 23 55 Scale of the map 1cm = 40km
  • 30. INTERIOR OF THE EARTH 29plane. It may get rested in different forms. In while the thick horizontal deposits arecase it develops into a saucer shape, concave called the sky body, it is called lapolith. A wavymass of intrusive rocks, at times, is found at Dykesthe base of synclines or at the top of anticline When the lava makes its way through cracksin folded igneous country. Such wavy materials and the fissures developed in the land, ithave a definite conduit to source beneath inthe form of magma chambers (subsequently solidifies almost perpendicular to the ground.developed as batholiths). These are called the It gets cooled in the same position to develop aphacoliths. wall-like structure. Such structures are called The near horizontal bodies of the dykes. These are the most commonly foundintrusive igneous rocks are called sill or intrusive forms in the western Maharashtra area.sheet, depending on the thickness of the These are considered the feeders for the eruptionsmaterial. The thinner ones are called sheets that led to the development of the Deccan traps. EXERCISES 1. Multiple choice questions. (i) Which one of the following earthquake waves is more destructive? (a) P-waves (c) Surface waves (b) S-waves (d) None of the above (ii) Which one of the following is a direct source of information about the interior of the earth? (a) Earthquake waves (c) Gravitational force (b) Volcanoes (d) Earth magnetism (iii) Which type of volcanic eruptions have caused Deccan Trap formations? (a) Shield (c) Composite (b) Flood (d) Caldera (iv) Which one of the following describes the lithosphere: (a) upper and lower mantle (c) crust and core (b) crust and upper mantle (d) mantle and core 2. Answer the following questions in about 30 words. (i) What are body waves? (ii) Name the direct sources of information about the interior of the earth. (iii) Why do earthquake waves develop shadow zone? (iv) Briefly explain the indirect sources of information of the interior of the earth other than those of seismic activity. 3. Answer the following questions in about 150 words. (i) What are the effects of propagation of earthquake waves on the rock mass through which they travel? (ii) What do you understand by intrusive forms? Briefly describe various intrusive forms.
  • 31. CHAPTER DISTRIBUTION OF OCEANS AND CONTINENTSIn the previous chapter, you have studied the theory” in 1912. This was regarding theinterior of the earth. You are already familiar distribution of the oceans and the continents.with the world map. You know that continents According to Wegener, all the continentscover 29 per cent of the surface of the earth formed a single continental mass, a mega oceanand the remainder is under oceanic waters. surrounded by the same. The super continentThe positions of the continents and the ocean was named PANGAEA, which meant all earth.bodies, as we see them in the map, have not The mega-ocean was called PANTHALASSA,been the same in the past. Moreover, it is now meaning all water. He argued that, around 200a well-accepted fact that oceans and million years ago, the super continent,continents will not continue to enjoy their Pangaea, began to split. Pangaea first brokepresent positions in times to come. If this is into two large continental masses as Laurasiaso, the question arises what were their positions and Gondwanaland forming the northern andin the past? Why and how do they change their southern components respectively. Subse-positions? Even if it is true that the continents quently, Laurasia and Gondwanalandand oceans have changed and are changing continued to break into various smallertheir positions, you may wonder as to how continents that exist today. A variety of evidencescientists know this. How have they determined was offered in support of the continental drift.their earlier positions? You will find the answers Some of these are given some of these and related questions in thischapter. Evidence in Support of the Continental DriftCONTINENTAL DRIFT The Matching of Continents (Jig-Saw-Fit)Observe the shape of the coastline of the Atlantic The shorelines of Africa and South AmericaOcean. You will be surprised by the symmetry facing each other have a remarkable andof the coastlines on either side of the ocean. No unmistakable match. It may be noted that awonder, many scientists thought of this map produced using a computer programmesimilarity and considered the possibility of the to find the best fit of the Atlantic margin wastwo Americas, Europe and Africa, to be once presented by Bullard in 1964. It proved to bejoined together. From the known records of the quite perfect. The match was tried at 1,000-history of science, it was Abraham Ortelius, a fathom line instead of the present shoreline.Dutch map maker, who first proposed such apossibility as early as 1596. Antonio Pellegrini Rocks of Same Age Across the Oceansdrew a map showing the three continents together.However, it was Alfred Wegener—a German The radiometric dating methods developed inmeteorologist who put forth a comprehensive the recent period have facilitated correlating theargument in the form of “the continental drift rock formation from different continents across
  • 32. DISTRIBUTION OF OCEANS AND CONTINENTS 31the vast ocean. The belt of ancient rocks of Force for Drifting2,000 million years from Brazil coast matches Wegener suggested that the movementwith those from western Africa. The earliest responsible for the drifting of the continentsmarine deposits along the coastline of South was caused by pole-fleeing force and tidal force.America and Africa are of the Jurassic age. The polar-fleeing force relates to the rotationThis suggests that the ocean did not exist prior of the earth. You are aware of the fact that theto that time. earth is not a perfect sphere; it has a bulge at the equator. This bulge is due to the rotationTillite of the earth. The second force that wasIt is the sedimentary rock formed out of suggested by Wegener—the tidal force—is duedeposits of glaciers. The Gondawana system to the attraction of the moon and the sun thatof sediments from India is known to have its develops tides in oceanic waters. Wegenercounter parts in six different landmasses of the believed that these forces would becomeSouthern Hemisphere. At the base the system effective when applied over many million years.has thick tillite indicating extensive and However, most of scholars considered theseprolonged glaciation. Counter parts of this forces to be totally inadequate.succession are found in Africa, Falkland Island,Madagascar, Antarctica and Australia besides Post-Drift StudiesIndia. Overall resemblance of the Gondawana It is interesting to note that for continental drift,type sediments clearly demonstrates that these most of the evidence was collected from thelandmasses had remarkably similar histories. continental areas in the form of distribution ofThe glacial tillite provides unambiguous flora and fauna or deposits like tillite. A numberevidence of palaeoclimates and also of drifting of discoveries during the post-war periodof continents. added new information to geological literature. Particularly, the information collected from thePlacer Deposits ocean floor mapping provided new dimensionsThe occurrence of rich placer deposits of gold for the study of distribution of oceans andin the Ghana coast and the absolute absence continents.of source rock in the region is an amazing fact. Convectional Current TheoryThe gold bearing veins are in Brazil and it isobvious that the gold deposits of the Ghana Arthur Holmes in 1930s discussed theare derived from the Brazil plateau when the possibility of convection currents operating intwo continents lay side by side. the mantle portion. These currents are generated due to radioactive elements causingDistribution of Fossils thermal differences in the mantle portion. Holmes argued that there exists a system ofWhen identical species of plants and animals such currents in the entire mantle portion. Thisadapted to living on land or in fresh water are was an attempt to provide an explanation tofound on either side of the marine barriers, a the issue of force, on the basis of whichproblem arises regarding accounting for such contemporary scientists discarded thedistribution. The observations that Lemurs continental drift theory.occur in India, Madagascar and Africa led someto consider a contiguous landmass “Lemuria” Mapping of the Ocean Floorlinking these three landmasses. Mesosauruswas a small reptile adapted to shallow brackish Detailed research of the ocean configurationwater. The skeletons of these are found only revealed that the ocean floor is not just a vastin two localities : the Southern Cape province plain but it is full of relief. Expeditions to mapof South Africa and Iraver formations of Brazil. the oceanic floor in the post-war periodThe two localities presently are 4,800 km apart provided a detailed picture of the ocean reliefwith an ocean in between them. and indicated the existence of submerged
  • 33. 32 FUNDAMENTALS OF PHYSICAL GEOGRAPHYmountain ranges as well as deep trenches, Abyssal Plainsmostly located closer to the continent margins.The mid-oceanic ridges were found to be most These are extensive plains that lie between theactive in terms of volcanic eruptions. The dating continental margins and mid-oceanic ridges.of the rocks from the oceanic crust revealed The abyssal plains are the areas where thethe fact that the latter is much younger than continental sediments that move beyond thethe continental areas. Rocks on either side of margins get deposited.the crest of oceanic ridges and having equi-distant locations from the crest were found to Mid-Oceanic Ridgeshave remarkable similarities both in terms of This forms an interconnected chain oftheir constituents and their age. mountain system within the ocean. It is the longest mountain-chain on the surface of theOcean Floor Configuration earth though submerged under the oceanicIn this section we shall note a few things related waters. It is characterised by a central riftto the ocean floor configuration that help us in system at the crest, a fractionated plateau andthe understanding of the distribution of flank zone all along its length. The rift systemcontinents and oceans. You will be studying at the crest is the zone of intense volcanicthe details of ocean floor relief in Chapter activity. In the previous chapter, you have been13. The ocean floor may be segmented into introduced to this type of volcanoes as mid-three major divisions based on the depth oceanic well as the forms of relief. These divisionsare continental margins, deep-sea basins and Distribution of Earthquakes and Volcanoesmid-ocean ridges. Study the maps showing the distribution of seismic activity and volcanoes given in Figure 4.2. You will notice a line of dots in the central parts of the Atlantic Ocean almost parallel to the coastlines. It further extends into the Indian Ocean. It bifurcates a little south of the Indian subcontinent with one branch moving into East Africa and the other meeting a similar line from Myanmar to New Guiana. You will notice that this line of dots coincides with the mid- oceanic ridges. The shaded belt showing another area of concentration coincides with the Alpine-Himalayan system and the rim of the Pacific Ocean. In general, the foci of the earthquake in the areas of mid-oceanic ridges Figure 4.1 : Ocean Floor are at shallow depths whereas along the Alpine-Himalayan belt as well as the rim of theContinental Margins Pacific, the earthquakes are deep-seated ones. The map of volcanoes also shows a similarThese form the transition between continental pattern. The rim of the Pacific is also called rimshores and deep-sea basins. They include of fire due to the existence of active volcanoes incontinental shelf, continental slope, continental this area.rise and deep-oceanic trenches. Of these, thedeep-sea trenches are the areas which are of CONCEPT OF SEA FLOOR SPREADINGconsiderable interest in so far as thedistribution of oceans and continents is As mentioned above, the post-drift studiesconcerned. provided considerable information that was not
  • 34. DISTRIBUTION OF OCEANS AND CONTINENTS 33 Figure 4. 2 : Distribution of earthquakes and volcanoesavailable at the time Wegener put forth his (iv) The sediments on the ocean floor areconcept of continental drift. Particularly, the unexpectedly very thin. Scientists weremapping of the ocean floor and palaeomagnetic expecting, if the ocean floors were as oldstudies of rocks from oceanic regions revealed as the continent, to have a completethe following facts : sequence of sediments for a period of much(i) It was realised that all along the mid- longer duration. However, nowhere was the oceanic ridges, volcanic eruptions are sediment column found to be older than common and they bring huge amounts of 200 million years. lava to the surface in this area. (v) The deep trenches have deep-seated(ii) The rocks equidistant on either sides of the earthquake occurrences while in the mid- crest of mid-oceanic ridges show oceanic ridge areas, the quake foci have remarkable similarities in terms of period shallow depths. of formation, chemical compositions and These facts and a detailed analysis of magnetic magnetic properties. Rocks closer to the properties of the rocks on either sides of the mid-oceanic ridges are normal polarity and mid-oceanic ridge led Hess (1961) to propose are the youngest. The age of the rocks his hypothesis, known as the “sea floor increases as one moves away from the spreading”. Hess argued that constant crest. eruptions at the crest of oceanic ridges cause(iii) The ocean crust rocks are much younger the rupture of the oceanic crust and the new than the continental rocks. The age of rocks lava wedges into it, pushing the oceanic crust in the oceanic crust is nowhere more than on either side. The ocean floor, thus spreads. 200 million years old. Some of the continental The younger age of the oceanic crust as well rock formations are as old as 3,200 million as the fact that the spreading of one ocean does years. not cause the shrinking of the other, made Hess
  • 35. 34 FUNDAMENTALS OF PHYSICAL GEOGRAPHY Figure 4. 3 : Sea floor spreadingthink about the consumption of the oceanic PLATE TECTONICScrust. He further maintained that the ocean Since the advent of the concept of sea floorfloor that gets pushed due to volcanic spreading, the interest in the problem oferuptions at the crest, sinks down at the distribution of oceans and continents wasoceanic trenches and gets consumed. revived. It was in 1967, McKenzie and Parker The basic concept of sea floor spreading has and also Morgan, independently collected thebeen depicted in Figure 4.3. available ideas and came out with another The motions of the continents during the past 540 million years. 1. Africa; 2. South America; 3. Antarctica; 4. Australia; 5. India; 6. China; 7. North America; 8. Europe; 9. and 10. Siberia (Emilani, 1992) Figure 4.4 : Position of continents through geological past
  • 36. DISTRIBUTION OF OCEANS AND CONTINENTS 35concept termed Plate Tectonics. A tectonic (ii) North American (with western Atlanticplate (also called lithospheric plate) is a floor separated from the South Americanmassive, irregularly-shaped slab of solid rock, plate along the Caribbean islands) plategenerally composed of both continental and (iii) South American (with western Atlanticoceanic lithosphere. Plates move horizontally floor separated from the North Americanover the asthenosphere as rigid units. The plate along the Caribbean islands) platelithosphere includes the crust and top mantlewith its thickness range varying between 5-100 (iv) Pacific platekm in oceanic parts and about 200 km in the (v) India-Australia-New Zealand platecontinental areas. A plate may be referred to (vi) Africa with the eastern Atlantic floor plateas the continental plate or oceanic plate (vii) Eurasia and the adjacent oceanic plate.depending on which of the two occupy a larger Some important minor plates are listedportion of the plate. Pacific plate is largely an below:oceanic plate whereas the Eurasian plate may be (i) Cocos plate : Between Central Americacalled a continental plate. The theory of plate and Pacific platetectonics proposes that the earth’s lithosphere is (ii) Nazca plate : Between South Americadivided into seven major and some minor plates. and Pacific plateYoung Fold Mountain ridges, trenches, and/or (iii) Arabian plate : Mostly the Saudi Arabianfaults surround these major plates (Figure 4.5). landmassThe major plates are as follows : (i) Antarctica and the surrounding oceanic (iv) Philippine plate : Between the Asiatic and plate Pacific plate Figure 4.5 : Major and minor plates of the world
  • 37. 36 FUNDAMENTALS OF PHYSICAL GEOGRAPHY (v) Caroline plate : Between the Philippine Transform Boundaries and Indian plate (North of New Guinea) Where the crust is neither produced nor (vi) Fuji plate : North-east of Australia. destroyed as the plates slide horizontally past These plates have been constantly moving each other. Transform faults are the planes ofover the globe throughout the history of the separation generally perpendicular to the mid-earth. It is not the continent that moves as oceanic ridges. As the eruptions do not takebelieved by Wegener. Continents are part of a all along the entire crest at the same time, thereplate and what moves is the plate. Moreover, it is a differential movement of a portion of themay be noted that all the plates, without plate away from the axis of the earth. Also, theexception, have moved in the geological past, rotation of the earth has its effect on theand shall continue to move in the future period separated blocks of the plate well. Wegener had thought of all thecontinents to have initially existed as a super How do you think the rate of platecontinent in the form of Pangaea. However, movement is determined?later discoveries reveal that the continentalmasses, resting on the plates, have been Rates of Plate Movementwandering all through the geological period,and Pangaea was a result of converging of The strips of normal and reverse magnetic fielddifferent continental masses that were parts that parallel the mid-oceanic ridges helpof one or the other plates. Scientists using the scientists determine the rates of platepalaeomagnetic data have determined the movement. These rates vary considerably. Thepositions held by each of the present continental Arctic Ridge has the slowest rate (less than 2.5landmass in different geological periods. cm/yr), and the East Pacific Rise near EasterPosition of the Indian sub-continent (mostly Island, in the South Pacific about 3,400 kmPeninsular India) is traced with the help of the west of Chile, has the fastest rate (more thanrocks analysed from the Nagpur area. 15 cm/yr). There are three types of plate boundaries: Force for the Plate MovementDivergent Boundaries At the time that Wegener proposed his theoryWhere new crust is generated as the plates pull of continental drift, most scientists believedaway from each other. The sites where the that the earth was a solid, motionless body.plates move away from each other are called However, concepts of sea floor spreading andspreading sites. The best-known example of the unified theory of plate tectonics havedivergent boundaries is the Mid-Atlantic Ridge. emphasised that both the surface of the earthAt this, the American Plate(s) is/are separated and the interior are not static and motionlessfrom the Eurasian and African Plates. but are dynamic. The fact that the plates move is now a well-accepted fact. The mobile rockConvergent Boundaries beneath the rigid plates is believed to beWhere the crust is destroyed as one plate dived moving in a circular manner. The heatedunder another. The location where sinking of material rises to the surface, spreads anda plate occurs is called a subduction zone. begins to cool, and then sinks back into deeperThere are three ways in which convergence can depths. This cycle is repeated over and over tooccur. These are: (i) between an oceanic and generate what scientists call a convection cellcontinental plate; (ii) between two oceanic or convective flow. Heat within the earth comesplates; and (iii) between two continental from two main sources: radioactive decay andplates. residual heat. Arthur Holmes first considered
  • 38. DISTRIBUTION OF OCEANS AND CONTINENTS 37this idea in the 1930s, which later influencedHarry Hess’ thinking about seafloor spreading.The slow movement of hot, softened mantlethat lies below the rigid plates is the drivingforce behind the plate movement.MOVEMENT OF THE INDIAN PLATEThe Indian plate includes Peninsular Indiaand the Australian continental portions. Thesubduction zone along the Himalayas formsthe northern plate boundary in the form ofcontinent— continent convergence. In the east,it extends through Rakinyoma Mountains ofMyanmar towards the island arc along theJava T rench. The eastern margin is aspreading site lying to the east of Australia inthe form of an oceanic ridge in SW Pacific. TheWestern margin follows Kirthar Mountain ofPakistan. It further extends along the Makranacoast and joins the spreading site from theRed Sea rift southeastward along the ChagosArchipelago. The boundary between Indiaand the Antarctic plate is also marked byoceanic ridge (divergent boundary) runningin roughly W-E direction and merging into thespreading site, a little south of New Zealand. India was a large island situated off theAustralian coast, in a vast ocean. The TethysSea separated it from the Asian continent tillabout 225 million years ago. India is supposedto have started her northward journey about200 million years ago at the time when Pangaeabroke. India collided with Asia about 40-50 Figure 4.6 : Movement of the Indian platemillion years ago causing rapid uplift of the plate towards the Asiatic plate, a major eventHimalayas. The positions of India since about that occurred was the outpouring of lava and71 million years till the present are shown in formation of the Deccan Traps. This startedthe Figure 4.6. It also shows the position of somewhere around 60 million years ago andthe Indian subcontinent and the Eurasian continued for a long period of time. Note thatplate. About 140 million years before the the subcontinent was still close to the equator.present, the subcontinent was located as From 40 million years ago and thereafter, thesouth as 50oS. latitude. The two major plates event of formation of the Himalayas took place.were separated by the Tethys Sea and the Scientists believe that the process is stillTibetan block was closer to the Asiatic continuing and the height of the Himalayas islandmass. During the movement of the Indian rising even to this date.
  • 39. 38 FUNDAMENTALS OF PHYSICAL GEOGRAPHY EXERCISES 1. Multiple choice questions. (i) Who amongst the following was the first to consider the possibility of Europe, Africa and America having been located side by side. (a) Alfred Wegener (c) Abraham Ortelius (b) Antonio Pellegrini (d) Edmond Hess (ii) Polar fleeing force relates to: (a) Revolution of the Earth (c) Rotation of the earth (b) Gravitation (d) Tides (iii) Which one of the following is not a minor plate? (a) Nazca (c) Philippines (b) Arabia (d) Antarctica (iv) Which one of the following facts was not considered by those while discussing the concept of sea floor spreading? (a) Volcanic activity along the mid-oceanic ridges. (b) Stripes of normal and reverse magnetic field observed in rocks of ocean floor. (c) Distribution of fossils in different continents. (d) Age of rocks from the ocean floor. (v) Which one of the following is the type of plate boundary of the Indian plate along the Himalayan mountains? (a) Ocean-continent convergence (b) Divergent boundary (c) Transform boundary (d) Continent-continent convergence 2. Answer the following questions in about 30 words. (i) What were the forces suggested by Wegener for the movement of the continents? (ii) How are the convectional currents in the mantle initiated and maintained? (iii) What is the major difference between the transform boundary and the convergent or divergent boundaries of plates? (iv) What was the location of the Indian landmass during the formation of the Deccan Traps? 3. Answer the following questions in about 150 words. (i) What are the evidences in support of the continental drift theory? (ii) Bring about the basic difference between the drift theory and Plate tectonics. (iii) What were the major post-drift discoveries that rejuvenated the interest of scientists in the study of distribution of oceans and continents? Project Work Prepare a collage related to damages caused by an earthquake.
  • 40. UNIT III LANDFORMSThis unit deals with• Rocks and minerals — major types of rocks and their characteristics• Landforms and their evolution• Geomorphic processes — weathering, mass wasting, erosion and deposition; soils — formation
  • 41. CHAPTER MINERALS AND ROCKST he earth is composed of various kinds Though the number of elements making of elements. These elements are in solid up the lithosphere are limited they are form in the outer layer of the earth and combined in many different ways to make upin hot and molten form in the interior. many varieties of minerals. There are at least About 98 per cent of the total crust of the 2,000 minerals that have been named andearth is composed of eight elements like identified in the earth crust; but almost all theoxygen, silicon, aluminium, iron, calcium, commonly occurring ones are related to sixsodium, potassium and magnesium (Table 5.1), major mineral groups that are known as majorand the rest is constituted by titanium,hydrogen, phosphorous, manganese, sulphur, rock forming minerals.carbon, nickel and other elements. The basic source of all minerals is the hot magma in the interior of the earth. When Table 5.1 : The Major Elements of the Earth’s Crust magma cools, crystals of minerals appear and Sl. No. Elements By Weight(%) a systematic series of minerals are formed in 1. Oxygen 46.60 sequence to solidify so as to form rocks. 2. Silicon 27.72 Minerals such as coal, petroleum and natural 3. Aluminium 8.13 4. Iron 5.00 gas are organic substances found in solid, 5. Calcium 3.63 liquid and gaseous forms respectively. 6. Sodium 2.83 A brief information about some important 7. Potassium 2.59 minerals in terms of their nature and physical 8. Magnesium 2.09 9. Others 1.41 characteristics is given below : The elements in the earth’s crust are rarelyfound exclusively but are usually combined with PHYSICAL CHARACTERISTICSother elements to make various substances. (i) Exter nal crystal for m — deter -These substances are recognised as minerals. mined by internal arrangement of the molecules — cubes, octahe- Thus, a mineral is a naturally occurring drons, hexagonal prisms, etc. inorganic substance, having an orderly (ii) Cleavage — tendency to break in atomic structure and a definite chemical given directions producing composition and physical properties. A relatively plane surfaces — result mineral is composed of two or more of internal arrangement of the elements. But, sometimes single element molecules — may cleave in one or more directions and at any angle minerals like sulphur, copper, silver, gold, to each other. graphite etc. are found.
  • 42. MINERALS AND ROCKS 41 SOME MAJOR MINERALS AND (iii) Fracture — internal molecular THEIR CHARACTERISTICS arrangement so complex there are no planes of molecules; the crystal Feldspar will break in an irregular manner, not along planes of cleavage. Silicon and oxygen are common elements in (iv) Lustre — appearance of a material all types of feldspar and sodium, potassium, without regard to colour; each calcium, aluminium etc. are found in specific mineral has a distinctive lustre like feldspar variety. Half of the earth’s crust is metallic, silky, glossy etc. composed of feldspar. It has light cream to (v) Colour — some minerals have salmon pink colour. It is used in ceramics and characteristic colour determined glass making. by their molecular structure — malachite, azurite, chalcopyrite etc., Quartz and some minerals are coloured by It is one of the most important components of impurities. For example, because sand and granite. It consists of silica. It is a of impurities quartz may be white, green, red, yellow etc. hard mineral virtually insoluble in water. It is (vi) Streak — colour of the ground powder white or colourless and used in radio and radar. of any mineral. It may be of the It is one of the most important components of same colour as the mineral or may granite. differ — malachite is green and gives green streak, fluorite is purple or Pyroxene green but gives a white streak. Pyroxene consists of calcium, aluminum, (vii) Transparency — transparent: light magnesium, iron and silica. Pyroxene forms rays pass through so that objects 10 per cent of the earth’s crust. It is commonly can be seen plainly; translucent found in meteorites. It is in green or black — light rays pass through but will colour. get diffused so that objects cannot be seen; opaque — light will not pass Amphibole at all. (viii) Structure — particular arrange- Aluminium, calcium, silica, iron, magnesium ment of the individual crystals; are the major elements of amphiboles. They fine, medium or coarse grained; form 7 per cent of the earth’s crust. It is in fibrous — separable, divergent, green or black colour and is used in asbestos radiating. industry. Hornblende is another form of (ix) Hardness — relative resistance amphiboles. being scratched; ten minerals are selected to measure the degree of Mica hardness from 1-10. They are: It comprises of potassium, aluminium, 1. talc; 2. gypsum; 3. calcite; magnesium, iron, silica etc. It forms 4 per cent 4. fluorite; 5. apatite; 6. feldspar; of the earth’s crust. It is commonly found in 7. quartz; 8. topaz; 9. corundum; igneous and metamorphic rocks. It is used in 10. diamond. Compared to this for electrical instruments. example, a fingernail is 2.5 and glass or knife blade is 5.5. Olivine (x) Specific gravity — the ratio between the weight of a given object and Magnesium, iron and silica are major elements the weight of an equal volume of of olivine. It is used in jewellery. It is usually a water; object weighed in air and greenish crystal, often found in basaltic rocks. then weighed in water and divide Besides these main minerals, other minerals weight in air by the difference of the like chlorite, calcite, magnetite, haematite, two weights. bauxite and barite are also present in some quantities in the rocks.
  • 43. 42 FUNDAMENTALS OF PHYSICAL GEOGRAPHYMetallic Minerals Igneous RocksThese minerals contain metal content and can As igneous rocks form out of magma and lavabe sub-divided into three types: from the interior of the earth, they are known (i) Precious metals : gold, silver, platinum as primary rocks. The igneous rocks (Ignis – etc. in Latin means ‘Fire’) are formed when magma (ii) Ferrous metals : iron and other metals cools and solidifies. You already know what often mixed with iron to form various magma is. When magma in its upward kinds of steel. movement cools and turns into solid form it is (iii) Non-ferrous metals : include metals called igneous rock. The process of cooling and like copper, lead, zinc, tin, aluminium solidification can happen in the earth’s crust etc. or on the surface of the earth. Igneous rocks are classified based onNon-Metallic Minerals texture. Texture depends upon size and arrangement of grains or other physicalThese minerals do not contain metal content. conditions of the materials. If molten materialSulphur, phosphates and nitrates are examples is cooled slowly at great depths, mineral grainsof non-metallic minerals. Cement is a mixture may be very large. Sudden cooling (at theof non-metallic minerals. surface) results in small and smooth grains. Intermediate conditions of cooling would resultROCKS in intermediate sizes of grains making upThe earth’s crust is composed of rocks. A igneous rocks. Granite, gabbro, pegmatite,rock is an aggregate of one or more minerals. basalt, volcanic breccia and tuff are some ofRock may be hard or soft and in varied the examples of igneous rocks.colours. For example, granite is hard, soapstoneis soft. Gabbro is black and quartzite can be Sedimentary Rocksmilky white. Rocks do not have definite The word ‘sedimentary’ is derived from the Latincomposition of mineral constituents. word sedimentum, which means settling. RocksFeldspar and quartz are the most common (igneous, sedimentary and metamorphic) of theminerals found in rocks. earth’s surface are exposed to denudational agents, and are broken up into various sizes Petrology is science of rocks. A petrologist of fragments. Such fragments are transported studies rocks in all their aspects viz., by different exogenous agencies and mineral composition, texture, structure, deposited. These deposits through compaction origin, occurrence, alteration and turn into rocks. This process is called relationship with other rocks. lithification. In many sedimentary rocks, the layers of deposits retain their characteristics As there is a close relation between rocks even after lithification. Hence, we see a numberand landforms, rocks and soils, a geographer of layers of varying thickness in sedimentaryrequires basic knowledge of rocks. There are rocks like sandstone, shale etc.many different kinds of rocks which are Depending upon the mode of formation,grouped under three families on the basis of sedimentary rocks are classified into three majortheir mode of formation. They are: (i) Igneous groups: (i) mechanically formed — sandstone,Rocks — solidified from magma and lava; conglomerate, limestone, shale, loess etc. are(ii) Sedimentary Rocks — the result of examples; (ii) organically formed — geyserite,deposition of fragments of rocks by exogenous chalk, limestone, coal etc. are some examples;processes; (iii) Metamorphic Rocks — formed out (iii) chemically formed — chert, limestone, halite,of existing rocks undergoing recrystallisation. potash etc. are some examples.
  • 44. MINERALS AND ROCKS 43Metamorphic Rocks major groups — foliated rocks and non-foliated rocks. Gneissoid, granite, syenite, slate, schist,The word metamorphic means ‘change of form’. marble, quartzite etc. are some examples ofThese rocks form under the action of pressure, metamorphic rocks.volume and temperature (PVT) changes.Metamorphism occurs when rocks are forced ROCK CYCLEdown to lower levels by tectonic processes orwhen molten magma rising through the crust Rocks do not remain in their original form forcomes in contact with the crustal rocks or the long but may undergo transformation. Rockunderlying rocks are subjected to great cycle is a continuous process through whichamounts of pressure by overlying rocks. old rocks are transformed into new ones.Metamorphism is a process by which already Igneous rocks are primary rocks and otherconsolidated rocks undergo recrystallisation rocks (sedimentary and metamorphic) formand reorganisation of materials within original from these primary rocks. Igneous rocks canrocks. be changed into metamorphic rocks. The Mechanical disruption and reorganisation fragments derived out of igneous andof the original minerals within rocks due to metamorphic rocks form into sedimentarybreaking and crushing without anyappreciable chemical changes is called dynamicmetamorphism. The materials of rockschemically alter and recrystallise due tothermal metamorphism. There are two typesof thermal metamorphism — contact meta-morphism and regional metamorphism. Incontact metamorphism the rocks come incontact with hot intruding magma and lavaand the rock materials recrystallise under hightemperatures. Quite often new materials formout of magma or lava are added to the rocks.In regional metamorphism, rocks undergorecrystallisation due to deformation caused by Fig 5.1 : Rock Cycletectonic shearing together with hightemperature or pressure or both. In the process rocks. Sedimentary rocks themselves can turnof metamorphism in some rocks grains or into fragments and the fragments can be aminerals get arranged in layers or lines. Such source for formation of sedimentary rocks. Thean arrangement of minerals or grains in crustal rocks (igneous, metamorphic andmetamorphic rocks is called foliation orlineation. Sometimes minerals or materials of sedimentary) once formed may be carrieddifferent groups are arranged into alternating down into the mantle (interior of the earth)thin to thick layers appearing in light and dark through subduction process (parts or wholeshades. Such a structure in metamorphic of crustal plates going down under anotherrocks is called banding and rocks displaying plate in zones of plate convergence) and thebanding are called banded rocks. Types of same melt down due to increase inmetamorphic rocks depend upon original temperature in the interior and turn intorocks that were subjected to metamorphism. molten magma, the original source forMetamorphic rocks are classified into two igneous rocks (Figure 5.1).
  • 45. 44 FUNDAMENTALS OF PHYSICAL GEOGRAPHY EXERCISES 1. Multiple choice questions. (i) Which one of the following are the two main constituents of granite? (a) Iron and nickel (c) Silica and aluminium (b) Iron and silver (d) Iron Oxide and potassium (ii) Which one of the following is the salient feature of metamorphic rocks? (a) Changeable (c) Crystalline (b) Quite (d) Foliation (iii) Which one of the following is not a single element mineral? (a) Gold (c) Mica (b) Silver (d) Graphite (iv) Which one of the following is the hardest mineral? (a) Topaz (c) Quartz (b) Diamond (d) Feldspar (v) Which one of the following is not a sedimentary rock? (a) Tillite (c) Breccia (b) Borax (d) Marble 2. Answer the following questions in about 30 words. (i) What do you mean by rocks? Name the three major classes of rocks. (ii) What is an igneous rock? Describe the method of formation and characteristics of igneous rock. (iii) What is meant by sedimentary rock? Describe the mode of formation of sedimentary rock. (iv) What relationship explained by rock cycle between the major type of rock? 3. Answer the following questions in about 150 words. (i) Define the term ‘mineral’ and name the major classes of minerals with their physical characteristics. (ii) Describe the nature and mode of origin of the chief types of rock at the earth’s crust. How will you distinguish them? (iii) What are metamorphic rocks? Describe the types of metamorphic rock and how are they formed? Project Work Collect different rock samples and try to recognise them from their physical characteristics and identify their family.
  • 46. CHAPTERGEOMORPHIC PROCESSESA fter learning about how the earth was forces continuously elevate or build up parts born, how it evolved its crust and other of the earth’s surface and hence the exogenic inner layers, how its crustal plates processes fail to even out the relief variationsmoved and are moving, and other information of the surface of the earth. So, variations remainon earthquakes, the forms of volcanism and as long as the opposing actions of exogenic andabout the rocks and minerals the crust is endogenic forces continue. In general terms,composed of, it is time to know in detail about the endogenic forces are mainly land buildingthe surface of the earth on which we live. Let forces and the exogenic processes are mainlyus start with this question. land wearing forces. The surface of the earth is sensitive. Humans depend on it for their Why is the surface of the earth uneven? sustenance and have been using it extensively and intensively. So, it is essential to understand First of all, the earth’s crust is dynamic. You its nature in order to use it effectively withoutare well aware that it has moved and moves disturbing its balance and diminishing itsvertically and horizontally. Of course, it moved potential for the future. Almost all organismsa bit faster in the past than the rate at which it contribute to sustain the earth’s moving now. The differences in the internal However, humans have caused over use offorces operating from within the earth which resources. Use we must, but must also leave itbuilt up the crust have been responsible for potential enough to sustain life through thethe variations in the outer surface of the crust. future. Most of the surface of the earth had andThe earth’s surface is being continuously has been shaped over very long periods of timesubjected to external forces induced basically (hundreds and thousands of years) andby energy (sunlight). Of course, the internal because of its use and misuse by humans itsforces are still active though with different potential is being diminished at a fast rate. Ifintensities. That means, the earth’s surface is the processes which shaped and are shapingbeing continuously subjected to by external the surface of the earth into varieties of formsforces originating within the earth’s atmosphere (shapes) and the nature of materials of whichand by internal forces from within the earth. it is composed of, are understood, precautionsThe external forces are known as exogenic can be taken to minimise the detrimental effectsforces and the internal forces are known as of human use and to preserve it for posterity.endogenic forces. The actions of exogenicforces result in wearing down (degradation) of GEOMORPHIC PROCESSESrelief/elevations and filling up (aggradation) ofbasins/depressions, on the earth’s surface. The You would like to know the meaning ofphenomenon of wearing down of relief geomorphic processes. The endogenic andvariations of the surface of the earth through exogenic forces causing physical stresses anderosion is known as gradation. The endogenic chemical actions on earth materials and
  • 47. 46 FUNDAMENTALS OF PHYSICAL GEOGRAPHYbringing about changes in the configuration ENDOGENIC PROCESSESof the surface of the earth are known as The energy emanating from within the earth isgeomorphic processes. Diastrophism and the main force behind endogenic geomorphicvolcanism are endogenic geomorphic processes. This energy is mostly generated byprocesses. These have already been discussed radioactivity, rotational and tidal friction andin brief in the preceding unit. Weathering, mass primordial heat from the origin of the earth.wasting, erosion and deposition are exogenic This energy due to geothermal gradients andgeomorphic processes. These exogenic heat flow from within induces diastrophismprocesses are dealt with in detail in this chapter. and volcanism in the lithosphere. Due to Any exogenic element of nature (like water, variations in geothermal gradients and heat flowice, wind, etc.,) capable of acquiring and from within, crustal thickness and strength,transporting earth materials can be called a the action of endogenic forces are not uniformgeomorphic agent. When these elements of and hence the tectonically controlled originalnature become mobile due to gradients, they crustal surface is uneven.remove the materials and transport them overslopes and deposit them at lower level. DiastrophismGeomorphic processes and geomorphic agents All processes that move, elevate or build upespecially exogenic, unless stated separately, portions of the earth’s crust come underare one and the same. diastrophism. They include: (i) orogenic A process is a force applied on earth processes involving mountain buildingmaterials affecting the same. An agent is a through severe folding and affecting long andmobile medium (like running water, moving ice narrow belts of the earth’s crust; (ii) epeirogenicmasses, wind, waves and currents etc.) which processes involving uplift or warping of largeremoves, transports and deposits earth parts of the earth’s crust; (iii) earthquakesmaterials. Running water, groundwater, involving local relatively minor movements;glaciers, wind, waves and currents, etc., can (iv) plate tectonics involving horizontalbe called geomorphic agents. movements of crustal plates. In the process of orogeny, the crust is Do you think it is essential to distinguish severely deformed into folds. Due to epeirogeny, geomorphic agents and geomorphic there may be simple deformation. Orogeny is processes? a mountain building process whereas epeirogeny is continental building process. Gravity besides being a directional force Through the processes of orogeny, epeirogeny,activating all downslope movements of matter earthquakes and plate tectonics, there can bealso causes stresses on the earth’s materials. faulting and fracturing of the crust. All theseIndirect gravitational stresses activate wave and processes cause pressure, volume andtide induced currents and winds. Without temperature (PVT) changes which in turngravity and gradients there would be no induce metamorphism of rocks.mobility and hence no erosion, transportationand deposition are possible. So, gravitational Epeirogeny and orogeny, cite thestresses are as important as the other differences.geomorphic processes. Gravity is the force thatis keeping us in contact with the surface and itis the force that switches on the movement of Volcanismall surface material on earth. All the movements Volcanism includes the movement of molteneither within the earth or on the surface of the rock (magma) onto or toward the earth’searth occur due to gradients — from higher surface and also formation of many intrusivelevels to lower levels, from high pressure to low and extrusive volcanic forms. Many aspects ofpressure areas etc. volcanism have already been dealt in detail
  • 48. GEOMORPHIC PROCESSES 47under volcanoes in the Unit II and under processes and their respective driving forces.igneous rocks in the preceding chapter in this It should become clear from this chart that forunit. each process there exists a distinct driving force or energy. What do the words volcanism and As there are different climatic regions on volcanoes indicate? the earth’s surface owing to thermal gradients created by latitudinal, seasonal and land and water spread variations, the exogenicEXOGENIC PROCESSES geomorphic processes vary from region toThe exogenic processes derive their energy region. The density, type and distribution offrom atmosphere determined by the ultimate vegetation which largely depend uponenergy from the sun and also the gradientscreated by tectonic factors. Why do you think that the slopes or gradients are created by tectonic factors? Gravitational force acts upon all earthmaterials having a sloping surface and tend toproduce movement of matter in down slopedirection. Force applied per unit area is calledstress. Stress is produced in a solid by pushing Figure 6.1 : Denudational processes and theiror pulling. This induces deformation. Forces driving forcesacting along the faces of earth materials are precipitation and temperature exert influenceshear stresses (separating forces). It is this indirectly on exogenic geomorphic processes.stress that breaks rocks and other earth Within different climatic regions there may bematerials. The shear stresses result in angular local variations of the effects of different climaticdisplacement or slippage. Besides the elements due to altitudinal differences, aspectgravitational stress earth materials become variations and the variation in the amount ofsubjected to molecular stresses that may be insolation received by north and south facingcaused by a number of factors amongst which slopes as compared to east and west facingtemperature changes, crystallisation and slopes. Further, due to differences in windmelting are the most common. Chemical velocities and directions, amount and kind ofprocesses normally lead to loosening of bonds precipitation, its intensity, the relation betweenbetween grains, dissolving of soluble minerals precipitation and evaporation, daily range ofor cementing materials. Thus, the basic reason temperature, freezing and thawing frequency,that leads to weathering, mass movements, depth of frost penetration, the geomorphicerosion and deposition is development of processes vary within any climatic region.stresses in the body of the earth materials. As there are different climatic regions on What is the sole driving force behind allthe earth’s surface the exogenic geomorphic the exogenic processes?processes vary from region to region.Temperature and precipitation are the two Climatic factors being equal, the intensityimportant climatic elements that control of action of exogenic geomorphic processesvarious processes. depends upon type and structure of rocks. The All the exogenic geomorphic processes are term structure includes such aspects of rockscovered under a general term, denudation. The as folds, faults, orientation and inclination ofword ‘denude’ means to strip off or to uncover. beds, presence or absence of joints, beddingWeathering, mass wasting/movements, erosion planes, hardness or softness of constituentand transportation are included in denudation. minerals, chemical susceptibility of mineralThe flow chart (Figure 6.1) gives the denudation constituents; the permeability or impermeability
  • 49. 48 FUNDAMENTALS OF PHYSICAL GEOGRAPHYetc. Different types of rocks with differences intheir structure offer varying resistances tovarious geomorphic processes. A particularrock may be resistant to one process and non-resistant to another. And, under varyingclimatic conditions, particular rocks mayexhibit different degrees of resistance togeomorphic processes and hence they operateat differential rates and give rise to differencesin topography. The effects of most of theexogenic geomorphic processes are small andslow and may be imperceptible in a short timespan, but will in the long run affect the rocksseverely due to continued fatigue. Finally, it boils down to one fact that the Figure 6.2 : Climatic regimes and depth of weatheringdifferences on the surface of the earth though mantles (adapted and modified from Strakhov, 1967)originally related to the crustal evolutioncontinue to exist in some form or the other dueto differences in the type and structure of earth Activitymaterials, differences in geomorphic processesand in their rates of operation. Mark the latitude values of different Some of the exogenic geomorphic processes climatic regimes in Figure 6.2 andhave been dealt in detail here. compare the details.WEATHERING There are three major groups of weatheringWeathering is action of elements of weather and processes : (i) chemical; (ii) physical orclimate over earth materials. There are a mechanical; (iii) biological weathering processes.number of processes within weathering which Very rarely does any one of these processes everact either individually or together to affect the operate completely by itself, but quite often aearth materials in order to reduce them to dominance of one process can be seen.fragmental state. Chemical Weathering Processes Weathering is defined as mechanical A group of weathering processes viz; solution, disintegration and chemical decom- carbonation, hydration, oxidation and position of rocks through the actions of reduction act on the rocks to decompose, various elements of weather and climate. dissolve or reduce them to a fine clastic state through chemical reactions by oxygen, surface As very little or no motion of materials and/or soil water and other acids. Water andtakes place in weathering, it is an in-situ or air (oxygen and carbon dioxide) along withon-site process. heat must be present to speed up all chemical reactions. Over and above the carbon dioxide Is this little motion which can occur present in the air, decomposition of plants and sometimes due to weathering synonymous animals increases the quantity of carbon with transportation? If not, why? dioxide underground. These chemical Weathering processes are conditioned by reactions on various minerals are very muchmany complex geological, climatic, topographic similar to the chemical reactions in a laboratory.and vegetative factors. Climate is of particularimportance. Not only weathering processes Solutiondiffer from climate to climate, but also the depth When something is dissolved in water or acids,of the weathering mantle (Figure 6.2). the water or acid with dissolved contents is
  • 50. GEOMORPHIC PROCESSES 49called solution. This process involves removal Many clay minerals swell and contract duringof solids in solution and depends upon wetting and drying and a repetition of thissolubility of a mineral in water or weak acids. process results in cracking of overlyingOn coming in contact with water many solids materials. Salts in pore spaces undergo rapiddisintegrate and mix up as suspension in and repeated hydration and help in rockwater. Soluble rock forming minerals like fracturing. The volume changes in mineralsnitrates, sulphates, and potassium etc. are due to hydration will also help in physicalaffected by this process. So, these minerals are weathering through exfoliation and granulareasily leached out without leaving any residue rainy climates and accumulate in dryregions. Minerals like calcium carbonate and Oxidation and Reductioncalcium magnesium bicarbonate present in In weathering, oxidation means a combinationlimestones are soluble in water containing of a mineral with oxygen to form oxides orcarbonic acid (formed with the addition of hydroxides. Oxidation occurs where there iscarbon dioxide in water), and are carried away ready access to the atmosphere andin water as solution. Carbon dioxide produced oxygenated waters. The minerals mostby decaying organic matter along with soil commonly involved in this process are iron,water greatly aids in this reaction. Common manganese, sulphur etc. In the process ofsalt (sodium chloride) is also a rock forming oxidation rock breakdown occurs due to themineral and is susceptible to this process of disturbance caused by addition of oxygen. Redsolution. colour of iron upon oxidation turns to brown or yellow. When oxidised minerals are placedCarbonation in an environment where oxygen is absent, reduction takes place. Such conditions existCarbonation is the reaction of carbonate and usually below the water table, in areas ofbicarbonate with minerals and is a common stagnant water and waterlogged ground. Redprocess helping the breaking down of colour of iron upon reduction turns to greenishfeldspars and carbonate minerals. Carbon or bluish grey.dioxide from the atmosphere and soil air is These weathering processes are inter-absorbed by water, to form carbonic acid that related. Hydration, carbonation and oxidationacts as a weak acid. Calcium carbonates and go hand in hand and hasten the weatheringmagnesium carbonates are dissolved in process.carbonic acid and are removed in a solutionwithout leaving any residue resulting in caveformation. Can we give iron rusting as an example of oxidation? How essential is water in Why are clay minerals easily erodible? chemical weathering processes? Can chemical weathering processes dominate in water scarce hot deserts?HydrationHydration is the chemical addition of water. Physical Weathering ProcessesMinerals take up water and expand; thisexpansion causes an increase in the volume of Physical or mechanical weathering processesthe material itself or rock. Calcium sulphate depend on some applied forces. The appliedtakes in water and turns to gypsum, which is forces could be: (i) gravitational forces such asmore unstable than calcium sulphate. This overburden pressure, load and shearing stress;process is reversible and long, continued (ii) expansion forces due to temperaturerepetition of this process causes fatigue in the changes, crystal growth or animal activity;rocks and may lead to their disintegration. (iii) water pressures controlled by wetting and
  • 51. 50 FUNDAMENTALS OF PHYSICAL GEOGRAPHYdrying cycles. Many of these forces are applied temperatures, this internal movement amongboth at the surface and within different earth the mineral grains of the superficial layers ofmaterials leading to rock fracture. Most of the rocks takes place regularly. This process isphysical weathering processes are caused by most effective in dry climates and highthermal expansion and pressure release. These elevations where diurnal temperature changesprocesses are small and slow but can cause are drastic. As has been mentioned earliergreat damage to the rocks because of though these movements are very small theycontinued fatigue the rocks suffer due to make the rocks weak due to continued fatigue.repetition of contraction and expansion. The surface layers of the rocks tend to expand more than the rock at depth and this leads toUnloading and Expansion the formation of stress within the rock resulting in heaving and fracturing parallel to theRemoval of overlying rock load because of surface. Due to differential heating andcontinued erosion causes vertical pressure resulting expansion and contraction of surfacerelease with the result that the upper layers of layers and their subsequent exfoliation fromthe rock expand producing disintegration of the surface results in smooth rounded surfacesrock masses. Fractures will develop roughly in rocks. In rocks like granites, smoothparallel to the ground surface. In areas of surfaced and rounded small to big boulderscurved ground surface, arched fractures tend called tors form due to such produce massive sheets or exfoliation slabsof rock. Exfoliation sheets resulting from What is the difference between exfoliationexpansion due to unloading and pressure domes and exfoliated tors?release may measure hundreds or eventhousands of metres in horizontal extent. Large,smooth rounded domes called exfoliation Freezing, Thawing and Frost Wedgingdomes (Figure 6.3) result due to this process. Frost weathering occurs due to growth of ice within pores and cracks of rocks during repeated cycles of freezing and melting. This process is most effective at high elevations in mid-latitudes where freezing and melting is often repeated. Glacial areas are subject to frost wedging daily. In this process, the rate of freezing is important. Rapid freezing of water causes its sudden expansion and high pressure. The resulting expansion affects joints, cracks and small inter granular fractures to become wider and wider till the rock breaks apart.Figure 6.3 : A large exfoliation dome in granite rock Salt Weatheringnear bhongir (Bhuvanagiri) town in Andhra Pradesh Salts in rocks expand due to thermal action,Temperature Changes and Expansion hydration and crystallisation. Many salts like calcium, sodium, magnesium, potassium andVarious minerals in rocks possess their own barium have a tendency to expand. Expansionlimits of expansion and contraction. With rise of these salts depends on temperature andin temperature, every mineral expands and their thermal properties. High temperaturepushes against its neighbour and as ranges between 30 and 50 oC of surfacetemperature falls, a corresponding contraction temperatures in deserts favour such salttakes place. Because of diurnal changes in the expansion. Salt crystals in near-surface pores
  • 52. GEOMORPHIC PROCESSES 51cause splitting of individual grains withinrocks, which eventually fall off. This process offalling off of individual grains may result ingranular disintegration or granular foliation. Salt crystallisation is most effective of allsalt-weathering processes. In areas withalternating wetting and drying conditions saltcrystal growth is favoured and the neighbouringgrains are pushed aside. Sodium chloride andgypsum crystals in desert areas heave upoverlying layers of materials and with the resultpolygonal cracks develop all over the heavedsurface. With salt crystal growth, chalk breaks Fig.6.4 : Exfoliation (Flacking) and granulardown most readily, followed by limestone, disintegrationsandstone, shale, gneiss and granite etc. temperature changes. Exfoliation domes and tors result due to unloading and thermalBIOLOGICAL ACTIVITY AND WEATHERING expansion respectively.Biological weathering is contribution to orremoval of minerals and ions from the SIGNIFICANCE OF WEATHERINGweathering environment and physical changes Weathering processes are responsible fordue to growth or movement of organisms. breaking down the rocks into smallerBurrowing and wedging by organisms like fragments and preparing the way for formationearthworms, termites, rodents etc., help in of not only regolith and soils, but also erosionexposing the new surfaces to chemical attack and mass movements. Biomes and bio-and assists in the penetration of moisture and diversity is basically a result of forestsair. Human beings by disturbing vegetation, (vegetation) and forests depend upon the depthploughing and cultivating soils, also help in of weathering mantles. Erosion cannot bemixing and creating new contacts between air, significant if the rocks are not weathered. Thatwater and minerals in the earth materials. means, weathering aids mass wasting, erosionDecaying plant and animal matter help in the and reduction of relief and changes inproduction of humic, carbonic and other acids landforms are a consequence of erosion.which enhance decay and solubility of some Weathering of rocks and deposits helps in theelements. Algae utilise mineral nutrients for enrichment and concentrations of certaingrowth and help in concentration of iron and valuable ores of iron, manganese, aluminium,manganese oxides. Plant roots exert a copper etc., which are of great importance fortremendous pressure on the earth materials the national economy. Weathering is anmechanically breaking them apart. important process in the formation of soils.SOME SPECIAL EFFECTS OF WEATHERING When rocks undergo weathering, some materials are removed through chemicalThis has already been explained under or physical leaching by groundwater andphysical weathering processes of unloading, thereby the concentration of remainingthermal contraction and expansion and salt (valuable) materials increases. Withoutweathering. Exfoliation is a result but not a such a weathering taking place, theprocess. Flaking off of more or less curved concentration of the same valuablesheets of shells from over rocks or bedrock material may not be sufficient andresults in smooth and rounded surfaces economically viable to exploit, process and(Figure 6.4). Exfoliation can occur due to refine. This is what is called enrichment.expansion and contraction induced by
  • 53. 52 FUNDAMENTALS OF PHYSICAL GEOGRAPHYMASS MOVEMENTS the three forms of movements. Figure 6.5 shows the relationships among different types of massThese movements transfer the mass of rock movements, their relative rates of movementdebris down the slopes under the direct and moisture limits.influence of gravity. That means, air, water orice do not carry debris with them from place toplace but on the other hand the debris maycarry with it air, water or ice. The movementsof mass may range from slow to rapid,affecting shallow to deep columns of materialsand include creep, flow, slide and fall. Gravityexerts its force on all matter, both bedrock andthe products of weathering. So, weathering isnot a pre-requisite for mass movement thoughit aids mass movements. Mass movements arevery active over weathered slopes rather thanover unweathered materials. Mass movements are aided by gravity andno geomorphic agent like running water,glaciers, wind, waves and currents participatein the process of mass movements. That means Figure 6.5 : Relationships among different types of mass movements, their relative rates of movementmass movements do not come under erosion and moisture limits (after Whitehead, 2001)though there is a shift (aided by gravity) ofmaterials from one place to another. Materials Mass movements can be grouped underover the slopes have their own resistance to three major classes: (i) slow movements;disturbing forces and will yield only when force (ii) rapid movements; (iii) greater than the shearing resistance of thematerials. Weak unconsolidated materials, Slow Movementsthinly bedded rocks, faults, steeply dipping Creep is one type under this category whichbeds, vertical cliffs or steep slopes, abundant can occur on moderately steep, soil coveredprecipitation and torrential rains and scarcity slopes. Movement of materials is extremelyof vegetation etc., favour mass movements. slow and imperceptible except through Several activating causes precede mass extended observation. Materials involved canmovements. They are : (i) removal of support be soil or rock debris. Have you ever seen fencefrom below to materials above through natural posts, telephone poles lean downslope fromor artificial means; (ii) increase in gradient and their vertical position and in their linearheight of slopes; (iii) overloading through alignment? If you have, that is due to the creepaddition of materials naturally or by artificial effect. Depending upon the type of material involved, several types of creep viz., soil creep,filling; (iv) overloading due to heavy rainfall, talus creep, rock creep, rock-glacier creep etc.,saturation and lubrication of slope materials; can be identified. Also included in this group(v) removal of material or load from over the is solifluction which involves slow downslopeoriginal slope surfaces; (vi) occurrence of flowing soil mass or fine grained rock debrisearthquakes, explosions or machinery; saturated or lubricated with water. This process(vii) excessive natural seepage; (viii) heavy is quite common in moist temperate areasdrawdown of water from lakes, reservoirs and where surface melting of deeply frozen groundrivers leading to slow outflow of water from and long continued rain respectively, occurunder the slopes or river banks; (ix) indis- frequently. When the upper portions getcriminate removal of natural vegetation. saturated and when the lower parts are Heave (heaving up of soils due to frost impervious to water percolation, flowing occursgrowth and other causes), flow and slide are in the upper parts.
  • 54. GEOMORPHIC PROCESSES 53Rapid Movements discontinuities in the rock, the degree of weathering and the steepness of the slope.These movements are mostly prevalent in Depending upon the type of movement ofhumid climatic regions and occur over gentle materials several types are identified in thisto steep slopes. Movement of water-saturated category.clayey or silty earth materials down low-angle Slump is slipping of one or several units ofterraces or hillsides is known as earthflow. rock debris with a backward rotation withQuite often, the materials slump making step- respect to the slope over which the movementlike terraces and leaving arcuate scarps at their takes place (Figure 6.6). Rapid rolling or slidingheads and an accumulation bulge at the toe.When slopes are steeper, even the bedrockespecially of soft sedimentary rocks like shaleor deeply weathered igneous rock may slidedownslope. Another type in this category is mudflow.In the absence of vegetation cover and withheavy rainfall, thick layers of weatheredmaterials get saturated with water and eitherslowly or rapidly flow down along definitechannels. It looks like a stream of mud withina valley. When the mudflows emerge out of Figure 6.6 : Slumping of debris with backward rotationchannels onto the piedmont or plains, they canbe very destructive engulfing roads, bridges of earth debris without backward rotation ofand houses. Mudflows occur frequently on the mass is known as debris slide. Debris fall isslopes of erupting or recently erupted volcanoes. nearly a free fall of earth debris from a verticalVolcanic ash, dust and other fragments turn or overhanging face. Sliding of individual rockinto mud due to heavy rains and flow down as masses down bedding, joint or fault surfacestongues or streams of mud causing great is rockslide. Over steep slopes, rock sliding isdestruction to human habitations. very fast and destructive. Figure 6.7 shows A third type is the debris avalanche, which landslide scars over steep slopes. Slides occuris more characteristic of humid regions with as planar failures along discontinuities likeor without vegetation cover and occurs in bedding planes that dip steeply. Rock fall isnarrow tracks on steep slopes. This debris free falling of rock blocks over any steep slopeavalanche can be much faster than the keeping itself away from the slope. Rock fallsmudflow. Debris avalanche is similar to snow occur from the superficial layers of the rockavalanche. In Andes mountains of South America and the Rockies mountains of North America, there are a few volcanoes which erupted during the last decade and very devastating mudflows occurred down their slopes during eruption as well as after eruption.LandslidesThese are known as relatively rapid andperceptible movements. The materials involvedare relatively dry. The size and shape of the Figure 6.7 : Landslide scars in Shiwalik Himalayan rangesdetached mass depends on the nature of near river Sarada at India-Nepal border, Uttar Pradesh
  • 55. 54 FUNDAMENTALS OF PHYSICAL GEOGRAPHYface, an occurrence that distinguishes it from erosion it is not a pre-condition for erosion torockslide which affects materials up to a take place. Weathering, mass-wasting andsubstantial depth. erosion are degradational processes. It is erosion that is largely responsible for Between mass wasting and mass continuous changes that the earth’s surface is movements, which term do you feel is undergoing. As indicated in Figure 6.1, most appropriate? Why? Can solifluction denudational processes like erosion and be included under rapid flow movements? transportation are controlled by kinetic energy. Why it can be and can’t be? The erosion and transportation of earth materials is brought about by wind, running water, glaciers, waves and ground water. Of In our country, debris avalanche and these the first three agents are controlled by landslides occur very frequently in the climatic conditions. Himalayas. There are many reasons for this. One, the Himalayas are tectonically Can you compare the three climatically active. They are mostly made up of controlled agents? sedimentary rocks and unconsolidated and semi-consolidated deposits. The slopes are very steep. Compared to the They represent three states of matter — Himalayas, the Nilgiris bordering gaseous (wind), liquid (running water) and Tamilnadu, Karnataka, Kerala and the solid (glacier) respectively. The erosion can be Western Ghats along the west coast are defined as “application of the kinetic energy relatively tectonically stable and are associated with the agent to the surface of the mostly made up of very hard rocks; but, land along which it moves”. Kinetic energy is still, debris avalanches and landslides computed as KE = 1/2 mv2 where ‘m’ is the mass occur though not as frequently as in the and ‘v’ is the velocity. Hence the energy Himalayas, in these hills. Why? Many available to perform work will depend on the slopes are steeper with almost vertical mass of the material and the velocity with cliffs and escarpments in the Western which it is moving. Obviously then you will find Ghats and Nilgiris. Mechanical weathering that though the glaciers move at very low due to temperature changes and ranges is pronounced. They receive heavy velocities due to tremendous mass are more amounts of rainfall over short periods. effective as the agents of erosion and wind, So, there is almost direct rock fall quite being in gaseous state, are less effective. frequently in these places along with The work of the other two agents of erosion- landslides and debris avalanches. waves and ground water is not controlled by climate. In case of waves it is the location along the interface of litho and hydro sphere —EROSION AND DEPOSITION coastal region — that will determine the work of waves, whereas the work of ground water isErosion involves acquisition and transportation determined more by the lithological characterof rock debris. When massive rocks break into of the region. If the rocks are permeable andsmaller fragments through weathering and soluble and water is available only then karstany other process, erosional geomorphic topography develops. In the next chapter weagents like running water, groundwater, shall be dealing with the landforms producedglaciers, wind and waves remove and by each of the agents of erosion.transport it to other places depending upon Deposition is a consequence of erosion. Thethe dynamics of each of these agents. Abrasion erosional agents loose their velocity and henceby rock debris carried by these geomorphic energy on gentler slopes and the materialsagents also aids greatly in erosion. By erosion, carried by them start to settle themselves. Inrelief degrades, i.e., the landscape is worn other words, deposition is not actually the workdown. That means, though weathering aids of any agent. The coarser materials get
  • 56. GEOMORPHIC PROCESSES 55deposited first and finer ones later. By of the weathered material) which is the basicdeposition depressions get filled up. The same input for soil to form. First, the weatherederosional agents viz., running water, glaciers, material or transported deposits are colonisedwind, waves and groundwater act as by bacteria and other inferior plant bodies likeaggradational or depositional agents also. mosses and lichens. Also, several minor What happens to the surface of the earth organisms may take shelter within the mantledue to erosion and deposition is elaborated in and deposits. The dead remains of organismsthe next chapter on landforms and their and plants help in humus accumulation. Minorevolution. grasses and ferns may grow; later, bushes and trees will start growing through seeds brought There is a shift of materials in mass in by birds and wind. Plant roots penetrate movements as well as in erosion from one down, burrowing animals bring up particles, place to the other. So, why can’t both be mass of material becomes porous and sponge- treated as one and the same? Can there like with a capacity to retain water and to permit be appreciable erosion without rocks the passage of air and finally a mature soil, a undergoing weathering? complex mixture of mineral and organic products forms.SOIL FORMATION Is weathering solely responsible for soilSoil and Soil Contents formation? If not, why?You see plants growing in soils. You play inthe ground and come into contact with soil. Pedology is soil science. A pedologist is aYou touch and feel soil and soil your clothes soil-scientist.while playing. Can you describe it? A pedologist who studies soils defines soilas a collection of natural bodies on the earth’s Soil-forming Factorssurface containing living matter andsupporting or capable of supporting plants. Five basic factors control the formation of soils: Soil is a dynamic medium in which many (i) parent material; (ii) topography; (iii) climate;chemical, physical and biological activities go (iv) biological activity; (v) time. In fact soilon constantly. Soil is a result of decay, it is also forming factors act in union and affect thethe medium for growth. It is a changing and action of one another.developing body. It has many characteristicsthat fluctuate with the seasons. It may be Parent Materialalternatively cold and warm or dry and moist. Parent material is a passive control factor inBiological activity is slowed or stopped if thesoil becomes too cold or too dry. Organic matter soil formation. Parent materials can be any in-increases when leaves fall or grasses die. The situ or on-site weathered rock debris (residualsoil chemistry, the amount of organic matter, soils) or transported deposits (transportedthe soil flora and fauna, the temperature and soils). Soil formation depends upon the texturethe moisture, all change with the seasons as (sizes of debris) and structure (disposition ofwell as with more extended periods of time. individual grains/particles of debris) as wellThat means, soil becomes adjusted to as the mineral and chemical composition of theconditions of climate, landform and vegetation rock debris/deposits.and will change internally when these Nature and rate of weathering and depth ofcontrolling conditions change. weathering mantle are important consideration under parent materials. There may beProcess of Soil Formation differences in soil over similar bedrock and dissimilar bedrocks may have similar soilsSoil formation or pedogenesis depends first on above them. But when soils are very youngweathering. It is this weathering mantle (depth and have not matured these show strong links
  • 57. 56 FUNDAMENTALS OF PHYSICAL GEOGRAPHYwith the type of parent rock. Also, in case of climates and in areas with intermediatesome limestone areas, where the weathering precipitation conditions, calcium carbonateprocesses are specific and peculiar, soils will nodules (kanker) are clear relation with the parent rock. Temperature acts in two ways — increasing or reducing chemical and biological activity.Topography Chemical activity is increased in higherTopography like parent materials is another temperatures, reduced in cooler temperaturespassive control factor. The influence of (with an exception of carbonation) and stopstopography is felt through the amount of in freezing conditions. That is why, tropical soilsexposure of a surface covered by parent with higher temperatures show deeper profilesmaterials to sunlight and the amount of and in the frozen tundra regions soils containsurface and sub-surface drainage over and largely mechanically broken materials.through the parent materials. Soils will be thin Biological Activityon steep slopes and thick over flat uplandareas. Over gentle slopes where erosion is slow The vegetative cover and organisms that occupyand percolation of water is good, soil formation the parent materials from the beginning and alsois very favourable. Soils over flat areas may at later stages help in adding organic matter,develop a thick layer of clay with good moisture retention, nitrogen etc. Dead plantsaccumulation of organic matter giving the soil provide humus, the finely divided organic matterdark colour. In middle latitudes, the south of the soil. Some organic acids which formfacing slopes exposed to sunlight have different during humification aid in decomposing theconditions of vegetation and soils and the north minerals of the soil parent materials.facing slopes with cool, moist conditions have Intensity of bacterial activity shows upsome other soils and vegetation. differences between soils of cold and warm climates. Humus accumulates in cold climatesClimate as bacterial growth is slow. With undecomposedClimate is an important active factor in soil organic matter because of low bacterial activity,formation. The climatic elements involved in soil layers of peat develop in sub-arctic and tundradevelopment are : (i) moisture in terms of its climates. In humid tropical and equatorialintensity, frequency and duration of climates, bacterial growth and action is intenseprecipitation - evaporation and humidity; and dead vegetation is rapidly oxidised leaving(ii) temperature in terms of seasonal and very low humus content in the soil. Further,diurnal variations. bacteria and other soil organisms take gaseous Precipitation gives soil its moisture content nitrogen from the air and convert it into awhich makes the chemical and biological chemical form that can be used by plants. Thisactivities possible. Excess of water helps in the process is known as nitrogen fixation.downward transportation of soil components Rhizobium, a type of bacteria, lives in the rootthrough the soil (eluviation) and deposits the nodules of leguminous plants and fixes nitrogensame down below (illuviation). In climates like beneficial to the host plant. The influence of largewet equatorial rainy areas with high rainfall, animals like ants, termites, earthworms, rodentsnot only calcium, sodium, magnesium, etc., is mechanical, but, it is neverthelesspotassium etc. but also a major part of silica is important in soil formation as they rework theremoved from the soil. Removal of silica from soil up and down. In case of earthworms, asthe soil is known as desilication. In dry climates, they feed on soil, the texture and chemistry ofbecause of high temperature, evaporation the soil that comes out of their body changes.exceeds precipitation and hence ground wateris brought up to the surface by capillary action Timeand in the process the water evaporates leaving Time is the third important controlling factorbehind salts in the soil. Such salts form into a in soil formation. The length of time the soilcrust in the soil known as hardpans. In tropical forming processes operate, determines
  • 58. GEOMORPHIC PROCESSES 57 maturation of soils and profile development. A Is it necessary to separate the process of soil becomes mature when all soil-forming soil formation and the soil forming control processes act for a sufficiently long time factors? developing a profile. Soils developing from recently deposited alluvium or glacial till are Why are time, topography and parent considered young and they exhibit no horizons material considered as passive control or only poorly developed horizons. No specific factors in soil formation? length of time in absolute terms can be fixed for soils to develop and mature. EXERCISES 1. Multiple choice questions. (i) Which one of the following processes is a gradational process? (a) Deposition (c) Volcanism (b) Diastrophism (d) Erosion (ii) Which one of the following materials is affected by hydration process? (a) Granite (c) Quartz (b) Clay (d) Salts (iii) Debris avalanche can be included in the category of: (a) Landslides (c) Rapid flow mass movements (b) Slow flow mass movements (d) Subsidence 2. Answer the following questions in about 30 words. (i) It is weathering that is responsible for bio-diversity on the earth. How? (ii) What are mass movements that are real rapid and perceptible? List. (iii) What are the various mobile and mighty exogenic geomorphic agents and what is the prime job they perform? (iv) Is weathering essential as a pre-requisite in the formation of soils? Why? 3. Answer the following questions in about 150 words. (i) “Our earth is a playfield for two opposing groups of geomorphic processes.” Discuss. (ii) Exogenic geomorphic processes derive their ultimate energy from the sun’s heat. Explain. (iii) Are physical and chemical weathering processes independent of each other? If not, why? Explain with examples. (iv) How do you distinguish between the process of soil formation and soil- forming factors? What is the role of climate and biological activity as two important control factors in the formation of soils? Project Work Depending upon the topography and materials around you, observe and record climate, possible weathering process and soil contents and characteristics.
  • 59. CHAPTER LANDFORMS AND THEIR EVOLUTIONA fter weathering processes have had means, each and every landform has a history their actions on the earth materials of development and changes through time. A making up the surface of the earth, the landmass passes through stages ofgeomorphic agents like running water, ground development somewhat comparable to thewater, wind, glaciers, waves perform erosion. stages of life — youth, mature and old age.It is already known to you that erosion causeschanges on the surface of the earth. Deposition What are the two important aspects offollows erosion and because of deposition too, the evolution of landforms?changes occur on the surface of the earth. As this chapter deals with landforms and The evolutionary history of the continuallytheir evolution first start with the question, changing surface of the earth is essential to bewhat is a landform? In simple words, small to understood in order to use it effectively withoutmedium tracts or parcels of the earth’s surface disturbing its balance and diminishing itsare called landforms. potential for the future. Geomorphology deals If landform is a small to medium sized part with the reconstruction of the history of theof the surface of the earth, what is a landscape? surface of the earth through a study of its Several related landforms together make forms, the materials of which it is made up ofup landscapes, (large tracts of earth’s surface). and the processes that shape it.Each landform has its own physical shape, size, Changes on the surface of the earth owematerials and is a result of the action of certain mostly to erosion by various geomorphicgeomorphic processes and agent(s). Actions agents. Of course, the process of deposition too,of most of the geomorphic processes and by covering the land surfaces and filling theagents are slow, and hence the results take a basins, valleys or depressions, brings changeslong time to take shape. Every landform has a in the surface of the land. Deposition followsbeginning. Landforms once formed may erosion and the depositional surfaces too arechange in their shape, size and nature slowly ultimately subjected to erosion. Running water,or fast due to continued action of geomorphic ground-water, glaciers, wind and waves areprocesses and agents. powerful erosional and depositional agents Due to changes in climatic conditions and shaping and changing the surface of the earthvertical or horizontal movements of land- aided by weathering and mass wastingmasses, either the intensity of processes or the processes. These geomorphic agents actingprocesses themselves might change leading to over long periods of time produce systematicnew modifications in the landforms. Evolution changes leading to sequential development ofhere implies stages of transformation of either landforms. Each geomorphic agent producesa part of the earth’s surface from one landform its own assemblage of landforms. Not only this,into another or transformation of individual each geomorphic process and agent leave theirlandforms after they are once formed. That distinct imprints on the landforms they
  • 60. LANDFORMS AND THEIR EVOLUTION 59produce. You know that most of the streams and rivers in valleys. Most of thegeomorphic processes are imperceptible erosional landforms made by running waterfunctions and can only be seen and measured are associated with vigorous and youthfulthrough their results. What are the results? rivers flowing along gradients. With time,These results are nothing but landforms and stream channels over steep gradients turntheir characteristics. Hence, a study of gentler due to continued erosion, and as alandforms, will reveal to us the process and consequence, lose their velocity, facilitatingagent which has made or has been making active deposition. There may be depositionalthose landforms. forms associated with streams flowing over steep slopes. But these phenomena will be on Most of the geomorphic processes are a small scale compared to those associated imperceptible. Cite a few processes which with rivers flowing over medium to gentle can be seen and a few which can’t be slopes. The gentler the river channels in seen. gradient or slope, the greater is the deposition. When the stream beds turn gentler due to As the geomorphic agents are capable of continued erosion, downward cutting becomeserosion and deposition, two sets — erosional less dominant and lateral erosion of banksor destructional and depositional or increases and as a consequence the hills andconstructional — of landforms are produced valleys are reduced to them. Many varieties of landforms developby the action of each of the geomorphic agents Is complete reduction of relief of a highdepending upon especially the type and land mass possible?structure i.e. folds, faults, joints, fractures,hardness and softness, permeability and Overland flow causes sheet erosion.impermeability, etc. come under structure of Depending upon irregularities of the landrocks. There are some other independent surface, the overland flow may concentrate intocontrols like (i) stability of sea level; (ii) tectonic narrow to wide paths. Because of the sheerstability of landmasses; (iii) climate, which friction of the column of flowing water, minorinfluence the evolution of landforms. Any or major quantities of materials from thedisturbance in any of these three controlling surface of the land are removed in the direction of flow and gradually small and narrow rillsfactors can upset the systematic and will form. These rills will gradually develop intosequential stages in the development and long and wide gullies; the gullies will furtherevolution of landforms. deepen, widen, lengthen and unite to give rise In the following pages, under each of the to a network of valleys. In the early stages,geomorphic regimes i.e. running water; down-cutting dominates during whichgroundwater, glaciers, waves, and winds, first irregularities such as waterfalls and cascadesa brief discussion is presented as to how will be removed. In the middle stages, streamslandmasses are reduced in their relief through cut their beds slower, and lateral erosion oferosion and then, development of some of the valley sides becomes severe. Gradually, theerosional and depositional landforms is dealt valley sides are reduced to lower and lowerwith. slopes. The divides between drainage basins are likewise lowered until they are almostRUNNING WATER completely flattened leaving finally, a lowlandIn humid regions, which receive heavy rainfall of faint relief with some low resistant remnantsrunning water is considered the most called monadnocks standing out here andimportant of the geomorphic agents in there. This type of plain forming as a result ofbringing about the degradation of the land stream erosion is called a peneplain (an almostsurface. There are two components of running plain). The characteristics of each of the stageswater. One is overland flow on general land of landscapes developing in running watersurface as a sheet. Another is linear flow as regimes may be summarised as follows:
  • 61. 60 FUNDAMENTALS OF PHYSICAL GEOGRAPHYYouthStreams are few during this stage with poorintegration and flow over original slopesshowing shallow V-shaped valleys with nofloodplains or with very narrow floodplainsalong trunk streams. Streams divides are broadand flat with marshes, swamp and lakes.Meanders if present develop over these broadupland surfaces. These meanders mayeventually entrench themselves into theuplands. Waterfalls and rapids may exist wherelocal hard rock bodies are exposed.MatureDuring this stage streams are plenty with goodintegration. The valleys are still V-shaped butdeep; trunk streams are broad enough to havewider floodplains within which streams mayflow in meanders confined within the valley.The flat and broad inter stream areas andswamps and marshes of youth disappear andthe stream divides turn sharp. Waterfalls andrapids disappear.OldSmaller tributaries during old age are few with Figure 7.1 : The Valley of Kaveri river near Hogenekal,gentle gradients. Streams meander freely over Dharmapuri district, Tamilnadu in the form of gorgevast floodplains showing natural levees, oxbowlakes, etc. Divides are broad and flat with lakes,swamps and marshes. Most of the landscapeis at or slightly above sea level.EROSIONAL LANDFORMSValleysValleys start as small and narrow rills; the rillswill gradually develop into long and widegullies; the gullies will further deepen, widenand lengthen to give rise to valleys. Depending Figure 7.2 : An entrenched meander loop of river Colorado in USA showing step-like side slopes of its valleyupon dimensions and shape, many types of typical of a canyonvalleys like V-shaped valley, gorge, canyon,etc. can be recognised. A gorge is a deep valley is wider at its top than at its bottom. In fact, awith very steep to straight sides (Figure 7.1) and canyon is a variant of gorge. Valley types dependa canyon is characterised by steep step-like upon the type and structure of rocks in whichside slopes (Figure 7.2) and may be as deep as they form. For example, canyons commonlya gorge. A gorge is almost equal in width at its form in horizontal bedded sedimentary rockstop as well as its bottom. In contrast, a canyon and gorges form in hard rocks.
  • 62. LANDFORMS AND THEIR EVOLUTION 61Potholes and Plunge Pools River TerracesOver the rocky beds of hill-streams more or less River terraces are surfaces marking old valleycircular depressions called potholes form floor or floodplain levels. They may be bedrockbecause of stream erosion aided by the abrasion surfaces without any alluvial cover or alluvialof rock fragments. Once a small and shallow terraces consisting of stream deposits. Riverdepression forms, pebbles and boulders get terraces are basically products of erosion ascollected in those depressions and get rotated they result due to vertical erosion by the streamby flowing water and consequently the into its own depositional floodplain. There candepressions grow in dimensions. A series of such be a number of such terraces at differentdepressions eventually join and the stream heights indicating former river bed levels. Thevalley gets deepened. At the foot of waterfalls river terraces may occur at the same elevationalso, large potholes, quite deep and wide, form on either side of the rivers in which case theybecause of the sheer impact of water and are called paired terraces (Figure 7.3).rotation of boulders. Such large and deep holesat the base of waterfalls are called plunge pools.These pools also help in the deepening of valleys.Waterfalls are also transitory like any otherlandform and will recede gradually and bringthe floor of the valley above waterfalls to thelevel below.INCISED OR ENTRENCHED MEANDERSIn streams that flow rapidly over steepgradients, normally erosion is concentrated onthe bottom of the stream channel. Also, in the Figure 7.3 : Paired and unpaired river terracescase of steep gradient streams, lateral erosionon the sides of the valleys is not much whencompared to the streams flowing on low and When a terrace is present only on one sidegentle slopes. Because of active lateral erosion, of the stream and with none on the other sidestreams flowing over gentle slopes, develop or one at quite a different elevation on the othersinuous or meandering courses. It is common side, the terraces are called non-pairedto find meandering courses over floodplains terraces. Unpaired terraces are typical in areasand delta plains where stream gradients are of slow uplift of land or where the water columnvery gentle. But very deep and wide meanders changes are not uniform along both the banks.can also be found cut in hard rocks. Such The terraces may result due to (i) receding watermeanders are called incised or entrenched after a peak flow; (ii) change in hydrologicalmeanders (Figure 7.2). Meander loops develop regime due to climatic changes; (iii) tectonicover original gentle surfaces in the initial stages uplift of land; (iv) sea level changes in case ofof development of streams and the same loops rivers closer to the sea.get entrenched into the rocks normally due toerosion or slow, continued uplift of the land DEPOSITIONAL LANDFORMSover which they start. They widen and deepenover time and can be found as deep gorges andcanyons in hard rock areas. They give an Alluvial Fansindication on the status of original land Alluvial fans (Figure 7.4) are formed whensurfaces over which streams have developed. streams flowing from higher levels break into foot slope plains of low gradient. Normally very What are the differences between incised coarse load is carried by streams flowing over meanders and meanders over flood and mountain slopes. This load becomes too heavy delta plains? for the streams to be carried over gentler
  • 63. 62 FUNDAMENTALS OF PHYSICAL GEOGRAPHYgradients and gets dumped and spread as a as a low cone. Unlike in alluvial fans, thebroad low to high cone shaped deposit called deposits making up deltas are very well sortedalluvial fan. Usually, the streams which flow with clear stratification. The coarsest materialsover fans are not confined to their original settle out first and the finer fractions like siltschannels for long and shift their position across and clays are carried out into the sea. As thethe fan forming many channels called delta grows, the river distributaries continuedistributaries. Alluvial fans in humid areas to increase in length (Figure 7.5) and deltashow normally low cones with gentle slope from continues to build up into the sea. Floodplains, Natural Levees and Point Bars Deposition develops a floodplain just as erosion makes valleys. Floodplain is a major landform of river deposition. Large sized materials are deposited first when stream channel breaks into a gentle slope. Thus, normally, fine sized materials like sand, silt and clay are carried by relatively slow moving waters in gentler channels usually found in the plains and deposited over the bed and when the waters spill over the banks during floodingFigure 7.4 : An alluvial fan deposited by a hill stream above the bed. A river bed made of river on the way to Amarnath, Jammu and Kashmir deposits is the active floodplain. The floodplain above the bank is inactive floodplain. Inactivehead to toe and they appear as high cones with floodplain above the banks basically containsteep slope in arid and semi-arid climates. two types of deposits — flood deposits and channel deposits. In plains, channels shiftDeltas laterally and change their courses occasionallyDeltas are like alluvial fans but develop at a leaving cut-off courses which get filled updifferent location. The load carried by the rivers gradually. Such areas over flood plains builtis dumped and spread into the sea. If this load up by abandoned or cut-off channels containis not carried away far into the sea or distributed coarse deposits. The flood deposits of spilledalong the coast, it spreads and accumulates waters carry relatively finer materials like silt and clay. The flood plains in a delta are called delta plains. Natural levees and point bars (Figure 7.6) are some of the important landforms found associated with floodplains. Natural levees are found along the banks of large rivers. They are low, linear and parallel ridges of coarse deposits along the banks of rivers, quite often cut into individual mounds. During flooding as the water spills over the bank, the velocity of the water comes down and large sized and high specific gravity materials get dumped in the immediate vicinity of the bank as ridges. They are high nearer the banks and slope gently away from the river. The levee deposits are coarser than the deposits spread by flood Figure 7.5 : A satellite view of part of Krishna river waters away from the river. When rivers shift delta, Andhra Pradesh laterally, a series of natural levees can form.
  • 64. LANDFORMS AND THEIR EVOLUTION 63 Meander is not a landform but is only a type of channel pattern. This is because of (i) propensity of water flowing over very gentle gradients to work laterally on the banks; (ii) unconsolidated nature of alluvial deposits making up the banks with many irregularities which can be used by water exerting pressure laterally; (iii) coriolis force acting on the fluid water deflecting it like it deflects the wind. When the gradient of the channel becomes extremely low, water flows leisurely and starts working Figure 7.6 : Natural levee and point bars laterally. Slight irregularities along the banks slowly get transformed into a small curvaturePoint bars are also known as meander bars. in the banks; the curvature deepens due toThey are found on the convex side of meanders deposition on the inside of the curve andof large rivers and are sediments deposited in erosion along the bank on the outside. If therea linear fashion by flowing waters along the is no deposition and no erosion or undercutting,bank. They are almost uniform in profile and inwidth and contain mixed sizes of sediments. If the tendency to meander is reduced. Normally,there more than one ridge, narrow and elongated in meanders of large rivers, there is activedepressions are found in between the point bars. deposition along the convex bank andRivers build a series of them depending upon undercutting along the concave bank.the water flow and supply of sediment. As therivers build the point bars on the convex side,the bank on the concave side will erode actively. In what way do natural levees differ from point bars?MeandersIn large flood and delta plains, rivers rarely flowin straight courses. Loop-like channel patternscalled meanders develop over flood and deltaplains (Figure 7.7). Figure 7.7 : A satellite scene showing meanderingBurhi Gandak river near Muzaffarpur, Bihar, showing Figure 7.8 : Meander growth and cut-off loops and a number of oxbow lakes and cut-offs slip-off and undercut banks
  • 65. 64 FUNDAMENTALS OF PHYSICAL GEOGRAPHYThe concave bank is known as cut-off bank is more in the valley, channel bars and islandswhich shows up as a steep scarp and the of sand, gravel and pebbles develop on the floorconvex bank presents a long, gentle profile and of the channel and the water flow is dividedis known as slip-off bank (Figure 7.8). As into multiple threads. These thread-like streamsmeanders grow into deep loops, the same may of water rejoin and subdivide repeatedly to giveget cut-off due to erosion at the inflection points a typical braided pattern (Figure 7.9).and are left as ox-bow lakes.Braided ChannelsWhen rivers carry coarse material, there can beselective deposition of coarser materials causingformation of a central bar which diverts the flowtowards the banks; and this flow increaseslateral erosion on the banks. As the valleywidens, the water column is reduced and moreand more materials get deposited as islandsand lateral bars developing a number ofseparate channels of water flow. Depositionand lateral erosion of banks are essential for Figure 7.9 : Satellite scenes showing braided channelthe for mation of braided patter n. Or, segments of Gandak (left) and Son (right) riversalternatively, when discharge is less and load Arrows show the direction of flow Figure 7.10 : Various karst features
  • 66. LANDFORMS AND THEIR EVOLUTION 65GROUNDWATER as solution forms first and if the bottom of a sinkhole forms the roof of a void or caveHere the interest is not on groundwater as a underground, it might collapse leaving a largeresource. Our focus is on the work of hole opening into a cave or a void belowgroundwater in the erosion of landmasses and (collapse sinks). Quite often, sinkholes areevolution of landforms. The surface water covered up with soil mantle and appear aspercolates well when the rocks are permeable, shallow water pools. Anybody stepping overthinly bedded and highly jointed and cracked. such pools would go down like it happens inAfter vertically going down to some depth, the quicksands in deserts. The term doline iswater under the ground flows horizontally sometimes used to refer the collapse sinks.through the bedding planes, joints or through Solution sinks are more common than collapsethe materials themselves. It is this downward sinks. Quite often the surface run-off simplyand horizontal movement of water which goes down swallow and sink holes and flow ascauses the rocks to erode. Physical or underground streams and re-emerge at amechanical removal of materials by moving distance downstream through a cave opening.groundwater is insignificant in developing When sink holes and dolines join togetherlandforms. That is why, the results of the work because of slumping of materials along theirof groundwater cannot be seen in all types of margins or due to roof collapse of caves, long,rocks. But in rocks like limestones or dolomites narrow to wide trenches called valley sinks orrich in calcium carbonate, the surface water Uvalas form. Gradually, most of the surface ofas well as groundwater through the chemical the limestone is eaten away by these pits andprocess of solution and precipitation trenches, leaving it extremely irregular with adeposition develop varieties of landforms. These maze of points, grooves and ridges or lapies.two processes of solution and precipitation are Especially, these ridges or lapies form due toactive in limestones or dolomites occurring differential solution activity along parallel toeither exclusively or interbedded with other sub-parallel joints. The lapie field mayrocks. Any limestone or dolomitic region eventually turn into somewhat smoothshowing typical landforms produced by the limestone pavements.action of groundwater through the processesof solution and deposition is called Karst Cavestopography after the typical topographydeveloped in limestone rocks of Karst region In areas where there are alternating beds ofin the Balkans adjacent to Adriatic sea. rocks (shales, sandstones, quartzites) with The karst topography is also characterised limestones or dolomites in between or in areasby erosional and depositional landforms. where limestones are dense, massive and occurring as thick beds, cave formation isEROSIONAL LANDFORMS prominent. Water percolates down either through the materials or through cracks and joints and moves horizontally along beddingPools, Sinkholes, Lapies and planes. It is along these bedding planes thatLimestone Pavements the limestone dissolves and long and narrow to wide gaps called caves result. There can beSmall to medium sized round to sub-rounded a maze of caves at different elevationsshallow depressions called swallow holes form depending upon the limestone beds andon the surface of limestones through solution. intervening rocks. Caves normally have anSinkholes are very common in limestone/karst opening through which cave streams areareas. A sinkhole is an opening more or less discharged. Caves having openings at both thecircular at the top and funnel-shapped towards ends are called tunnels.the bottom with sizes varying in area from afew sq. m to a hectare and with depth from a Depositional Landformsless than half a metre to thirty metres or more.Some of these form solely through solution Many depositional forms develop within theaction (solution sinks) and others might start limestone caves. The chief chemical in limestone
  • 67. 66 FUNDAMENTALS OF PHYSICAL GEOGRAPHYis calcium carbonate which is easily soluble in GLACIERScarbonated water (carbon dioxide absorbedrainwater). This calcium carbonate is deposited Masses of ice moving as sheets over the landwhen the water carrying it in solution (continental glacier or pidmont glacier if a vastevaporates or loses its carbon dioxide as it sheet of ice is spread over the plains at the foottrickles over rough rock surfaces. of mountains) or as linear flows down the slopes of mountains in broad trough-likeStalactites, Stalagmites and Pillars valleys (mountain and valley glaciers) are called glaciers (Figure 7.12). The movement of glaciersStalactites hang as icicles of differentdiameters. Normally they are broad at theirbases and taper towards the free ends showingup in a variety of forms. Stalagmites rise upfrom the floor of the caves. In fact, stalagmitesform due to dripping water from the surface orthrough the thin pipe, of the stalactite,immediately below it (Figure 7.11). Figure 7.12 : A glacier in its valley is slow unlike water flow. The movement could be a few centimetres to a few metres a day or even less or more. Glaciers move basically because of the force of gravity. We have many glaciers in our country moving down the slopes and valleys in Himalayas. Higher reaches of Uttaranchal, Himachal Pradesh and Jammu and Kashmir, are places to see some of them. Do you know where one can see river Bhagirathi is basically fed by meltwaters from under the snout (Gaumukh) of the Gangotri glacier. In fact, Alkapuri glacier feeds waters to Alakananda river. Rivers Alkananda and Bhagirathi join to make river Ganga near Deoprayag. Erosion by glaciers is tremendous because of friction caused by sheer weight of the ice.Figure 7.11 : Stalactites and stalagmites in a limestone cave The material plucked from the land by glaciers (usually large-sized angular blocks and Stalagmites may take the shape of a fragments) get dragged along the floors or sidescolumn, a disc, with either a smooth, rounded of the valleys and cause great damage throughbulging end or a miniature crater like abrasion and plucking. Glaciers can causedepression. The stalagmite and stalactites significant damage to even un-weathered rockseventually fuse to give rise to columns and and can reduce high mountains into low hillspillars of different diameters. and plains.
  • 68. LANDFORMS AND THEIR EVOLUTION 67 As glaciers continue to move, debris gets the glacier disappears. Such lakes are calledremoved, divides get lowered and eventually cirque or tarn lakes. There can be two or morethe slope is reduced to such an extent that cirques one leading into another down belowglaciers will stop moving leaving only a mass in a stepped sequence.of low hills and vast outwash plains along withother depositional features. Figures 7.13 and Horns and Serrated Ridges7.14 show various glacial erosional and Horns form through head ward erosion of thedepositional forms described in the text. cirque walls. If three or more radiating glaciers cut headward until their cirques meet, high,EROSIONAL LANDFORMS sharp pointed and steep sided peaks called horns form. The divides between cirque sideCirque walls or head walls get narrow because ofCirques are the most common of landforms in progressive erosion and turn into serrated orglaciated mountains. The cirques quite often saw-toothed ridges sometimes referred to asare found at the heads of glacial valleys. The arêtes with very sharp crest and a zig-zagaccumulated ice cuts these cirques while outline.moving down the mountain tops. They aredeep, long and wide troughs or basins with The highest peak in the Alps, Matterhorn and the highest peak in the Himalayas,very steep concave to vertically dropping high Everest are in fact horns formed throughwalls at its head as well as sides. A lake of water headward erosion of radiating cirques.can be seen quite often within the cirques after Figure 7.13 : Some glacial erosional and depositional forms (adapted and modified from Spencer, 1962)
  • 69. 68 FUNDAMENTALS OF PHYSICAL GEOGRAPHYGlacial Valleys/Troughs Some amount of rock debris small enough to be carried by such melt-water streams isGlaciated valleys are trough-like and U-shaped washed down and deposited. Such glacio-with broad floors and relatively smooth, and fluvial deposits are called outwash deposits.steep sides. The valleys may contain littered Unlike till deposits, the outwash deposits aredebris or debris shaped as moraines with roughly stratified and assorted. The rockswampy appearance. There may be lakes fragments in outwash deposits are somewhatgouged out of rocky floor or formed by debris rounded at their edges. Figure 7.14 shows awithin the valleys. There can be hanging valleys few depositional landforms commonly foundat an elevation on one or both sides of the main in glaciated areas.glacial valley. The faces of divides or spurs ofsuch hanging valleys opening into main glacial Morainesvalleys are quite often truncated to give theman appearance like triangular facets. Very deep They are long ridges of deposits of glacial till.glacial troughs filled with sea water and Terminal moraines are long ridges of debrismaking up shorelines (in high latitudes) are deposited at the end (toe) of the glaciers. Lateralcalled fjords/fiords. moraines form along the sides parallel to the glacial valleys. The lateral moraines may join a What are the basic differences between terminal moraine forming a horse-shoe shaped glacial valleys and river valleys? ridge. There can be many lateral moraines on either side in a glacial valley. These moraines partly or fully owe their origin to glacio-fluvialDepositional Landforms waters pushing up materials to the sides ofThe unassorted coarse and fine debris dropped glaciers. Many valley glaciers retreating rapidlyby the melting glaciers is called glacial till. Most leave an irregular sheet of till over their valleyof the rock fragments in till are angular to sub- floors. Such deposits varying greatly in thicknessangular in form. Streams form by melting ice and in surface topography are called groundat the bottom, sides or lower ends of glaciers. moraines. The moraine in the centre of the Figure 7.14 : A panoramic diagram of glacial landscape with various depositional landforms (adapted and modified from Spencer, 1962)
  • 70. LANDFORMS AND THEIR EVOLUTION 69glacial valley flanked by lateral moraines is What is the difference between till andcalled medial moraine. They are imperfectly alluvium?formed as compared to lateral moraines.Sometimes medial moraines are indistinguishablefrom ground moraines. WAVES AND CURRENTSEskers Coastal processes are the most dynamic and hence most destructive. So, don’t you think itWhen glaciers melt in summer, the water flows is important to know about the coastalon the surface of the ice or seeps down along processes and forms?the margins or even moves through holes in Some of the changes along the coasts takethe ice. These waters accumulate beneath the place very fast. At one place, there can beglacier and flow like streams in a channel erosion in one season and deposition inbeneath the ice. Such streams flow over the another. Most of the changes along the coastsground (not in a valley cut in the ground) with are accomplished by waves. When waves break,ice forming its banks. Very coarse materials like the water is thrown with great force onto theboulders and blocks along with some minor shore, and simultaneously, there is a greatfractions of rock debris carried into this stream churning of sediments on the sea bottom.settle in the valley of ice beneath the glacier Constant impact of breaking waves drasticallyand after the ice melts can be found as a affects the coasts. Storm waves and tsunamisinuous ridge called esker. waves can cause far-reaching changes in a short period of time than normal breakingOutwash Plains waves. As wave environment changes, the intensity of the force of breaking waves changes.The plains at the foot of the glacial mountainsor beyond the limits of continental ice sheets Do you know about the generating forcesare covered with glacio-fluvial deposits in the behind waves and currents? If not, referform of broad flat alluvial fans which may join to the chapter on movements in oceanto form outwash plains of gravel, silt, sand and waters.clay. Other than the action of waves, the coastal Distinguish between river alluvial plains landforms depend upon (i) the configuration and glacial outwash plains. of land and sea floor; (ii) whether the coast is advancing (emerging) seaward or retreating (submerging) landward. Assuming sea level toDrumlins be constant, two types of coasts are consideredDrumlins are smooth oval shaped ridge-like to explain the concept of evolution of coastalfeatures composed mainly of glacial till with landforms: (i) high, rocky coasts (submergedsome masses of gravel and sand. The long axes coasts); (ii) low, smooth and gently slopingof drumlins are parallel to the direction of ice sedimentary coasts (emerged coasts).movement. They may measure up to 1 km inlength and 30 m or so in height. One end of HIGH ROCKY COASTSthe drumlins facing the glacier called the stoss Along the high rocky coasts, the rivers appearend is blunter and steeper than the other end to have been drowned with highly irregularcalled tail. The drumlins form due to dumping coastline. The coastline appears highlyof rock debris beneath heavily loaded ice indented with extension of water into the landthrough fissures in the glacier. The stoss end where glacial valleys (fjords) are present. Thegets blunted due to pushing by moving ice. hill sides drop off sharply into the water. ShoresDrumlins give an indication of direction of do not show any depositional landformsglacier movement. initially. Erosion features dominate.
  • 71. 70 FUNDAMENTALS OF PHYSICAL GEOGRAPHY Along high rocky coasts, waves break with Storm and tsunami waves cause drasticgreat force against the land shaping the hill changes irrespective of supply of sediments.sides into cliffs. With constant pounding by Large rivers which bring lots of sediments buildwaves, the cliffs recede leaving a wave-cut deltas along low sedimentary coasts.platform in front of the sea cliff. Wavesgradually minimise the irregularities along the The west coast of our country is a highshore. rocky retreating coast. Erosional forms The materials which fall off, and removed dominate in the west coast. The eastfrom the sea cliffs, gradually break into smaller coast of India is a low sedimentary coast.fragments and roll to roundness, will get Depositional forms dominate in the eastdeposited in the offshore. After a considerable coast.period of cliff development and retreat whencoastline turns somewhat smooth, with theaddition of some more material to this deposit What are the various differences betweenin the offshore, a wave-built terrace would a high rocky coast and a low sedimentarydevelop in front of wave-cut terrace. As the coast in terms of processes anderosion along the coast takes place a good landforms?supply material becomes available to longshorecurrents and waves to deposit them as beaches EROSIONAL LANDFORMSalong the shore and as bars (long ridges of sandand/or shingle parallel to the coast) in the Cliffs, Terraces, Caves and Stacksnearshore zone. Bars are submerged featuresand when bars show up above water, they are Wave-cut cliffs and terraces are two formscalled barrier bars. Barrier bar which get keyed usually found where erosion is the dominantup to the headland of a bay is called a spit. shore process. Almost all sea cliffs are steepWhen barrier bars and spits form at the mouth and may range from a few m to 30 m or evenof a bay and block it, a lagoon forms. The more. At the foot of such cliffs there may be alagoons would gradually get filled up by flat or gently sloping platform covered by rocksediments from the land giving rise to a coastal debris derived from the sea cliff behind. Suchplain. platforms occurring at elevations above the average height of waves is called a wave-cutLOW SEDIMENTARY COASTS terrace. The lashing of waves against the base of the cliff and the rock debris that getsAlong low sedimentary coasts the rivers appear smashed against the cliff along with lashingto extend their length by building coastal waves create hollows and these hollows getplains and deltas. The coastline appears widened and deepened to form sea caves. Thesmooth with occasional incursions of water in roofs of caves collapse and the sea cliffs recedethe form of lagoons and tidal creeks. The land further inland. Retreat of the cliff may leaveslopes gently into the water. Marshes and some remnants of rock standing isolated asswamps may abound along the coasts. small islands just off the shore. Such resistantDepositional features dominate. masses of rock, originally parts of a cliff or hill When waves break over a gently sloping are called sea stacks. Like all other features,sedimentary coast, the bottom sediments get sea stacks are also temporary and eventuallychurned and move readily building bars, coastal hills and cliffs will disappear becausebarrier bars, spits and lagoons. Lagoons of wave erosion giving rise to narrow coastalwould eventually turn into a swamp which plains, and with onrush of deposits from overwould subsequently turn into a coastal plain. the land behind may get covered up byThe maintenance of these depositional features alluvium or may get covered up by shingle ordepends upon the steady supply of materials. sand to form a wide beach.
  • 72. LANDFORMS AND THEIR EVOLUTION 71DEPOSITIONAL LANDFORMS develop attached to headlands/hills. The barriers, bars and spits at the mouth of theBeaches and Dunes bay gradually extend leaving only a small opening of the bay into the sea and the bayBeaches are characteristic of shorelines that are will eventually develop into a lagoon. Thedominated by deposition, but may occur as lagoons get filled up gradually by sedimentpatches along even the rugged shores. Most of coming from the land or from the beach itselfthe sediment making up the beaches comes (aided by wind) and a broad and wide coastalfrom land carried by the streams and rivers or plain may develop replacing a lagoon.from wave erosion. Beaches are temporaryfeatures. The sandy beach which appears so Do you know, the coastal off-shore barspermanent may be reduced to a very narrow offer the first buffer or defence againststrip of coarse pebbles in some other season. storm or tsunami by absorbing most ofMost of the beaches are made up of sand sized their destructive force. Then come thematerials. Beaches called shingle beaches barriers, beaches, beach dunes andcontain excessively small pebbles and even mangroves, if any, to absorb thecobbles. destructive force of storm and tsunami Just behind the beach, the sands lifted and waves. So, if we do anything whichwinnowed from over the beach surfaces will be disturbs the ‘sediment budget’ and thedeposited as sand dunes. Sand dunes forming mangroves along the coast, these coastallong ridges parallel to the coastline are very forms will get eroded away leaving humancommon along low sedimentary coasts. habitations to bear first strike of storm and tsunami waves.Bars, Barriers and SpitsA ridge of sand and shingle formed in the sea WINDSin the off-shore zone (from the position of low Wind is one of the two dominant agents in hottide waterline to seaward) lying approximately deserts. The desert floors get heated up tooparallel to the coast is called an off-shore bar. much and too quickly because of being dryAn off-shore bar which is exposed due to and barren. The heated floors heat up the airfurther addition of sand is termed a barrier directly above them and result in upwardbar. The off-shore bars and barriers commonly movements in the hot lighter air withform across the mouth of a river or at the turbulence, and any obstructions in its pathentrance of a bay. Sometimes such barrier bars sets up eddies, whirlwinds, updrafts andget keyed up to one end of the bay when they downdrafts. Winds also move along the desertare called spits (Figure 7.15). Spits may also floors with great speed and the obstructions in their path create turbulence. Of course, there are storm winds which are very destructive. Winds cause deflation, abrasion and impact. Deflation includes lifting and removal of dust and smaller particles from the surface of rocks. In the transportation process sand and silt act as effective tools to abrade the land surface. The impact is simply sheer force of momentum which occurs when sand is blown into or against a rock surface. It is similar to sand- blasting operation. The wind action creates a number of interesting erosional andFigure 7.15 : A satellite picture of a part of Godavari depositional features in the deserts. river delta showing a spit In fact, many features of deserts owe their
  • 73. 72 FUNDAMENTALS OF PHYSICAL GEOGRAPHYformation to mass wasting and running water deposition of sediment from basin margins, aas sheet floods. Though rain is scarce in deserts, nearly level plain forms at the centre of theit comes down torrentially in a short period of basin. In times of sufficient water, this plain istime. The desert rocks devoid of vegetation, covered up by a shallow water body. Suchexposed to mechanical and chemical types of shallow lakes are called as playasweathering processes due to drastic diurnal where water is retained only for short durationtemperature changes, decay faster and the due to evaporation and quite often the playastorrential rains help in removing the weathered contain good deposition of salts. The playamaterials easily. That means, the weathered plain covered up by salts is called alkali flats.debris in deserts is moved by not only windbut also by rain/sheet wash. The wind moves Deflation Hollows and Cavesfine materials and general mass erosion is Weathered mantle from over the rocks or bareaccomplished mainly through sheet floods or soil, gets blown out by persistent movementsheet wash. Stream channels in desert areas of wind currents in one direction. This processare broad, smooth and indefinite and flow for may create shallow depressions calleda brief time after rains. deflation hollows. Deflation also creates numerous small pits or cavities over rockEROSIONAL LANDFORMS surfaces. The rock faces suffer impact and abrasion of wind-borne sand and first shallowPediments and Pediplains depressions called blow outs are created, andLandscape evolution in deserts is primarily some of the blow outs become deeper andconcerned with the formation and extension of wider fit to be called caves.pediments. Gently inclined rocky floors closeto the mountains at their foot with or without Mushroom, Table and Pedestal Rocksa thin cover of debris, are called pediments. Many rock-outcrops in the deserts easilySuch rocky floors form through the erosion of susceptible to wind deflation and abrasion aremountain front through a combination of worn out quickly leaving some remnants oflateral erosion by streams and sheet flooding. resistant rocks polished beautifully in the Erosion starts along the steep margins of shape of mushroom with a slender stalk and athe landmass or the steep sides of the broad and rounded pear shaped cap above.tectonically controlled steep incision features Sometimes, the top surface is broad like a tableover the landmass. Once, pediments are formed top and quite often, the remnants stand outwith a steep wash slope followed by cliff or free like pedestals.face above it, the steep wash slope and free faceretreat backwards. This method of erosion is List the erosional features carved out bytermed as parallel retreat of slopes through wind action and action of sheet floods.backwasting. So, through parallel retreat ofslopes, the pediments extend backwards at the Depositional Landformsexpense of mountain front, and gradually, themountain gets reduced leaving an inselberg Wind is a good sorting agent. Depending uponwhich is a remnant of the mountain. That’s how the velocity of wind, different sizes of grains arethe high relief in desert areas is reduced to low moved along the floors by rolling or saltationfeatureless plains called pediplains. and carried in suspension and in this process of transportation itself, the materials get sorted.Playas When the wind slows or begins to die down,Plains are by far the most prominent landforms depending upon sizes of grains and theirin the deserts. In basins with mountains and critical velocities, the grains will begin to settle.hills around and along, the drainage is towards So, in depositional landforms made by wind,the centre of the basin and due to gradual good sorting of grains can be found. Since
  • 74. LANDFORMS AND THEIR EVOLUTION 73wind is there everywhere and wherever there are equally important. There can be a greatis good source of sand and with constant wind variety of dune forms (Figure 7.16).directions, depositional features in arid regionscan develop anywhere. Barchans Crescent shaped dunes called barchans withSand Dunes the points or wings directed away from windDry hot deserts are good places for sand dune direction i.e., downwind, form where the windformation. Obstacles to initiate dune formation direction is constant and moderate and where the original surface over which sand is moving is almost uniform. Parabolic dunes form when sandy surfaces are partially covered with vegetation. That means parabolic dunes are reversed barchans with wind direction being the same. Seif is similar to barchan with a small difference. Seif has only one wing or point. This happens when there is shift in wind conditions. The lone wings of seifs can grow very long and high. Longitudinal dunes form when supply of sand is poor and wind direction is constant. They appear as long ridges of considerable length but low in height. Transverse dunes are aligned perpendicular to wind direction. These dunes form when the wind direction is constant and the source of sand is an elongated feature at right angles to the wind direction. They may be very long and low in height. When sand is plenty, quite often, the regular shaped dunes coalesce and lose their individual characteristics. Most of the dunes Figure 7.16 : Various types of sand dunes in the deserts shift and a few of them will get Arrows indicate wind direction stabilised especially near human habitations. EXERCISES 1. Multiple choice questions. (i) In which of the following stages of landform development, downward cutting is dominated? (a) Youth stage (c) Early mature stage (b) Late mature stage (d) Old stage (ii) A deep valley characterised by steep step-like side slopes is known as (a) U-shaped valley (c) Blind valley (b) Gorge (d) Canyon (iii) In which one of the following regions the chemical weathering process is more dominant than the mechanical process? (a) Humid region (c) Arid region (b) Limestone region (d) Glacier region
  • 75. 74 FUNDAMENTALS OF PHYSICAL GEOGRAPHY (iv) Which one of the following sentences best defines the term ‘Lapies’ ? (a) A small to medium sized shallow depression (b) A landform whose opening is more or less circular at the top and funnel shaped towards bottom (c) A landform forms due to dripping water from surface (d) An irregular surface with sharp pinnacles, grooves and ridges (v) A deep, long and wide trough or basin with very steep concave high walls at its head as well as in sides is known as: (a) Cirque (c) Lateral Moraine (b) Glacial valley (d) Esker 2. Answer the following questions in about 30 words. (i) What do incised meanders in rocks and meanders in plains of alluvium indicate? (ii) Explain the evolution of valley sinks or uvalas. (iii) Underground flow of water is more common than surface run-off in limestone areas. Why? (iv) Glacial valleys show up many linear depositional forms. Give their locations and names. (v) How does wind perform its task in desert areas? Is it the only agent responsible for the erosional features in the deserts? 3. Answer the following questions in about 150 words. (i) Running water is by far the most dominating geomorphic agent in shaping the earth’s surface in humid as well as in arid climates. Explain. (ii) Limestones behave differently in humid and arid climates. Why? What is the dominant and almost exclusive geomorphic process in limestone areas and what are its results? (iii) How do glaciers accomplish the work of reducing high mountains into low hills and plains? Project Work Identify the landforms, materials and processes around your area.
  • 76. UNIT IV CLIMATEThis unit deals with• Atmosphere — compositions and structure; elements of weather and climate• Insolation — angle of incidence and distribution; heat budget of the earth — heating and cooling of atmosphere (conduction, convection, terrestrial radiation, advection); temperature — factors controlling temperature; distribution of temperature — horizontal and vertical; inversion of temperature• Pressure — pressure belts; winds-planetary seasonal and local, air masses and fronts; tropical and extra tropical cyclones• Precipitation — evaporation; condensation — dew, frost, fog, mist and cloud; rainfall — types and world distributon• World climates — classification (Koeppen), greenhouse effect, global warming and climatic changes
  • 77. CHAPTER COMPOSITION AND STRUCTURE OF ATMOSPHEREC an a person live without air? We eat Table 8.1 : Permanent Gases of the Atmosphere food two - three times a day and drink Constituent Formula Percentage by Volume water more frequently but breatheevery few seconds. Air is essential to the Nitrogen N2 78.08survival of all organisms. Some organisms like Oxygen O2 20.95 Argon Ar 0.93humans may survive for some time without Carbon dioxide CO2 0.036food and water but can’t survive even a few Neon Ne 0.002minutes without breathing air. That shows the Helium He 0.0005reason why we should understand the Krypto Kr 0.001atmosphere in greater detail. Atmosphere is a Xenon Xe 0.00009mixture of different gases and it envelopes the Hydrogen H2 0.00005earth all round. It contains life-giving gases likeoxygen for humans and animals and carbon Gasesdioxide for plants. The air is an integral part of Carbon dioxide is meteorologically a verythe earth’s mass and 99 per cent of the total important gas as it is transparent to themass of the atmosphere is confined to the incoming solar radiation but opaque to theheight of 32 km from the earth’s surface. The outgoing terrestrial radiation. It absorbs a partair is colourless and odourless and can be felt of terrestrial radiation and reflects back someonly when it blows as wind. part of it towards the earth’s surface. It is largely responsible for the green house effect. Can you imagine what will happen The volume of other gases is constant but the to us in the absence of ozone in the volume of carbon dioxide has been rising in atmosphere? the past few decades mainly because of the burning of fossil fuels. This has also increased the temperature of the air. Ozone is anotherCOMPOSITION OF THE ATMOSPHERE important component of the atmosphere foundThe atmosphere is composed of gases, water between 10 and 50 km above the earth’svapour and dust particles. Table 8.1 shows surface and acts as a filter and absorbs thedetails of various gases in the air, particularly ultra-violet rays radiating from the sun andin the lower atmosphere. The proportion of prevents them from reaching the surface of thegases changes in the higher layers of the earth.atmosphere in such a way that oxygen will bealmost in negligible quantity at the height of Water Vapour120 km. Similarly, carbon dioxide and water Water vapour is also a variable gas in thevapour are found only up to 90 km from the atmosphere, which decreases with altitude. Insurface of the earth. the warm and wet tropics, it may account for
  • 78. COMPOSITION AND STRUCTURE OF ATMOSPHERE 77four per cent of the air by volume, while in the The zone separating the tropsophere fromdry and cold areas of desert and polar regions, stratosphere is known as the tropopause. Theit may be less than one per cent of the air. Water air temperature at the tropopause is aboutvapour also decreases from the equator minus 800C over the equator and about minustowards the poles. It also absorbs parts of the 45oC over the poles. The temperature here isinsolation from the sun and preserves the nearly constant, and hence, it is called theearth’s radiated heat. It thus, acts like a blanket tropopause. The stratosphere is found aboveallowing the earth neither to become too cold the tropopause and extends up to a height ofnor too hot. Water vapour also contributes to 50 km. One important feature of thethe stability and instability in the air. stratosphere is that it contains the ozone layer. This layer absorbs ultra-violet radiation andDust Particles shields life on the earth from intense, harmful form of energy.Atmosphere has a sufficient capacity to keep The mesosphere lies above the stratosphere,small solid particles, which may originate from which extends up to a height of 80 km. In thisdifferent sources and include sea salts, fine soil, layer, once again, temperature startssmoke-soot, ash, pollen, dust and disintegrated decreasing with the increase in altitude andparticles of meteors. Dust particles are reaches up to minus 100°C at the height of 80generally concentrated in the lower layers of km. The upper limit of mesosphere is knownthe atmosphere; yet, convectional air currents as the mesopause. The ionosphere is locatedmay transport them to great heights. The between 80 and 400 km above the mesopause.higher concentration of dust particles is found It contains electrically charged particles knownin subtropical and temperate regions due to as ions, and hence, it is known as ionosphere.dry winds in comparison to equatorial and Radio waves transmitted from the earth arepolar regions. Dust and salt particles act as reflected back to the earth by this layer.hygroscopic nuclei around which water vapour Temperature here starts increasing with height.condenses to produce clouds. The uppermost layer of the atmosphere aboveSTRUCTURE OF THE ATMOSPHEREThe atmosphere consists of different layers withvarying density and temperature. Density ishighest near the surface of the earth anddecreases with increasing altitude. The columnof atmosphere is divided into five differentlayers depending upon the temperaturecondition. They are: troposphere, stratosphere,mesosphere, ionosphere and exosphere. The troposphere is the lowermost layer ofthe atmosphere. Its average height is 13 kmand extends roughly to a height of 8 km nearthe poles and about 18 km at the equator.Thickness of the troposphere is greatest at theequator because heat is transported to greatheights by strong convectional currents. Thislayer contains dust particles and water vapour.All changes in climate and weather take placein this layer. The temperature in this layerdecreases at the rate of 1°C for every 165m ofheight. This is the most important layer for allbiological activity. Figure 8.1 : Structure of atmosphere
  • 79. 78 FUNDAMENTALS OF PHYSICAL GEOGRAPHYthe ionosphere is known as the exosphere. This Elements of Weather and Climateis the highest layer but very little is known aboutit. Whatever contents are there, these are The main elements of atmosphere which areextremely rarefied in this layer, and it gradually subject to change and which influence humanmerges with the outer space. Although all life on earth are temperature, pressure, winds,layers of the atmosphere must be exercising humidity, clouds and precipitation. Theseinfluence on us, geographers are concerned elements have been dealt in detail in Chapterswith the first two layers of the atmosphere. 9, 10 and 11. EXERCISES 1. Multiple choice questions. (i) Which one of the following gases constitutes the major portion of the atmosphere? (a) Oxygen (c) Argon (b) Nitrogen (d) Carbon dioxide (ii) Atmospheric layer important for human beings is: (a) Stratosphere (c) Troposphere (b) Mesosphere (d) Ionosphere (iii) Sea salt, pollen, ash, smoke soot, fine soil — these are associated with: (a) Gases (c) Water vapour (b) Dust particles (d) Meteors (iv) Oxygen gas is in negligible quantity at the height of atmosphere: (a) 90 km (c) 100 km (b) 120 km (d) 150 km (v) Which one of the following gases is transparent to incoming solar radiation and opaque to outgoing terrestrial radiation? (a) Oxygen (c) Helium (b) Nitrogen (d) Carbon dioxide 2. Answer the following questions in about 30 words. (i) What do you understand by atmosphere? (ii) What are the elements of weather and climate? (iii) Describe the composition of atmosphere. (iv) Why is troposphere the most important of all the layers of the atmosphere? 3. Answer the following questions in about 150 words. (i) Describe the composition of the atmosphere. (ii) Draw a suitable diagram for the structure of the atmosphere and label it and describe it.
  • 80. CHAPTERSOLAR RADIATION, HEAT BALANCEAND TEMPERATURED o you feel air around you? Do you The solar output received at the top of the know that we live at the bottom of a atmosphere varies slightly in a year due to the huge pile of air? We inhale and exhale variations in the distance between the earth andbut we feel the air when it is in motion. It means the sun. During its revolution around the sun,air in motion is wind. You have already learnt the earth is farthest from the sun (152 millionabout the fact that earth is surrounded by air km on 4th July). This position of the earth isall around. This envelop of air is atmosphere called aphelion. On 3rd January, the earth iswhich is composed of numerous gases. These the nearest to the sun (147 million km). Thisgases support life over the earth’s surface. position is called perihelion. Therefore, the The earth receives almost all of its energy annual insolation received by the earth on 3rdfrom the sun. The earth in turn radiates back January is slightly more than the amountto space the energy received from the sun. As received on 4th July. However, the effect of thisa result, the earth neither warms up nor does variation in the solar output is masked byit get cooled over a period of time. Thus, the other factors like the distribution of land andamount of heat received by different parts of sea and the atmospheric circulation. Hence, thisthe earth is not the same. This variation causes variation in the solar output does not havepressure differences in the atmosphere. This great effect on daily weather changes on theleads to transfer of heat from one region to the surface of the earth.other by winds. This chapter explains the Variability of Insolation atprocess of heating and cooling of the the Surface of the Earthatmosphere and the resultant temperaturedistribution over the earth’s surface. The amount and the intensity of insolation vary during a day, in a season and in a year. The factorsSOLAR RADIATION that cause these variations in insolation are : (i) the rotation of earth on its axis; (ii) the angle ofThe earth’s surface receives most of its energy inclination of the sun’s rays; (iii) the length of thein short wavelengths. The energy received by day; (iv) the transparency of the atmosphere; (v)the earth is known as incoming solar radiation the configuration of land in terms of its aspect.which in short is termed as insolation. The last two however, have less influence. As the earth is a geoid resembling a sphere, The fact that the earth’s axis makes an anglethe sun’s rays fall obliquely at the top of the of 66½ with the plane of its orbit round the sunatmosphere and the earth intercepts a very has a greater influence on the amount ofsmall portion of the sun’s energy. On an insolation received at different latitudes. Note theaverage the earth receives 1.94 calories per sq. variations in the duration of the day at differentcm per minute at the top of its atmosphere. latitudes on solstices given in Table 9.1.
  • 81. 80 FUNDAMENTALS OF PHYSICAL GEOGRAPHY The second factor that determines the colour of the sky are the result of scattering ofamount of insolation received is the angle of light within the atmosphere.Table 9.1 : Length of the Day in Hours and Minutes on Winter and Summer Solstices in the Northern Hemisphere Latitude 0° 20° 40° 60° 90° December 22 12h 00m 10h 48m 9h 8m 5h 33m 0 June 21 12 h 13h 12m 14h 52m 18h 27m 6 monthsinclination of the rays. This depends on the Spatial Distribution of Insolationlatitude of a place. The higher the latitude the at the Earth’s Surfaceless is the angle they make with the surface of The insolation received at the surface variesthe earth resulting in slant sun rays. The areacovered by vertical rays is always less than the from about 320 Watt/m2 in the tropics to aboutslant rays. If more area is covered, the energy 70 Watt/m2 in the poles. Maximum insolationgets distributed and the net energy received is received over the subtropical deserts, whereper unit area decreases. Moreover, the slant rays the cloudiness is the least. Equator receivesare required to pass through greater depth of comparatively less insolation than the tropics.the atmosphere resulting in more absorption, Generally, at the same latitude the insolationscattering and diffusion. is more over the continent than over the oceans. In winter, the middle and higher latitudes receive less radiation than in summer. HEATING AND COOLING OF ATMOSPHERE There are different ways of heating and cooling of the atmosphere. The earth after being heated by insolation transmits the heat to the atmospheric layers near to the earth in long wave form. The air in contact with the land gets heated slowly and the upper layers in contact with the lower layers also get heated. This process is called conduction. Conduction takes place when two bodies of unequal temperature are in contact with one Figure 9.1 : Summer Solstice another, there is a flow of energy from the warmerThe Passage of Solar Radiation to cooler body. The transfer of heat continues untilthrough the Atmosphere both the bodies attain the same temperature or the contact is broken. Conduction is importantThe atmosphere is largely transparent to short in heating the lower layers of the atmosphere.wave solar radiation. The incoming solar The air in contact with the earth risesradiation passes through the atmospherebefore striking the earth’s surface. Within the vertically on heating in the form of currentstroposphere water vapour, ozone and other and further transmits the heat of thegases absorb much of the near infrared atmsphere. This process of vertical heating ofradiation. the atmosphere is known as convection. The Very small-suspended particles in the convective transfer of energy is confined onlytroposphere scatter visible spectrum both to to the troposphere.the space and towards the earth surface. This The transfer of heat through horizontalprocess adds colour to the sky. The red colour movement of air is called advection. Horizontalof the rising and the setting sun and the blue movement of the air is relatively more important
  • 82. SOLAR RADIATION, HEAT BALANCE AND TEMPERATURE 81than the vertical movement. In middle latitudes, accumulate or loose heat. It maintains itsmost of dirunal (day and night) variation in temperature. This can happen only if thedaily weather are caused by advection alone. amount of heat received in the form of insolationIn tropical regions particularly in northern equals the amount lost by the earth throughIndia during summer season local winds called terrestrial radiation.‘loo’ is the outcome of advection process. Consider that the insolation received at the top of the atmosphere is 100 per cent. WhileTerrestrial Radiation passing through the atmosphere some amountThe insolation received by the earth is in short of energy is reflected, scattered and absorbed.waves forms and heats up its surface. The earth Only the remaining part reaches the earthafter being heated itself becomes a radiating surface. Roughly 35 units are reflected backbody and it radiates energy to the atmosphere to space even before reaching the earth’sin long wave form. This energy heats up the surface. Of these, 27 units are reflected backatmosphere from below. This process is known from the top of the clouds and 2 units from theas terrestrial radiation. snow and ice-covered areas of the earth. The The long wave radiation is absorbed by the reflected amount of radiation is called theatmospheric gases particularly by carbon albedo of the earth.dioxide and the other green house gases. Thus, The remaining 65 units are absorbed, 14the atmosphere is indirectly heated by the units within the atmosphere and 51 units byearth’s radiation. the earth’s surface. The earth radiates back The atmosphere in turn radiates and 51 units in the form of terrestrial radiation.transmits heat to the space. Finally the amount Of these, 17 units are radiated to spaceof heat received from the sun is returned to directly and the remaining 34 units arespace, thereby maintaining constant temperature absorbed by the atmosphere (6 unitsat the earth’s surface and in the atmosphere. absorbed directly by the atmosphere, 9 units through convection and turbulence and 19Heat Budget of the Planet Earth units through latent heat of condensation).Figure 9.2 depicts the heat budget of the planet 48 units absorbed by the atmosphereearth. The earth as a whole does not (14 units from insolation +34 units from Figure 9.2 : Heat budget of the earth
  • 83. 82 FUNDAMENTALS OF PHYSICAL GEOGRAPHYterrestrial radiation) are also radiated back heat which is measured in terms ofinto space. Thus, the total radiation temperature. While heat represents thereturning from the earth and the atmosphere molecular movement of particles comprising arespectively is 17+48=65 units which substance, the temperature is the measurementbalance the total of 65 units received from in degrees of how hot (or cold) a thing (or athe sun. This is termed the heat budget or place) is.heat balance of the earth. This explains, why the earth neither warms Factors Controlling Temperature Distributionup nor cools down despite the huge transfer of The temperature of air at any place is influencedheat that takes place. by (i) the latitude of the place; (ii) the altitude of the place; (iii) distance from the sea, the air-Variation in the Net Heat Budget at the mass circulation; (iv) the presence of warm andEarth’s Surface cold ocean currents; (v) local aspects.As explained earlier, there are variations in theamount of radiation received at the earth’s The latitude : The temperature of a placesurface. Some part of the earth has surplus depends on the insolation received. It has beenradiation balance while the other part has explained earlier that the insolation variesdeficit. according to the latitude hence the Figure 9.3 depicts the latitudinal variation temperature also varies the net radiation balance of the earth — the The altitude : The atmosphere is indirectlyatmosphere system. The figure shows that heated by terrestrial radiation from below.there is a surplus of net radiation balance Therefore, the places near the sea-level recordbetween 40 degrees north and south and the higher temperature than the places situatedregions near the poles have a deficit. The at higher elevations. In other words, thesurplus heat energy from the tropics is temperature generally decreases withredistributed pole wards and as a result the increasing height. The rate of decrease oftropics do not get progressively heated up due temperature with height is termed as theto the accumulation of excess heat or the high normal lapse rate. It is 6.5°C per 1,000 m.latitudes get permanently frozen due to excessdeficit. Distance from the sea : Another factor that influences the temperature is the location of a place with respect to the sea. Compared to land, the sea gets heated slowly and loses heat slowly. Land heats up and cools down quickly. Therefore, the variation in temperature over the sea is less compared to land. The places situated near the sea come under the moderating influence of the sea and land breezes which moderate the temperature. Air-mass and Ocean currents : Like the land Figure 9.3 : Latitudinal variation in net and sea breezes, the passage of air masses also radiation balance affects the temperature. The places, which come under the influence of warm air-massesTemperature experience higher temperature and the placesThe interaction of insolation with the that come under the influence of cold air-atmosphere and the earth’s surface creates masses experience low temperature. Similarly,
  • 84. SOLAR RADIATION, HEAT BALANCE AND TEMPERATURE 83the places located on the coast where the warm northern hemisphere the land surface area isocean currents flow record higher temperature much larger than in the southern hemisphere.than the places located on the coast where the Hence, the effects of land mass and the oceancold currents flow. currents are well pronounced. In January the isotherms deviate to the north over the oceanDistribution of Temperature and to the south over the continent. This can be seen on the North Atlantic Ocean. TheThe global distribution of temperature can well presence of warm ocean currents, Gulf Streambe understood by studying the temperature and North Atlantic drift, make the Northerndistribution in January and July. The Atlantic Ocean warmer and the isotherms bendtemperature distribution is generally shown towards the north. Over the land theon the map with the help of isotherms. The temperature decreases sharply and theIsotherms are lines joining places having equal isotherms bend towards south in Europe.temperature. Figure 9.4 (a) and (b) show the It is much pronounced in the Siberiandistribution of surface air temperature in the plain. The mean January temperature alongmonth of January and July. 60° E longitude is minus 20° C both at 80° N In general the effect of the latitude on and 50° N latitudes. The mean monthlytemperature is well pronounced on the map, temperature for January is over 27° C, inas the isotherms are generally parallel to the equatorial oceans over 24° C in the tropicslatitude. The deviation from this general trend and 2° C - 0° C in the middle latitudesis more pronounced in January than in July, and –18° C to –48° C in the Eurasianespecially in the northern hemisphere. In the continental interior. Figure 9.4 (a) : The distribution of surface air temperature in the month of January
  • 85. 84 FUNDAMENTALS OF PHYSICAL GEOGRAPHY Figure 9.4 (b) : The distribution of surface air temperature in the month of July Figure 9.5 : The range of temperature between January and July
  • 86. SOLAR RADIATION, HEAT BALANCE AND TEMPERATURE 85 The effect of the ocean is well pronounced the subtropical continental region of Asia,in the southern hemisphere. Here the isotherms along the 30° N latitude. Along the 40° N runsare more or less parallel to the latitudes and the isotherm of 10° C and along the 40° S thethe variation in temperature is more gradual temperature is 10° C.than in the northern hemisphere. The isotherm Figure 9.5 shows the range ofof 20° C, 10° C, and 0° C runs parallel to 35° S, temperature between January and July. The45° S and 60° S latitudes respectively. highest range of temperature is more than 60° In July the isotherms generally run C over the north-eastern part of Eurasianparallel to the latitude. The equatorial oceans continent. This is due to continentality. Therecord warmer temperature, more than 27°C. least range of temperature, 3°C, is foundOver the land more than 30°C is noticed in between 20° S and 15° N. EXERCISES 1. Multiple choice questions. (i) The sun is directly overhead at noon on 21st June at: (a) The equator (c) 23.5° N (b) 23.5° S (d) 66.5° N (ii) In which one of the following cities, are the days the longest? (a) Tiruvanantpuram (c) Hyderabad (b) Chandigarh (d) Nagpur (iii) The atmosphere is mainly heated by the: (a) Short wave solar radiation (c) Long wave terrestrial radiation (b) Reflected solar radiation (d) Scattered solar radiation (iv) Make correct pairs from the following two columns. (i) Insolation (a) The difference between the mean temperature of the warmest and the coldest months (ii) Albedo (b) The lines joining the places of equal temperature (iii) Isotherm (c) The incoming solar radiation (iv) Annual range (d) The percentage of visible light reflected by an object (v) The main reason that the earth experiences highest temperatures in the subtropics in the northern hemisphere rather than at the equator is : (a) Subtropical areas tend to have less cloud cover than equatorial areas. (b) Subtropical areas have longer day hours in the summer than the equatorial. (c) Subtropical areas have an enhanced “green house effect” compared to equatorial areas. (d) Subtropical areas are nearer to the oceanic areas than the equatorial locations.
  • 87. 86 FUNDAMENTALS OF PHYSICAL GEOGRAPHY 2. Answer the following questions in about 30 words. (i) How does the unequal distribution of heat over the planet earth in space and time cause variations in weather and climate? (ii) What are the factors that control temperature distribution on the surface of the earth? (iii) In India, why is the day temperature maximum in May and why not after the summer solstice? (iv) Why is the annual range of temperature high in the Siberian plains? 3. Answer the following questions in about 150 words. (i) How do the latitude and the tilt in the axis of rotation of the earth affect the amount of radiation received at the earth’s surface? (ii) Discuss the processes through which the earth-atmosphere system maintains heat balance. (iii) Compare the global distribution of temperature in January over the northern and the southern hemisphere of the earth. Project Work Select a meteorological observatory located in your city or near your town. Tabulate the temperature data as given in the climatological table of observatories : (i) Note the altitude, latitude of the observatory and the period for which the mean is calculated. (ii) Define the terms related to temperature as given in the table. (iii) Calculate the daily mean monthly temperature. (iv) Draw a graph to show the daily mean maximum, the daily mean minimum and the mean temperature. (v) Calculate the annual range of temperature. (vi) Find out in which months the daily range of temperature is the highest and the lowest. (vii) List out the factors that determine the temperature of the place and explain the possible causes for temperature variation in the months of January, May, July and October. Example Observatory : New Delhi (Safdarjung) Latitude : 28°35°’ N Based on observations : 1951 - 1980 Altitude above mean sea level : 216 m Month Mean of Mean of Highest Lowest Daily Daily Recorded Recorded Max.(°C) Min.(°C) (°C) (°C) January 21.1 7.3 29.3 0.6 May 39.6 25.9 47.2 17.5
  • 88. SOLAR RADIATION, HEAT BALANCE AND TEMPERATURE 87 Daily mean monthly temperature 21.1+7.3 January = 14.2OC 2 39.6+25.9 May = 32.75OC 2 Annual range of temperature Mean Max. Temperature in May - Mean Temperature in January Annual range of temperature = 32.75°C – 14.2°C = 18.55°C
  • 89. CHAPTER ATMOSPHERIC CIRCULATION AND WEATHER SYSTEMSE arlier Chapter 9 described the uneven the air at the surface is denser and hence has distribution of temperature over the higher pressure. Air pressure is measured with surface of the earth. Air expands when the help of a mercury barometer or the aneroidheated and gets compressed when cooled. This barometer. Consult your book, Practical Workresults in variations in the atmospheric in Geography — Part I (NCERT, 2006) andpressure. The result is that it causes the learn about these instruments. The pressuremovement of air from high pressure to low decreases with height. At any elevation it variespressure, setting the air in motion. You already from place to place and its variation is theknow that air in horizontal motion is wind. primary cause of air motion, i.e. wind whichAtmospheric pressure also determines when moves from high pressure areas to lowthe air will rise or sink. The wind redistributes pressure areas.the heat and moisture across the planet,thereby, maintaining a constant temperature Vertical Variation of Pressurefor the planet as a whole. The vertical rising of In the lower atmosphere the pressuremoist air cools it down to form the clouds and decreases rapidly with height. The decreasebring precipitation. This chapter has been amounts to about 1 mb for each 10 m increasedevoted to explain the causes of pressure in elevation. It does not always decrease at thedifferences, the forces that control the same rate. Table 10.1 gives the averageatmospheric circulation, the turbulent pattern pressure and temperature at selected levels ofof wind, the formation of air masses, the elevation for a standard atmosphere.disturbed weather when air masses interactwith each other and the phenomenon of violent Table 10.1 : Standard Pressure and Temperature attropical storms. Selected Levels Level Pressure in mb Temperature °CATMOSPHERIC PRESSURE Sea Level 1,013.25 15.2Do you realise that our body is subjected to a 1 km 898.76 8.7lot of air pressure. As one moves up the airgets varified and one feels breathless. 5 km 540.48 –17. 3 The weight of a column of air contained in 10 km 265.00 – 49.7a unit area from the mean sea level to the topof the atmosphere is called the atmospheric The vertical pressure gradient force is muchpressure. The atmospheric pressure is larger than that of the horizontal pressureexpressed in units of mb and Pascals. The gradient. But, it is generally balanced by awidely used unit is kilo Pascal written as hPa. nearly equal but opposite gravitational force.At sea level the average atmospheric pressure Hence, we do not experience strong upwardis 1,013.2 mb or 1,013.2 hPa. Due to gravity winds.
  • 90. ATMOSPHERIC CIRCULATION AND WEATHER SYSTEMS 89Horizontal Distribution of Pressure purposes of comparison. The sea level pressure distribution is shown on weather maps.Small differences in pressure are highly Figure 10.1 shows the patterns of isobarssignificant in terms of the wind direction and corresponding to pressure systems. Low- pressure system is enclosed by one or more isobars with the lowest pressure in the centre. High-pressure system is also enclosed by one or more isobars with the highest pressure in the centre. World Distribution of Sea Level Pressure The world distribution of sea level pressure in January and July has been shown in Figures 10.2 and 10.3. Near the equator the sea levelFigure 10.1 : Isobars, pressure and wind systems in pressure is low and the area is known as Northern Hemisphere equatorial low. Along 30° N and 30o S arevelocity. Horizontal distribution of pressure is found the high-pressure areas known as thestudied by drawing isobars at constant levels. subtropical highs. Further pole wards alongIsobars are lines connecting places having 60o N and 60o S, the low-pressure belts areequal pressure. In order to eliminate the effect termed as the sub polar lows. Near the polesof altitude on pressure, it is measured at any the pressure is high and it is known as the polarstation after being reduced to sea level for high. These pressure belts are not permanent Figure 10.2 : Distribution of pressure (in millibars) — January
  • 91. 90 FUNDAMENTALS OF PHYSICAL GEOGRAPHY Figure 10.3 : Distribution of pressure (in millibars) — Julyin nature. They oscillate with the apparent Pressure Gradient Forcemovement of the sun. In the northern The differences in atmospheric pressurehemisphere in winter they move southwards produces a force. The rate of change of pressureand in the summer northwards. with respect to distance is the pressureForces Affecting the Velocity gradient. The pressure gradient is strong whereand Direction of Wind the isobars are close to each other and is weak where the isobars are apart.You already know that the air is set in motiondue to the differences in atmospheric pressure. Frictional ForceThe air in motion is called wind. The wind It affects the speed of the wind. It is greatest atblows from high pressure to low pressure. The the surface and its influence generally extendswind at the surface experiences friction. In upto an elevation of 1 - 3 km. Over the seaaddition, rotation of the earth also affects the surface the friction is minimal.wind movement. The force exerted by therotation of the earth is known as the Coriolis Coriolis Forceforce. Thus, the horizontal winds near theearth surface respond to the combined effect The rotation of the earth about its axis affectsof three forces – the pressure gradient force, the direction of the wind. This force is calledthe frictional force and the Coriolis force. In the Coriolis force after the French physicist whoaddition, the gravitational force acts described it in 1844. It deflects the wind to thedownward. right direction in the northern hemisphere and
  • 92. ATMOSPHERIC CIRCULATION AND WEATHER SYSTEMS 91to the left in the southern hemisphere. The The wind circulation around a low isdeflection is more when the wind velocity is called cyclonic circulation. Around a highhigh. The Coriolis force is directly proportional it is called anti cyclonic circulation. Theto the angle of latitude. It is maximum at the direction of winds around such systemspoles and is absent at the equator. changes according to their location in The Coriolis force acts perpendicular to the different hemispheres (Table 10.2).pressure gradient force. The pressure gradient The wind circulation at the earth’s surfaceforce is perpendicular to an isobar. The higher around low and high on many occasions isthe pressure gradient force, the more is the closely related to the wind circulation at highervelocity of the wind and the larger is the level. Generally, over low pressure area the airdeflection in the direction of wind. As a result of will converge and rise. Over high pressure areathese two forces operating perpendicular to each the air will subside from above and diverge atother, in the low-pressure areas the wind blows the surface (Figure10.5). Apart fromaround it. At the equator, the Coriolis force is convergence, some eddies, convectionzero and the wind blows perpendicular to the currents, orographic uplift and uplift alongisobars. The low pressure gets filled instead of fronts cause the rising of air, which is essentialgetting intensified. That is the reason why tropical for the formation of clouds and precipitation.cyclones are not formed near the equator.Pressure and WindThe velocity and direction of the wind are thenet result of the wind generating forces. Thewinds in the upper atmosphere, 2 - 3 km abovethe surface, are free from frictional effect of thesurface and are controlled by the pressuregradient and the Coriolis force. When isobarsare straight and when there is no friction, the Figure 10.5 : Convergence and divergence of windspressure gradient force is balanced by theCoriolis force and the resultant wind blowsparallel to the isobar. This wind is known as General circulation of the atmospherethe geostrophic wind (Figure 10.4). The pattern of planetary winds largely depends on : (i) latitudinal variation of atmospheric heating; (ii) emergence of pressure belts; (iii) the migration of belts following apparent path of the sun; (iv) the distribution of continents and oceans; (v) the rotation of earth. The pattern of the movement of the planetary winds is called the general circulation of the atmosphere. The general circulation of the atmosphere also sets in motion the ocean water circulation which influences the earth’s Figure 10.4 : Geostropic Wind Table 10.2 : Pattern of Wind Direction in Cyclones and Anticyclones Pressure System Pressure Condition Pattern of Wind Direction at the Centre Northern Hemisphere Southern Hemisphere Cyclone Low Anticlockwise Clockwise Anticyclone High Clockwise Anticlockwise
  • 93. 92 FUNDAMENTALS OF PHYSICAL GEOGRAPHYclimate. A schematic description of the general The general circulation of the atmospherecirculation is shown in Figure 10.6. also affects the oceans. The large-scale winds of the atmosphere initiate large and slow moving currents of the ocean. Oceans in turn provide input of energy and water vapour into the air. These interactions take place rather slowly over a large part of the ocean. General Atmospheric Circulation and its Effects on Oceans Warming and cooling of the Pacific Ocean is most important in terms of general atmospheric circulation. The warm water of the central Pacific Ocean slowly drifts towards South American coast and replaces the cool Peruvian current. Such appearance of warm water off the coast Figure 10. 6 : Simplified general circulation of Peru is known as the El Nino. The El of the atmosphere Nino event is closely associated with the pressure changes in the Central Pacific The air at the Inter Tropical Convergence and Australia. This change in pressureZone (ITCZ) rises because of convection caused condition over Pacific is known as theby high insolation and a low pressure is southern oscillation. The combinedcreated. The winds from the tropics converge phenomenon of southern oscillation andat this low pressure zone. The converged air El Nino is known as ENSO. In the yearsrises along with the convective cell. It reaches when the ENSO is strong, large-scalethe top of the troposphere up to an altitude of variations in weather occur over the14 km. and moves towards the poles. This world. The arid west coast of Southcauses accumulation of air at about 30o N and America receives heavy rainfall, droughtS. Part of the accumulated air sinks to the occurs in Australia and sometimes inground and forms a subtropical high. Another India and floods in China. Thisreason for sinking is the cooling of air when it phenomenon is closely monitored and isreaches 30o N and S latitudes. Down below used for long range forecasting in majornear the land surface the air flows towards the parts of the world.equator as the easterlies. The easterlies fromeither side of the equator converge in the Inter Seasonal WindTropical Convergence Zone (ITCZ). Suchcirculations from the surface upwards and The pattern of wind circulation is modified invice-versa are called cells. Such a cell in the different seasons due to the shifting of regionstropics is called Hadley Cell. In the middle of maximum heating, pressure and wind belts.latitudes the circulation is that of sinking cold The most pronounced effect of such a shift isair that comes from the poles and the rising noticed in the monsoons, especially overwarm air that blows from the subtropical high. southeast Asia. You would be studying theAt the surface these winds are called westerlies details of monsoon in the book India : Physicaland the cell is known as the Ferrel cell. At polar Environment (NCERT, 2006). The other locallatitudes the cold dense air subsides near the deviations from the general circulation systempoles and blows towards middle latitudes as are as follows.the polar easterlies. This cell is called the polar Local Windscell. These three cells set the pattern for thegeneral circulation of the atmosphere. The Differences in the heating and cooling of earthtransfer of heat energy from lower latitudes to surfaces and the cycles those develop daily orhigher latitudes maintains the general annually can create several common, local orcirculation. regional winds.
  • 94. ATMOSPHERIC CIRCULATION AND WEATHER SYSTEMS 93Land and Sea Breezes as the valley breeze. During the night the slopes get cooled and the dense air descendsAs explained earlier, the land and sea absorb into the valley as the mountain wind. The cooland transfer heat differently. During the day the air, of the high plateaus and ice fields drainingland heats up faster and becomes warmer than into the valley is called katabatic wind. Anotherthe sea. Therefore, over the land the air rises type of warm wind occurs on the leeward sidegiving rise to a low pressure area, whereas the of the mountain ranges. The moisture in thesesea is relatively cool and the pressure over sea winds, while crossing the mountain rangesis relatively high. Thus, pressure gradient from condense and precipitate. When it descendssea to land is created and the wind blows from down the leeward side of the slope the dry airthe sea to the land as the sea breeze. In the night gets warmed up by adiabatic process. This drythe reversal of condition takes place. The land air may melt the snow in a short time.loses heat faster and is cooler than the sea. Thepressure gradient is from the land to the sea Air Massesand hence land breeze results (Figure 10.7). When the air remains over a homogenous area for a sufficiently longer time, it acquires the characteristics of the area. The homogenous regions can be the vast ocean surface or vast plains. The air with distinctive characteristics in terms of temperature and humidity is called an airmass. It is defined as a large body of air having little horizontal variation in temperature and moisture. The homogenous surfaces, over which air masses form, are called the source regions. The air masses are classified according to the source regions. There are five major source regions. These are: (i) Warm tropical and subtropical oceans; (ii) The subtropical hot deserts; (iii) The relatively cold high latitude oceans; (iv) The very cold snow covered continents in high latitudes; (v) Permanently ice covered continents in the Arctic and Antarctica. Accordingly, following types of air- masses are recognised: (i) Maritime tropical (mT); (ii) Continental tropical (cT); (iii) Maritime polar (mP); (iv) Continental polar (cP); (v) Continental arctic (cA). Tropical air masses are warm and polar air masses are cold. Fronts When two different air masses meet, the Figure 10.7 : Land and sea breezes boundary zone between them is called a front. The process of formation of the fronts is knownMountain and Valley Winds as frontogenesis. There are four types ofIn mountainous regions, during the day the fronts: (a) Cold; (b) Warm; (c) Stationary;slopes get heated up and air moves upslope (d) Occluded [(Figure10.8 (a), (b), (c)]. When theand to fill the resulting gap the air from the front remains stationary, it is called avalley blows up the valley. This wind is known stationary front. When the cold air moves
  • 95. 94 FUNDAMENTALS OF PHYSICAL GEOGRAPHY anticlockwise cyclonic circulation. The cyclonic circulation leads to a well developed extra tropical cyclone, with a warm front and a cold front. The plan and cross section of a well developed cyclone is given in Figure 10.9. There are pockets of warm air or warm sector wedged between the forward and the rear cold air or cold sector. The warm air glides over the cold air and a sequence of clouds appear over the sky ahead of the warm front and cause precipitation. The cold front approaches the warm air from behind and pushes the warm air up. As a result, cumulus clouds develop along the cold front. The cold front moves faster than the warm front ultimately overtaking the warm front. The warm air is completely lifted up and the front is occluded and the cyclone dissipates. The processes of wind circulation both at the surface and aloft are closely interlinked. Figure 10.8 : Vertical Sections of : (a) Warm Front; The extra tropical cyclone differs from the (b) Cold Front; (c) Occluded Front tropical cyclone in number of ways. The extra tropical cyclones have a clear frontal systemtowards the warm air mass, its contact zone iscalled the cold front, whereas if the warm airmass moves towards the cold air mass, thecontact zone is a warm front. If an air mass isfully lifted above the land surface, it is calledthe occluded front. The fronts occur in middlelatitudes and are characterised by steep gradientin temperature and pressure. They bringabrupt changes in temperature and cause theair to rise to form clouds and cause precipitation.Extra Tropical CyclonesThe systems developing in the mid and highlatitude, beyond the tropics are called themiddle latitude or extra tropical cyclones. Thepassage of front causes abrupt changes in theweather conditions over the area in the middleand high latitudes. Extra tropical cyclones form along the polarfront. Initially, the front is stationary. In thenorthern hemisphere, warm air blows from thesouth and cold air from the north of the front.When the pressure drops along the front, thewarm air moves northwards and the cold airmove towards, south setting in motion an Figure 10. 9 : Extra tropical cyclones
  • 96. ATMOSPHERIC CIRCULATION AND WEATHER SYSTEMS 95which is not present in the tropical cyclones. A schematic representation of the verticalThey cover a larger area and can originate over structure of a mature tropical cyclonic stormthe land and sea. Whereas the tropical cyclones is shown in Figure 10.10.originate only over the seas and on reaching A mature tropical cyclone is characterisedthe land they dissipate. The extra tropical by the strong spirally circulating wind aroundcyclone affects a much larger area as the centre, called the eye. The diameter of thecompared to the tropical cyclone. The wind circulating system can vary between 150 andvelocity in a tropical cyclone is much higher 250 km.and it is more destructive. The extra tropical The eye is a region of calm with subsidingcyclones move from west to east but tropical air. Around the eye is the eye wall, where therecyclones, move from east to west. is a strong spiralling ascent of air to greater height reaching the tropopause. The wind reaches maximum velocity in this region,Tropical Cyclones reaching as high as 250 km per hour.Tropical cyclones are violent storms that Torrential rain occurs here. From the eye walloriginate over oceans in tropical areas and rain bands may radiate and trains of cumulusmove over to the coastal areas bringing about and cumulonimbus clouds may drift into thelarge scale destruction caused by violent outer region. The diameter of the storm overwinds, very heavy rainfall and storm surges. the Bay of Bengal, Arabian sea and IndianThis is one of the most devastating natural ocean is between 600 - 1200 km. The systemcalamities. They are known as Cyclones in the moves slowly about 300 - 500 km per day.Indian Ocean, Hurricanes in the Atlantic, The cyclone creates storm surges and theyTyphoons in the Western Pacific and South inundate the coastal low lands. The stormChina Sea, and Willy-willies in the Western peters out on the land.Australia. Tropical cyclones originate and intensifyover warm tropical oceans. The conditionsfavourable for the formation and intensificationof tropical storms are: (i) Large sea surface withtemperature higher than 27° C; (ii) Presenceof the Coriolis force; (iii) Small variations in thevertical wind speed; (iv) A pre-existing weak-low-pressure area or low-level-cycloniccirculation; (v) Upper divergence above the sealevel system. The energy that intensifies the storm, comesfrom the condensation process in the toweringcumulonimbus clouds, surrounding thecentre of the storm. With continuous supplyof moisture from the sea, the storm is furtherstrengthened. On reaching the land themoisture supply is cut off and the stormdissipates. The place where a tropical cyclonecrosses the coast is called the landfall of thecyclone. The cyclones, which cross 20o Nlatitude generally, recurve and they are more Figure 10.10 : Vertical section of the tropical cyclonedestructive. (after Rama Sastry)
  • 97. 96 FUNDAMENTALS OF PHYSICAL GEOGRAPHYThunderstorms and Tornadoes greater height. This causes precipitation. Later, downdraft brings down to earth the cool airOther severe local storms are thunderstorms and the rain. From severe thunderstormsand tornadoes. They are of short duration, sometimes spiralling wind descends like aoccurring over a small area but are violent. trunk of an elephant with great force, with veryThunderstor ms are caused by intense low pressure at the centre, causing massiveconvection on moist hot days. A thunderstorm destruction on its way. Such a phenomenon isis a well-grown cumulonimbus cloud called a tornado. Tornadoes generally occurproducing thunder and lightening. When the in middle latitudes. The tornado over the seaclouds extend to heights where sub-zero is called water sprouts.temperature prevails, hails are formed and they These violent storms are the manifestationcome down as hailstorm. If there is insufficient of the atmosphere’s adjustments to varyingmoisture, a thunderstorm can generate dust- energy distribution. The potential and heatstorms. A thunderstorm is characterised by energies are converted into kinetic energy inintense updraft of rising warm air, which these storms and the restless atmosphere againcauses the clouds to grow bigger and rise to returns to its stable state. EXERCISES 1. Multiple choice questions. (i) If the surface air pressure is 1,000 mb, the air pressure at 1 km above the surface will be: (a) 700 mb (c) 900 mb (b) 1,100 mb (d) 1,300 mb (ii) The Inter Tropical Convergence Zone normally occurs: (a) near the Equator (b) near the Tropic of Cancer (c) near the Tropic of Capricorn (d) near the Arctic Circle (iii) The direction of wind around a low pressure in northern hemisphere is: (a) clockwise (c) anti-clock wise (b) perpendicular to isobars (d) parallel to isobars (iv) Which one of the following is the source region for the formation of air masses? (a) the Equatorial forest (c) the Siberian Plain (b) the Himalayas (d) the Deccan Plateau 2. Answer the following questions in about 30 words. (i) What is the unit used in measuring pressure? Why is the pressure measured at station level reduced to the sea level in preparation of weather maps? (ii) While the pressure gradient force is from north to south, i.e. from the subtropical high pressure to the equator in the northern hemisphere, why are the winds north easterlies in the tropics. (iii) What are the geotrophic winds? (iv) Explain the land and sea breezes.
  • 98. ATMOSPHERIC CIRCULATION AND WEATHER SYSTEMS 97 3. Answer the following questions in about 150 words. (i) Discuss the factors affecting the speed and direction of wind. (ii) Draw a simplified diagram to show the general circulation of the atmosphere over the globe. What are the possible reasons for the formation of subtropical high pressure over 30o N and S latitudes? (iii) Why does tropical cyclone originate over the seas? In which part of the tropical cyclone do torrential rains and high velocity winds blow and why? Project Work (i) Collect weather information over media such as newspaper, TV and radio for understanding the weather systems. (ii) Read the section on weather in any newspaper, preferably, one having a map showing a satellite picture. Mark the area of cloudiness. Attempt to infer the atmospheric circulation from the distribution of clouds. Compare the forecast given in the newspaper with the TV coverage, if you have access to TV. Estimate, how many days in a week was the forecast were accurate.
  • 99. CHAPTER WATER IN THE ATMOSPHEREY ou have already learnt that the air EVAPORATION AND CONDENSATION contains water vapour. It varies from zero to four per cent by volume of the The amount of water vapour in the atmosphereatmosphere and plays an important role in the is added or withdrawn due to evaporation andweather phenomena. Water is present in the condensation respectively. Evaporation is aatmosphere in three forms namely – gaseous, process by which water is transformed fromliquid and solid. The moisture in the liquid to gaseous state. Heat is the main causeatmosphere is derived from water bodies for evaporation. The temperature at which thethrough evaporation and from plants through water starts evaporating is referred to as thetranspiration. Thus, there is a continuous latent heat of of water between the atmosphere, the Increase in temperature increases wateroceans and the continents through the absorption and retention capacity of the givenprocesses of evaporation, transpiration, parcel of air. Similarly, if the moisture contentcondensation and precipitation. is low, air has a potentiality of absorbing and Water vapour present in the air is known retaining moisture. Movement of air replacesas humidity. It is expressed quantitatively in the saturated layer with the unsaturated layer.different ways. The actual amount of the water Hence, the greater the movement of air, thevapour present in the atmosphere is known as greater is the evaporation.the absolute humidity. It is the weight of water The transformation of water vapour intovapour per unit volume of air and is expressed water is called condensation. Condensation isin terms of grams per cubic metre. The ability caused by the loss of heat. When moist air isof the air to hold water vapour depends entirely cooled, it may reach a level when its capacityon its temperature. The absolute humidity to hold water vapour ceases. Then, the excessdiffers from place to place on the surface of the water vapour condenses into liquid form. If itearth. The percentage of moisture present in directly condenses into solid form, it is knownthe atmosphere as compared to its full capacity as sublimation. In free air, condensation resultsat a given temperature is known as the relative from cooling around very small particleshumidity. With the change of air temperature, termed as hygroscopic condensation nuclei.the capacity to retain moisture increases or Particles of dust, smoke and salt from the oceandecreases and the relative humidity is also are particularly good nuclei because theyaffected. It is greater over the oceans and least absorb water. Condensation also takes placeover the continents. when the moist air comes in contact with some The air containing moisture to its full colder object and it may also take place whencapacity at a given temperature is said to be the temperature is close to the dew point.saturated. It means that the air at the given Condensation, therefore, depends upon thetemperature is incapable of holding any amount of cooling and the relative humidity ofadditional amount of moisture at that stage. the air. Condensation is influenced by theThe temperature at which saturation occurs volume of air, temperature, pressure andin a given sample of air is known as dew point. humidity. Condensation takes place: (i) when
  • 100. WATER IN THE ATMOSPHERE 99the temperature of the air is reduced to dew condition when fog is mixed with smoke, ispoint with its volume remaining constant; (ii) described as smog. The only difference betweenwhen both the volume and the temperature are the mist and fog is that mist contains morereduced; (iv) when moisture is added to the air moisture than the fog. In mist each nucelithrough evaporation. However, the most contains a thicker layer of moisture. Mists arefavourable condition for condensation is the frequent over mountains as the rising warmdecrease in air temperature. air up the slopes meets a cold surface. Fogs After condensation the water vapour or the are drier than mist and they are prevalent wheremoisture in the atmosphere takes one of the warm currents of air come in contact with coldfollowing forms — dew, frost, fog and clouds. currents. Fogs are mini clouds in whichForms of condensation can be classified on the condensation takes place around nucleibasis of temperature and location. provided by the dust, smoke, and the saltCondensation takes place when the dew point lower than the freezing point as well ashigher than the freezing point. Clouds Cloud is a mass of minute water droplets orDew tiny crystals of ice formed by the condensationWhen the moisture is deposited in the form of of the water vapour in free air at considerablewater droplets on cooler surfaces of solid elevations. As the clouds are formed at someobjects (rather than nuclei in air above the height over the surface of the earth, they takesurface) such as stones, grass blades and plant various shapes. According to their height,leaves, it is known as dew. The ideal conditions expanse, density and transparency orfor its formation are clear sky, calm air, high opaqueness clouds are grouped under fourrelative humidity, and cold and long nights. types : (i) cirrus; (ii) cumulus; (iii) stratus;For the formation of dew, it is necessary that (iv) nimbus.the dew point is above the freezing point. CirrusFrost Cirrus clouds are formed at high altitudesFrost forms on cold surfaces when (8,000 - 12,000m). They are thin and detatchedcondensation takes place below freezing point clouds having a feathery appearance. They are(00C), i.e. the dew point is at or below the always white in colour.freezing point. The excess moisture is depositedin the form of minute ice crystals instead of Cumuluswater droplets. The ideal conditions for the Cumulus clouds look like cotton wool. Theyformation of white frost are the same as those are generally formed at a height of 4,000 -for the formation of dew, except that the air 7,000 m. They exist in patches and can be seentemperature must be at or below the freezing scattered here and there. They have a flat base.point. StratusFog and Mist As their name implies, these are layered cloudsWhen the temperature of an air mass covering large portions of the sky. These cloudscontaining a large quantity of water vapour falls are generally formed either due to loss of heatall of a sudden, condensation takes place within or the mixing of air masses with differentitself on fine dust particles. So, the fog is a cloud temperatures.with its base at or very near to the ground.Because of the fog and mist, the visibility Nimbusbecomes poor to zero. In urban and industrialcentres smoke provides plenty of nuclei which Nimbus clouds are black or dark gray. Theyhelp the formation of fog and mist. Such a form at middle levels or very near to the surface
  • 101. 100 FUNDAMENTALS OF PHYSICAL GEOGRAPHYof the earth. These are extremely dense and Precipitationopaque to the rays of the sun. Sometimes, the The process of continuous condensation in freeclouds are so low that they seem to touch the air helps the condensed particles to grow inground. Nimbus clouds are shapeless masses size. When the resistance of the air fails to holdof thick vapour. them against the force of gravity, they fall on to the earth’s surface. So after the condensation of water vapour, the release of moisture is known as precipitation. This may take place in liquid or solid form. The precipitation in the form of water is called rainfall, when the temperature is lower than the 00C, precipitation takes place in the form of fine flakes of snow and is called snowfall. Moisture is released in the form of hexagonal crystals. These crystals form flakes of snow. Besides rain and snow, other forms of precipitation are sleet and hail, Figure 11.1 though the latter are limited in occurrence and are sporadic in both time and space. Sleet is frozen raindrops and refrozen melted snow-water. When a layer of air with the temperature above freezing point overlies a subfreezing layer near the ground, precipitation takes place in the form of sleet. Raindrops, which leave the warmer air, encounter the colder air below. As a result, they solidify and reach the ground as small pellets of ice not bigger than the raindrops from which they are formed. Sometimes, drops of rain after being released by the clouds become solidified into small rounded solid pieces of ice and which reach the surface of the earth are called hailstones. These are formed by the rainwater passing through the colder layers. Hailstones have several concentric layers of ice one over the other. Types of Rainfall Figure 11.2 On the basis of origin, rainfall may be classified into three main types – the convectional, Identify these cloud types which are orographic or relief and the cyclonic or frontal. shown in Figure 11.1 and 11.2. Conventional Rain A combination of these four basic types can The, air on being heated, becomes light andgive rise to the following types of clouds: highclouds – cirrus, cirrostratus, cirrocumulus; rises up in convection currents. As it rises, itmiddle clouds – altostratus and altocumulus; expands and loses heat and consequently,low clouds – stratocumulus and nimbostratus condensation takes place and cumulousand clouds with extensive vertical clouds are formed. With thunder and lightening,development – cumulus and cumulonimbus. heavy rainfall takes place but this does not last
  • 102. WATER IN THE ATMOSPHERE 101long. Such rain is common in the summer or the continents. The rainfall is more over thein the hotter part of the day. It is very common oceans than on the landmasses of the worldin the equatorial regions and interior parts of because of being great sources of water.the continents, particularly in the northern Between the latitudes 350 and 400 N and S ofhemisphere. the equator, the rain is heavier on the eastern coasts and goes on decreasing towards theOrographic Rain west. But, between 450 and 650 N and S of equator, due to the westerlies, the rainfall isWhen the saturated air mass comes across a first received on the western margins of themountain, it is forced to ascend and as it rises, continents and it goes on decreasing towardsit expands; the temperature falls, and the the east. Wherever mountains run parallel tomoisture is condensed. The chief characteristic the coast, the rain is greater on the coastalof this sort of rain is that the windward slopes plain, on the windward side and it decreasesreceive greater rainfall. After giving rain on the towards the leeward side.windward side, when these winds reach the On the basis of the total amount of annualother slope, they descend, and their precipitation, major precipitation regimes of thetemperature rises. Then their capacity to take world are identified as moisture increases and hence, these leeward The equatorial belt, the windward slopesslopes remain rainless and dry. The area of the mountains along the western coasts insituated on the leeward side, which gets less the cool temperate zone and the coastal areasrainfall is known as the rain-shadow area. It of the monsoon land receive heavy rainfall ofis also known as the relief rain. over 200 cm per annum. Interior continental areas receive moderate rainfall varying fromCyclonic Rain 100 - 200 cm per annum. The coastal areas of the continents receive moderate amount ofYou have already read about extra tropical rainfall. The central parts of the tropical landcyclones and cyclonic rain in Chapter 10. and the eastern and interior parts of thePlease consult Chapter 10 to understand temperate lands receive rainfall varyingcyclonic rainfall. between 50 - 100 cm per annum. Areas lying in the rain shadow zone of the interior of theWorld Distribution of Rainfall continents and high latitudes receive very lowDifferent places on the earth’s surface receive rainfall-less than 50 cm per annum. Seasonaldifferent amounts of rainfall in a year and that distribution of rainfall provides an importanttoo in different seasons. aspect to judge its effectiveness. In some In general, as we proceed from the equator regions rainfall is distributed evenlytowards the poles, rainfall goes on decreasing throughout the year such as in the equatorialsteadily. The coastal areas of the world receive belt and in the western parts of cool temperategreater amounts of rainfall than the interior of regions. EXERCISES 1. Multiple choice questions. (i) Which one of the following is the most important constituent of the atmosphere for human beings? (a) Water vapour (c) Dust particle (b) Nitrogen (d) Oxygen
  • 103. 102 FUNDAMENTALS OF PHYSICAL GEOGRAPHY (ii) Which one of the following process is responsible for transforming liquid into vapour? (a) Condensation (c) Evaporation (b) Transpiration (d) Precipitation (iii) The air that contains moisture to its full capacity : (a) Relative humidity (c) Absolute humidity (b) Specific humidity (d) Saturated air (iv) Which one of the following is the highest cloud in the sky? (a) Cirrus (c) Nimbus (b) Stratus (d) Cumulus 2. Answer the following questions in about 30 words. (i) Name the three types of precipitation. (ii) Explain relative humidity. (iii) Why does the amount of water vapour decreases rapidly with altitude? (iv) How are clouds formed? Classify them. 3. Answer the following questions in about 150 words. (i) Discuss the salient features of the world distribution of precipitation. (ii) What are forms of condensation? Describe the process of dew and frost formation. Project Work Browse through the newspaper from 1st June to 31st December and note the news about extreme rainfall in different parts of the country.
  • 104. CHAPTERWORLD CLIMATE ANDCLIMATE CHANGET he world climate can be studied by related them to the distribution of vegetation organising information and data on and used these values for classifying the climate and synthesising them in climates. It is an empirical classification basedsmaller units for easy understanding, on mean annual and mean monthlydescription and analysis. Three broad temperature and precipitation data. Heapproaches have been adopted for classifying introduced the use of capital and small lettersclimate. They are empirical, genetic and to designate climatic groups and types.applied. Empirical classification is based on Although developed in 1918 and modified overobserved data, particularly on temperature a period of time, Koeppen’s scheme is stilland precipitation. Genetic classification popular and in use.attempts to organise climates according to their Koeppen recognised five major climaticcauses. Applied classification is for specific groups, four of them are based on temperaturepurpose. and one on precipitation. Table 12.1 lists the climatic groups and their characteristicsKOEPP E N ’ S SCHEME OF CLASSIFICATION according to Koeppen. The capital letters : A,C,OFCLIMATE D and E delineate humid climates and B dry climates.The most widely used classification of climate The climatic groups are subdivided intois the empirical climate classification scheme types, designated by small letters, based ondeveloped by V. Koeppen. Koeppen identified seasonality of precipitation and temperaturea close relationship between the distribution characteristics. The seasons of dryness areof vegetation and climate. He selected certain indicated by the small letters : f, m, w and s,values of temperature and precipitation and where f corresponds to no dry season, Table 12.1 : Climatic Groups According to Koeppen Group Characteristics A - Tropical Average temperature of the coldest month is 18° C or higher B - Dry Climates Potential evaporation exceeds precipitation C - Warm Temperate The average temperature of the coldest month of the (Mid-latitude) climates years is higher than minus 3°C but below 18°C D - Cold Snow Forest Climates The average temperature of the coldest month is minus 3° C or below E - Cold Climates Average temperature for all months is below 10° C H - High Land Cold due to elevation
  • 105. 104 FUNDAMENTALS OF PHYSICAL GEOGRAPHYm - monsoon climate, w- winter dry season and islands of East Indies. Significant amount ofs - summer dry season. The small letters a, b, rainfall occurs in every month of the year asc and d refer to the degree of severity of thunder showers in the afternoon. Thetemperature. The B- Dry Climates are temperature is uniformly high and the annualsubdivided using the capital letters S for steppe range of temperature is negligible. Theor semi-arid and W for deserts. The climatic maximum temperature on any day is around 30°C while the minimum temperature istypes are listed in Table 12.2. The distribution around 20°C. Tropical evergreen forests withof climatic groups and types is shown in dense canopy cover and large biodiversity areTable 12.1. found in this climate. Table 12.2 : Climatic Types According to Koeppen Group Type Letter Code Characteristics Tropical wet Af No dry season A-Tropical Humid Climate Tropical monsoon Am Monsoonal, short dry season Tropical wet and dry Aw Winter dry season Subtropical steppe BSh Low-latitude semi arid or dry Subtropical desert BWh Low-latitude arid or dry B-Dry Climate Mid-latitude steppe BSk Mid-latitude semi arid or dry Mid-latitude desert BWk Mid-latitude arid or dry Humid subtropical Cfa No dry season, warm summer C-Warm temperate (Mid- Mediterranean Cs Dry hot summer latitude) Climates Marine west coast Cfb No dry season, warm and cool summer D-Cold Snow- Humid continental Df No dry season, severe winter forest Climates Subarctic Dw Winter dry and very severe Tundra ET No true summer E-Cold Climates Polar ice cap EF Perennial ice H-Highland Highland H Highland with snow coverGroup A : Tropical Humid Climates Tropical Monsoon Climate (Am)Tropical humid climates exist between Tropic Tropical monsoon climate (Am) is found overof Cancer and Tropic of Capricorn. The sun the Indian sub-continent, North Eastern partbeing overhead throughout the year and the of South America and Northern Australia.presence of Inter Tropical Convergence Zone Heavy rainfall occurs mostly in summer. Winter(INTCZ) make the climate hot and humid. is dry. The detailed climatic account of thisAnnual range of temperature is very low and climatic type is given in the book on India:annual rainfall is high. The tropical group is Physical Environment.divided into three types, namely (i) Af- Tropicalwet climate; (ii) Am - Tropical monsoon climate; Tropical Wet and Dry Climate (Aw)(iii) Aw- Tropical wet and dry climate. Tropical wet and dry climate occurs north and south of Af type climate regions. It borders withTropical Wet Climate (Af) dry climate on the western part of the continentTropical wet climate is found near the equator. and Cf or Cw on the eastern part. ExtensiveThe major areas are the Amazon Basin in South Aw climate is found to the north and south ofAmerica, western equatorial Africa and the the Amazon forest in Brazil and adjoining parts
  • 106. WORLD CLIMATE AND CLIMATE CHANGE 105of Bolivia and Paraguay in South America, often causing famine. Rain occurs in shortSudan and south of Central Africa. The annual intense thundershowers in deserts and israinfall in this climate is considerably less than ineffective in building soil moisture. Fog isthat in Af and Am climate types and is variable common in coastal deserts bordering coldalso. The wet season is shorter and the dry currents. Maximum temperature in the summerseason is longer with the drought being more is very high. The highest shade temperature ofsevere. Temperature is high throughout the 58° C was recorded at Al Aziziyah, Libya onyear and diurnal ranges of temperature are the 13 September 1922. The annual and diurnalgreatest in the dry season. Deciduous forest and ranges of temperature are also high.tree-shredded grasslands occur in this climate. Warm Temperate (Mid-Latitude) Climates-CDry Climates : B Warm temperate (mid-latitude) climates extendDry climates are characterised by very low from 30° - 50° of latitude mainly on the easternrainfall that is not adequate for the growth of and western margins of continents. Theseplants. These climates cover a very large area climates generally have warm summers withof the planet extending over large latitudes from mild winters. They are grouped into four types:15° - 60° north and south of the equator. At (i) Humid subtropical, i.e. dry in winter andlow latitudes, from 15° - 30°, they occur in the hot in summer (Cwa); (ii) Mediterranean (Cs);area of subtropical high where subsidence and (iii) Humid subtropical, i.e. no dry season andinversion of temperature do not produce mild winter (Cfa); (iv) Marine west coast climaterainfall. On the western margin of the (Cfb).continents, adjoining the cold current,particularly over the west coast of South Humid Subtropical Climate (Cwa)America, they extend more equatorwards and Humid subtropical climate occurs poleward ofoccur on the coast land. In middle latitudes, Tropic of Cancer and Capricorn, mainly infrom 35° - 60° north and south of equator, they North Indian plains and South China interiorare confined to the interior of continents where plains. The climate is similar to Aw climatemaritime-humid winds do not reach and to except that the temperature in winter is warm.areas often surrounded by mountains. Dry climates are divided into steppe or Mediterranean Climate (Cs)semi-arid climate (BS) and desert climate (BW).They are further subdivided as subtropical As the name suggests, Mediterranean climatesteppe (BSh) and subtropical desert (BWh) at occurs around Mediterranean sea, along thelatitudes from 15° - 35° and mid-latitude west coast of continents in subtropical latitudessteppe (BSk) and mid-latitude desert (BWk) at between 30° - 40° latitudes e.g. — Centrallatitudes between 35° - 60°. California, Central Chile, along the coast in south eastern and south western Australia.Subtropical Steppe (BSh) and Subtropical These areas come under the influence of subDesert (BWh) Climates tropical high in summer and westerly wind in winter. Hence, the climate is characterised bySubtropical steppe (BSh) and subtropical hot, dry summer and mild, rainy winter. Monthlydesert (BWh) have common precipitation and average temperature in summer is aroundtemperature characteristics. Located in the 25° C and in winter below 10°C. The annualtransition zone between humid and dry precipitation ranges between 35 - 90 cm.climates, subtropical steppe receives slightlymore rainfall than the desert, adequate enough Humid Subtropical (Cfa) Climatefor the growth of sparse grasslands. The rainfallin both the climates is highly variable. The Humid subtropical climate lies on the easternvariability in the rainfall affects the life in the parts of the continent in subtropical latitudes.steppe much more than in the desert, more In this region the air masses are generally
  • 107. 106 FUNDAMENTALS OF PHYSICAL GEOGRAPHYunstable and cause rainfall throughout the Cold Climate with Dry Winters (Dw)year. They occur in eastern United States of Cold climate with dry winter occurs mainlyAmerica, southern and eastern China, over Northeastern Asia. The development ofsouthern Japan, northeastern Argentina, pronounced winter anti cyclone and itscoastal south Africa and eastern coast of weakening in summer sets in monsoon likeAustralia. The annual averages of precipitation reversal of wind in this region. Polewardvary from 75-150 cm. Thunderstorms in summer temperatures are lower and wintersummer and frontal precipitation in winter are temperatures are extremely low with manycommon. Mean monthly temperature in locations experiencing below freezing pointsummer is around 27°C, and in winter it varies temperatures for up to seven months in a year.from 5°-12° C. The daily range of temperature Precipitation occurs in summer. The annualis small. precipitation is low from 12-15 cm.Marine West Coast Climate (Cfb) Polar Climates (E)Marine west coast climate is located poleward Polar climates exist poleward beyond 70°from the Mediterranean climate on the west latitude. Polar climates consist of two types:coast of the continents. The main areas are: (i) Tundra (ET); (ii) Ice Cap (EF).Northwestern Europe, west coast of NorthAmerica, north of California, southern Chile, Tundra Climate (ET)southeastern Australia and New Zealand. Due The tundra climate (ET) is so called after theto marine influence, the temperature is types of vegetation, like low growing mosses,moderate and in winter, it is warmer than for lichens and flowering plants. This is the regionits latitude. The mean temperature in summer of permafrost where the sub soil is permanentlymonths ranges from 15°-20°C and in winter frozen. The short growing season and water4°-10°C. The annual and daily ranges of logging support only low growing plants.temperature are small. Precipitation occurs During summer, the tundra regions have verythroughout the year. Precipitation varies long duration of day light.greatly from 50-250cm. Ice Cap Climate (EF)Cold Snow Forest Climates (D) The ice cap climate (EF) occurs over interiorCold snow forest climates occur in the large Greenland and Antartica. Even in summer, thecontinental area in the northern hemisphere temperature is below freezing point. This areabetween 40°-70° north latitudes in Europe, receives very little precipitation. The snow andAsia and North America. Cold snow forest ice get accumulated and the mounting pressureclimates are divided into two types: (i) Df- cold causes the deformation of the ice sheets andclimate with humid winter; (ii) Dw- cold climate they break. They move as icebergs that float inwith dry winter. The severity of winter is more the Arctic and Antarctic waters. Plateau Stationpronounced in higher latitudes. , Antarctica ,79°S, portray this climate.Cold Climate with Humid Winters (Df) Highland Climates (H)Cold climate with humid winter occurs Highland climates are governed by topography.poleward of marine west coast climate and mid In high mountains, large changes in meanlatitude steppe. The winters are cold and temperature occur over short distances.snowy. The frost free season is short. The Precipitation types and intensity also varyannual ranges of temperature are large. The spatially across high lands. There is verticalweather changes are abrupt and short. zonation of layering of climatic types withPoleward, the winters are more severe. elevation in the mountain environment.
  • 108. WORLD CLIMATE AND CLIMATE CHANGE 107CLIMATE CHANGE crop failures, of floods and migration of people tell about the effects of changing climate. AThe earlier chapters on climate summarised number of times Europe witnessed warm, wet,our understanding of climate as it prevails now. cold and dry periods, the significant episodesThe type of climate we experience now might were the warm and dry conditions in the tenthbe prevailing over the last 10,000 years with and eleventh centuries, when the Vikingsminor and occasionally wide fluctuations. The settled in Greenland. Europe witnessed “Littleplanet earth has witnessed many variations in Ice Age” from 1550 to about 1850. From aboutclimate since the beginning. Geological records 1885-1940 world temperature showed anshow alteration of glacial and inter-glacial upward trend. After 1940, the rate of increaseperiods. The geomorphological features, in temperature slowed down.especially in high altitudes and high latitudes,exhibit traces of advances and retreats of Causes of Climate Changeglaciers. The sediment deposits in glacial lakesalso reveal the occurrence of warm and cold The causes for climate change are many. Theyperiods. The rings in the trees provide clues can be grouped into astronomical andabout wet and dry periods. Historical records terrestrial causes. The astronomical causes aredescribe the vagaries in climate. All these the changes in solar output associated withevidences indicate that change in climate is a sunspot activities. Sunspots are dark andnatural and continuous process. cooler patches on the sun which increase and India also witnessed alternate wet and dry decrease in a cyclical manner. According toperiods. Archaeological findings show that the some meteorologists, when the number ofRajasthan desert experienced wet and cool sunspots increase, cooler and wetter weatherclimate around 8,000 B.C. The period 3,000- and greater storminess occur. A decrease in1,700 B.C. had higher rainfall. From about sunspot numbers is associated with warm and2,000-1,700 B.C., this region was the centre drier conditions. Yet, these findings are notof the Harappan civilisation. Dry conditions statistically significant.accentuated since then. An another astronomical theory is In the geological past, the earth was warm Millankovitch oscillations, which infer cyclessome 500-300 million years ago, through the in the variations in the earth’s orbitalCambrian, Ordovician and Silurian periods. characteristics around the sun, the wobblingDuring the Pleistocene epoch, glacial and of the earth and the changes in the earth’s axialinter-glacial periods occurred, the last major tilt. All these alter the amount of insolationpeak glacial period was about 18,000 years received from the sun, which in turn, mightago. The present inter-glacial period started have a bearing on the climate.10,000 years ago. Volcanism is considered as another cause for climate change. Volcanic eruption throwsClimate in the recent past up lots of aerosols into the atmosphere. TheseVariability in climate occurs all the time. The aerosols remain in the atmosphere for anineties decade of the last century witnessed considerable period of time reducing the sun’sextreme weather events. The 1990s recorded radiation reaching the Earth’s surface. After thethe warmest temperature of the century and recent Pinatoba and El Cion volcanicsome of the worst floods around the world. The eruptions, the average temperature of the earthworst devastating drought in the Sahel region, fell to some extent for some years.south of the Sahara desert, from 1967-1977 The most important anthropogenic effectis one such variability. During the 1930s, on the climate is the increasing trend in thesevere drought occurred in southwestern Great concentration of greenhouse gases in thePlains of the United States, described as the atmosphere which is likely to cause globaldust bowl. Historical records of crop yield or warming.
  • 109. 108 FUNDAMENTALS OF PHYSICAL GEOGRAPHYGlobal Warming it will take for earth’s atmospheric system to recover from any change brought about by theDue to the presence of greenhouse gases, the latter.atmosphere is behaving like a greenhouse. The The largest concentration of GHGs in theatmosphere also transmits the incoming solar atmosphere is carbon dioxide. The emissionradiation but absorbs the vast majority of long of CO 2 comes mainly from fossil fuelwave radiation emitted upwards by the earth’s combustion (oil, gas and coal). Forests andsurface. The gases that absorb long wave oceans are the sinks for the carbon dioxide.radiation are called greenhouse gases. The Forests use CO 2 in their growth. So,processes that warm the atmosphere are often deforestation due to changes in land use, alsocollectively referred to as the greenhouse effect. increases the concentration of Co2. The time taken for atmospheric CO2 to adjust to changes The term greenhouse is derived from the analogy to a greenhouse used in cold in sources to sinks is 20-50 years. It is rising areas for preserving heat. A greenhouse at about 0.5 per cent annually. Doubling of is made up of glass. The glass which is concentration of CO2 over pre-industrial level transparent to incoming short wave solar is used as an index for estimating the changes radiation is opaque to outgoing long wave in climate in climatic models. radiation. The glass, therefore, allows in Chlorofluorocarbons (CFCs) are products more radiation and prevents the long of human activity. Ozone occurs in the wave radiation going outside the glass stratosphere where ultra-violet rays convert house, causing the temperature inside oxygen into ozone. Thus, ultra violet rays do the glasshouse structure warmer than not reach the earth’s surface. The CFCs which outside. When you enter a car or a bus, during summers, where windows are drift into the stratosphere destroy the ozone. closed, you feel more heat than outside. Large depletion of ozone occurs over Antarctica. Likewise during winter the vehicles with The depletion of ozone concentration in the closed doors and windows remain warmer stratosphere is called the ozone hole. This than the temperature outside. This is allows the ultra violet rays to pass through the another example of the greenhouse effect. troposphere. International efforts have been initiated forGreenhouse Gases(GHGs) reducing the emission of GHGs into the atmosphere. The most important one is theThe primary GHGs of concern today are carbon Kyoto protocol proclaimed in 1997. Thisdioxide (CO2), Chlorofluorocarbons (CFCs), protocol went into effect in 2005, ratified bymethane (CH4), nitrous oxide (N2O) and ozone 141 nations. Kyoto protocol bounds the 35(O3). Some other gases such as nitric oxide (NO) industrialised countries to reduce theirand carbon monoxide (CO) easily react with emissions by the year 2012 to 5 per cent lessGHGs and affect their concentration in the than the levels prevalent in the year 1990.atmosphere. The increasing trend in the concentration The effectiveness of any given GHG of GHGs in the atmosphere may, in the longmolecule will depend on the magnitude of the run, warm up the earth. Once the globalincrease in its concentration, its life time in the warming sets in, it will be difficult to reverse it.atmosphere and the wavelength of radiation The effect of global warming may not bethat it absorbs. The chlorofluorocarbons uniform everywhere. Nevertheless, the adverse(CFCs) are highly effective. Ozone which effect due to global warming will adversely affectabsorbs ultra violet radiation in the the life supporting system. Rise in the sea levelstratosphere is very effective in absorbing due to melting of glaciers and ice-caps andterrestrial radiation when it is present in the thermal expansion of the sea may inundatelower troposphere. Another important point to large parts of the coastal area and islands,be noted is that the more time the GHG leading to social problems. This is anothermolecule remains in the atmosphere, the longer cause for serious concern for the world
  • 110. WORLD CLIMATE AND CLIMATE CHANGE 109community. Efforts have already been initiated annual near surface temperature over landto control the emission of GHGs and to arrest from 1856-2000, relative to the periodthe trend towards global warming. Let us hope 1961-90 as normal for the globe.the world community responds to this challenge An increasing trend in temperature wasand adopts a lifestyle that leaves behind a discernible in the 20th century. The greatestlivable world for the generations to come. warming of the 20th century was during the One of the major concerns of the world two periods, 1901-44 and 1977-99. Over eachtoday is global warming. Let us look at how of these two periods, global temperatures rosemuch the planet has warmed up from the by about 0.4°C. In between, there was a slighttemperature records. cooling, which was more marked in the Temperature data are available from the Northern Hemisphere.middle of the 19th century mostly for western The globally averaged annual meanEurope. The reference period for this study is temperature at the end of the 20th century was1961-90. The temperature anomalies for the about 0.6°C above that recorded at the end ofearlier and later periods are estimated from the the 19th century. The seven warmest yearsaverage temperature for the period 1961-90. during the 1856-2000 were recorded in theThe annual average near -surface air last decade. The year 1998 was the warmesttemperature of the world is approximately year, probably not only for the 20th century14°C. The time series show anomalies of but also for the whole millennium. Write an explanatory note on “global warming”.
  • 111. 110 FUNDAMENTALS OF PHYSICAL GEOGRAPHY EXERCISES 1. Multiple choice questions. (i) Which one of the following is suitable for Koeppen’s “A” type of climate? (a) High rainfall in all the months (b) Mean monthly temperature of the coldest month more than freezing point (c) Mean monthly temperature of all the months more than 18o C (d) Average temperature for all the months below 10° C (ii) Koeppen’s system of classification of climates can be termed as : (a) Applied (b) Systematic (c) Genetic (d) Empirical (iii) Most of the Indian Peninsula will be grouped according to Koeppen’s system under: (a) “Af” (b) “BSh” (c) “Cfb” (d) “Am” (iv) Which one of the following years is supposed to have recorded the warmest temperature the world over? (a) 1990 (b) 1998 (c) 1885 (d) 1950 (v) Which one of the following groups of four climates represents humid conditions? (a) A—B—C—E (b) A—C—D—E (c) B—C—D—E (d) A—C—D—F 2. Answer the following questions in about 30 words. (i) Which two climatic variables are used by Koeppen for classification of the climate? (ii) How is the “genetic” system of classification different from the “empirical one”? (iii) Which types of climates have very low range of temperature? (iv) What type of climatic conditions would prevail if the sun spots increase? 3. Answer the following questions in about 150 words. (i) Make a comparison of the climatic conditions between the “A” and “B” types of climate. (ii) What type of vegetation would you find in the “C” and “A” type(s) of climate? (iii) What do you understand by the term “Greenhouse Gases”? Make a list of greenhouse gases. Project Work Collect information about Kyoto declaration related to global climate changes.
  • 112. UNIT V WATER (OCEANS)This unit deals with• Hydrological Cycle• Oceans — submarine relief; distribution of temperature and salinity; movements of ocean water-waves, tides and currents
  • 113. CHAPTER WATER (OCEANS)C an we think of life without water? It is the ocean to land and land to ocean. The said that the water is life. Water is an hydrological cycle describes the movement of essential component of all life forms that water on, in, and above the earth. The waterexist over the surface of the earth. The creatures cycle has been working for billions of yearson the earth are lucky that it is a water planet, and all the life on earth depends on it. Next tootherwise we all would have no existence. Water air, water is the most important elementis a rare commodity in our solar system. There required for the existence of life on earth. Theis no water on the sun or anywhere else in the distribution of water on earth is quite system. The earth, fortunately has an Many locations have plenty of water whileabundant supply of water on its surface. Hence, others have very limited quantity. Theour planet is called the ‘Blue Planet’. hydrological cycle, is the circulation of water within the earth’s hydrosphere in differentHYDROLOGICAL CYCLE forms i.e. the liquid, solid and the gaseousWater is a cyclic resource. It can be used and phases. It also refers to the continuousre-used. Water also undergoes a cycle from exchange of water between the oceans, Figure 13.1 : Hydrological Cycle
  • 114. WATER (OCEANS) 113 Table 13.1 : Water on the Earth’s surface crisis in different parts of the world — spatially Reservoir Volume Percentage and temporally. The pollution of river waters (Million of the Total has further aggravated the crisis. How can you Cubic km ) intervene in improving the water quality and augmenting the available quantity of water? Oceans 1,370 97.25 Ice Caps 29 2.05 RELIEF OF THE OCEAN FLOOR and Glaciers Groundwater 9.5 0.68 The oceans are confined to the great Lakes 0.125 0.01 depressions of the earth’s outer layer. In this Soil Moisture 0.065 0.005 section, we shall see the nature of the ocean Atmosphere 0.013 0.001 basins of the earth and their topography. The Streams 0.0017 0.0001 oceans, unlike the continents, merge so and Rivers naturally into one another that it is hard to Biosphere 0.0006 0.00004 demarcate them. The geographers have divided the oceanic part of the earth into four oceans, namely the Pacific, the Atlantic, the Indian and Table 13.2 : Components and Processes the Arctic. The various seas, bays, gulfs and of the Water Cycle other inlets are parts of these four large oceans. Components Processes A major portion of the ocean floor is found between 3-6 km below the sea level. The ‘land’ Water storage Evaporation in oceans Evapotranspiration under the waters of the oceans, that is, the Sublimation ocean floor exhibits complex and varied Water in the Condensation features as those observed over the land atmosphere Precipitation (Figure 13.2). The floors of the oceans are rugged with the world’s largest mountain Water storage in Snowmelt runoff ice and snow to streams ranges, deepest trenches and the largest plains. These features are formed, like those of the Surface runoff Stream flow freshwater storage infiltration continents, by the factors of tectonic, volcanic and depositional processes. Groundwater storage Groundwater discharge springs Divisions of the Ocean Floorsatmosphere, landsurface and subsurface and The ocean floors can be divided into four majorthe organisms. divisions: (i) the Continental Shelf; (ii) the Table 13.1 shows distribution of water on Continental Slope; (iii) the Deep Sea Plain;the surface of the earth. About 71 per cent of (iv) the Oceanic Deeps. Besides, these divisionsthe planetary water is found in the oceans. The there are also major and minor relief featuresremaining is held as freshwater in glaciers and in the ocean floors like ridges, hills, seaicecaps, groundwater sources, lakes, soil mounts, guyots, trenches, canyons, etc.moisture, atmosphere, streams and within life.Nearly 59 per cent of the water that falls on Continental Shelfland returns to the atmosphere through The continental shelf is the extended marginevaporation from over the oceans as well as of each continent occupied by relativelyfrom other places. The remainder runs-off on shallow seas and gulfs. It is the shallowest partthe surface, infiltrates into the ground or a part of the ocean showing an average gradient ofof it becomes glacier (Figure 13.1). 1° or even less. The shelf typically ends at a It is to be noted that the renewable water very steep slope, called the shelf break.on the earth is constant while the demand is The width of the continental shelves varyincreasing tremendously. This leads to water from one ocean to another. The average width
  • 115. 114 FUNDAMENTALS OF PHYSICAL GEOGRAPHYof continental shelves is about 80 km. The Continental Slopeshelves are almost absent or very narrow along The continental slope connects the continentalsome of the margins like the coasts of Chile, shelf and the ocean basins. It begins where thethe west coast of Sumatra, etc. On the contrary, bottom of the continental shelf sharply dropsthe Siberian shelf in the Arctic Ocean, the off into a steep slope. The gradient of the slopelargest in the world, stretches to 1,500 km in region varies between 2-5°. The depth of thewidth. The depth of the shelves also varies. It slope region varies between 200 and 3,000 m.may be as shallow as 30 m in some areas while The slope boundary indicates the end of thein some areas it is as deep as 600 m. continents. Canyons and trenches are observed The continental shelves are covered with in this region.variable thicknesses of sediments broughtdown by rivers, glaciers, wind, from the landand distributed by waves and currents. Massive Deep Sea Plainsedimentary deposits received over a long time Deep sea plains are gently sloping areas of theby the continental shelves, become the source ocean basins. These are the flattest andof fossil fuels. smoothest regions of the world. The depths vary between 3,000 and 6,000m. These plains are covered with fine-grained sediments like clay and silt. Oceanic Deeps or Trenches These areas are the deepest parts of the oceans. The trenches are relatively steep sided, narrow basins. They are some 3-5 km deeper than the surrounding ocean floor. They occur at the bases of continental slopes and along island arcs and are associated with active volcanoes and strong earthquakes. That is why they are very significant in the study of plate movements. As many as 57 deeps have been explored so far; of which 32 are in the Pacific Ocean; 19 in the Atlantic Ocean and 6 in the Indian Ocean. Minor Relief Features Apart from the above mentioned major relief features of the ocean floor, some minor but significant features predominate in different parts of the oceans. Mid-Oceanic Ridges A mid-oceanic ridge is composed of two chains of mountains separated by a large depression. The mountain ranges can have peaks as high as 2,500 m and some even reach above the ocean’s surface. Iceland, a part of the mid- Figure 13.2 : Relief features of ocean floors Atlantic Ridge, is an example.
  • 116. WATER (OCEANS) 115Seamount receive more heat due to their contact with larger extent of land than the oceans inIt is a mountain with pointed summits, rising the southern hemisphere.from the seafloor that does not reach the surface (iii) Prevailing wind : the winds blowing fromof the ocean. Seamounts are volcanic in origin. the land towards the oceans drive warmThese can be 3,000-4,500 m tall. The Emperor surface water away form the coastseamount, an extension of the Hawaiian Islands resulting in the upwelling of cold waterin the Pacific Ocean, is a good example. from below. It results into the longitudinal variation in the temperature. Contrary toSubmarine Canyons this, the onshore winds pile up warmThese are deep valleys, some comparable to water near the coast and this raises thethe Grand Canyon of the Colorado river. They temperature.are sometimes found cutting across the (iv) Ocean currents : warm ocean currentscontinental shelves and slopes, often extending raise the temperature in cold areas whilefrom the mouths of large rivers. The Hudson the cold currents decrease theCanyon is the best known canyon in the world. temperature in warm ocean areas. Gulf stream (warm current) raises theGuyots temperature near the eastern coast ofIt is a flat topped seamount. They show North America and the West Coast ofevidences of gradual subsidence through Europe while the Labrador current (coldstages to become flat topped submerged current) lowers the temperature near themountains. It is estimated that more than north-east coast of North America.10,000 seamounts and guyots exist in the All these factors influence the temperaturePacific Ocean alone. of the ocean currents locally. The enclosed seas in the low latitudes record relatively higherAtoll temperature than the open seas; whereas the enclosed seas in the high latitudes have lowerThese are low islands found in the tropical temperature than the open seas.oceans consisting of coral reefs surroundinga central depression. It may be a part of the Horizontal and Vertical Distributionsea (lagoon), or sometimes form enclosing a of Temperaturebody of fresh, brackish, or highly saline water. The temperature-depth profile for the oceanTEMPERATURE OF OCEAN WATERS water shows how the temperature decreases with the increasing depth. The profile shows aThis section deals with the spatial and vertical boundary region between the surface watersvariations of temperature in various oceans. of the ocean and the deeper layers. TheOcean waters get heated up by the solar energy boundary usually begins around 100 - 400 mjust as land. The process of heating and cooling below the sea surface and extends severalof the oceanic water is slower than land. hundred of m downward (Figure 13.3). This boundary region, from where there is a rapidFactors Affecting Temperature Distribution decrease of temperature, is called theThe factors which affect the distribution of thermocline. About 90 per cent of the totaltemperature of ocean water are : volume of water is found below the thermocline (i) Latitude : the temperature of surface water in the deep ocean. In this zone, temperatures decreases from the equator towards the approach 0° C. poles because the amount of insolation The temperature structure of oceans over decreases poleward. middle and low latitudes can be described as (ii) Unequal distribution of land and water : a three-layer system from surface to the bottom. the oceans in the northern hemisphere The first layer represents the top layer of
  • 117. 116 FUNDAMENTALS OF PHYSICAL GEOGRAPHYwarm oceanic water and it is about 500m thick hemisphere record relatively higher temperaturewith temperatures ranging between 20° and than in the southern hemisphere. The highest25° C. This layer, within the tropical region, is temperature is not recorded at the equator butpresent throughout the year but in mid slightly towards north of it. The average annuallatitudes it develops only during summer. temperatures for the northern and southern The second layer called the thermocline hemisphere are around 19° C and 16° Clayer lies below the first layer and is characterised respectively. This variation is due to theby rapid decrease in temperature with increasing unequal distribution of land and water in thedepth. The thermocline is 500 -1,000 m thick. northern and southern hemispheres. Figure 13.4 shows the spatial pattern of surface temperature of the oceans. It is a well known fact that the maximum temperature of the oceans is always at their surfaces because they directly receive the heat from the sun and the heat is transmitted to the lower sections of the oceans through the process of conduction. It results into decrease of temperature with the increasing depth, but the rate of decrease is not uniform throughout. The temperature falls very rapidly up to the depth of 200 m and thereafter, the rate of decrease of temperature is slowed down. SALINITY OF OCEAN WATERS All waters in nature, whether rain water or ocean water, contain dissolved mineral salts. Salinity is the term used to define the total content of dissolved salts in sea water (Table 13.4). It is calculated as the amount of salt (in gm) dissolved in 1,000 gm (1 kg) of seawater. It is usually expressed as parts per thousand (o/oo) or ppt. Salinity is an important property of sea water. Salinity of 24.7 o/oo has been considered as the upper limit to Figure 13.3 : Thermocline demarcate ‘brackish water’. Factors affecting ocean salinity are The third layer is very cold and extends mentioned below:upto the deep ocean floor. In the Arctic and (i) The salinity of water in the surface layerAntartic circles, the surface water temperatures of oceans depend mainly on evaporationare close to 0° C and so the temperature change and precipitation.with the depth is very slight. Here, only one (ii) Surface salinity is greatly influenced inlayer of cold water exists, which extends from coastal regions by the fresh water flowsurface to deep ocean floor. from rivers, and in polar regions by the The average temperature of surface water processes of freezing and thawing of ice.of the oceans is about 27°C and it gradually (iii) Wind, also influences salinity of an areadecreases from the equator towards the poles. by transferring water to other areas.The rate of decrease of temperature with (iv) The ocean currents contribute to theincreasing latitude is generally 0.5°C per salinity variations. Salinity, temperaturelatitude. The average temperature is around and density of water are interrelated.22°C at 20° latitudes, 14° C at 40° latitudes Hence, any change in the temperature orand 0° C near poles. The oceans in the northern density influences the salinity of an area.
  • 118. WATER (OCEANS) 117 Figure 13.4 : Spatial pattern of surface temperature (°C) of the oceans Red Sea, it is as high as 41o/oo, while in the Highest salinity in water bodies estuaries and the Arctic, the salinity fluctuates Lake Van in Turkey (330 o/oo), Dead Sea (238 o/oo), from 0 - 35 o/oo, seasonally. In hot and dry Great Salt Lake (220 o/oo) regions, where evaporation is high, the salinity sometimes reaches to 70 o/oo. The salinity variation in the Pacific Ocean Table 13.4 : Dissolved Salts in Sea Water is mainly due to its shape and larger areal (gm of Salt per kg of Water) extent. Salinity decreases from 35 o/oo - 31 o/oo on the western parts of the northern Chlorine 18.97 hemisphere because of the influx of melted Sodium 10.47 water from the Arctic region. In the same way, Sulphate 2.65 Magnesium 1.28 after 15° - 20° south, it decreases to 33 o/oo . Calcium 0.41 The average salinity of the Atlantic Ocean Potassium 0.38 is around 36 o/oo. The highest salinity is Bicarbonate 0.14 recorded between 15° and 20° latitudes. Bromine 0.06 Maximum salinity (37 o/oo) is observed between Borate 0.02 20° N and 30° N and 20° W - 60° W. It gradually Strontium 0.01 decreases towards the north. The North Sea, in spite of its location in higher latitudes, records higher salinity due to more saline waterHORIZONTAL DISTRIBUTION OF SALINITY brought by the North Atlantic Drift. Baltic SeaThe salinity for normal open ocean ranges records low salinity due to influx of river watersbetween 33o/oo and 37 o/oo. In the land locked in large quantity. The Mediterranean Sea
  • 119. 118 FUNDAMENTALS OF PHYSICAL GEOGRAPHY Figure13.5 : Surface salinity of the World’s Oceansrecords higher salinity due to high evaporation. water to ice or evaporation, or decreased bySalinity is, however, very low in Black Sea due the input of fresh waters, such as from theto enormous fresh water influx by rivers. See rivers. Salinity at depth is very much fixed,the atlas to find out the rivers joining Black Sea. because there is no way that water is ‘lost’, or The average salinity of the Indian Ocean is the salt is ‘added.’ There is a marked difference o35 /oo. The low salinity trend is observed in in the salinity between the surface zones andthe Bay of Bengal due to influx of river water the deep zones of the oceans. The lower salinityby the river Ganga. On the contrary, the water rests above the higher salinity denseArabian Sea shows higher salinity due to high water. Salinity, generally, increases with depthevaporation and low influx of fresh water. Figure and there is a distinct zone called the halocline,13.5 shows the salinity of the World’s oceans. where salinity increases sharply. Other factors being constant, increasing salinity of seawaterVertical Distribution of Salinity causes its density to increase. High salinitySalinity changes with depth, but the way it seawater, generally, sinks below the lowerchanges depends upon the location of the sea. salinity water. This leads to stratification bySalinity at the surface increases by the loss of salinity. EXERCISES 1. Multiple choice questions. (i) Identify the element which is not a part of the hydrological cycle (a) Evaporation (c) Precipitation (b) Hydration (d) Condensation
  • 120. WATER (OCEANS) 119 (ii) The average depth of continental slope varies between (a) 2-20m (c) 20-200m (b) 200-2,000m (d) 2,000-20,000m (iii) Which one of the following is not a minor relief feature in the oceans: (a) Seamount (c) Oceanic Deep (b) Atoll (d) Guyot (iv) Salinity is expressed as the amount of salt in grams dissolved in sea water per (a) 10 gm (c) 100 gm (b) 1,000 gm (d) 10,000 gm (v) Which one of the following is the smallest ocean: (a) Indian Ocean (c) Atlantic Ocean (b) Arctic Ocean (d) Pacific Ocean 2. Answer the following questions in about 30 words. (i) Why do we call the earth a Blue Planet? (ii) What is a continental margin? (iii) List out the deepest trenches of various oceans. (iv) What is a thermocline? (v) When you move into the ocean what thermal layers would you encounter? Why the temperature varies with depth? (vi) What is salinity of sea water? 3. Answer the following questions in about 150 words. (i) How are various elements of the hydrological cycle interrelated? (ii) Examine the factors that influence the temperature distribution of the oceans. Project Work (i) Consult the atlas and show ocean floor relief on the outline of the world map. (ii) Identify the areas of mid oceanic ridges from the Indian Ocean.
  • 121. CHAPTER MOVEMENTS OF OCEAN WATERT he ocean water is dynamic. Its physical wavelength of the wave, the wave breaks. The characteristics like temperature, largest waves are found in the open oceans. salinity, density and the external Waves continue to grow larger as they moveforces like of the sun, moon and the winds and absorb energy from the wind.influence the movement of ocean water. The Most of the waves are caused by the windhorizontal and vertical motions are common driving against water. When a breeze of twoin ocean water bodies. The horizontal motion knots or less blows over calm water, smallrefers to the ocean currents and waves. The ripples form and grow as the wind speedvertical motion refers to tides. Ocean currents increases until white caps appear in theare the continuous flow of huge amount of breaking waves. Waves may travel thousandswater in a definite direction while the waves of km before rolling ashore, breaking andare the horizontal motion of water. Water moves dissolving as surf.ahead from one place to another through ocean A wave’s size and shape reveal its origin.currents while the water in the waves does not Steep waves are fairly young ones and aremove, but the wave trains move ahead. The probably formed by local wind. Slow andvertical motion refers to the rise and fall of water steady waves originate from far away places,in the oceans and seas. Due to attraction of possibly from another hemisphere. Thethe sun and the moon, the ocean water is raised maximum wave height is determined by theup and falls down twice a day. The upwelling strength of the wind, i.e. how long it blows andof cold water from subsurface and the sinking the area over which it blows in a single direction.of surface water are also forms of vertical Waves travel because wind pushes themotion of ocean water. water body in its course while gravity pulls the crests of the waves downward. The falling waterWAVES pushes the former troughs upward, and theWaves are actually the energy, not the wateras such, which moves across the ocean surface.Water particles only travel in a small circle as awave passes. Wind provides energy to thewaves. Wind causes waves to travel in the oceanand the energy is released on shorelines. Themotion of the surface water seldom affects thestagnant deep bottom water of the oceans. Asa wave approaches the beach, it slows down.This is due to the friction occurring betweenthe dynamic water and the sea floor. And, whenthe depth of water is less than half the Figure14.1 : Motion of waves and water molecules
  • 122. MOVEMENTS OF OCEAN WATER 121wave moves to a new position (Figure 14.1). attraction of the moon is less as it is fartherThe actual motion of the water beneath the away, the centrifugal force causes tidal bulgewaves is circular. It indicates that things are on the other side (Figure 14.2).carried up and forward as the wave The ‘tide-generating’ force is the differenceapproaches, and down and back as it passes. between these two forces; i.e. the gravitational attraction of the moon and the centrifugal force. Characteristics of Waves On the surface of the earth, nearest the moon, Wave crest and trough : The highest and pull or the attractive force of the moon is greater lowest points of a wave are called the crest than the centrifugal force, and so there is a net and trough respectively. force causing a bulge towards the moon. On Wave height : It is the vertical distance the opposite side of the earth, the attractive from the bottom of a trough to the top of force is less, as it is farther away from the moon, a crest of a wave. the centrifugal force is dominant. Hence, there Wave amplitude : It is one-half of the wave is a net force away from the moon. It creates height. the second bulge away from the moon. On the Wave period : It is merely the time interval surface of the earth, the horizontal tide between two successive wave crests or generating forces are more important than the troughs as they pass a fixed point. vertical forces in generating the tidal bulges. Wavelength : It is the horizontal distance between two successive crests. Wave speed : It is the rate at which the wave moves through the water, and is measured in knots. Wave frequency : It is the number of waves passing a given point during a one- second time interval.TIDESThe periodical rise and fall of the sea level, onceor twice a day, mainly due to the attraction ofthe sun and the moon, is called a tide.Movement of water caused by meteorologicaleffects (winds and atmospheric pressurechanges) are called surges. Surges are notregular like tides. The study of tides is verycomplex, spatially and temporally, as it has greatvariations in frequency, magnitude and height. Figure14.2 : Relation between gravitational The moon’s gravitational pull to a great forces and tidesextent and to a lesser extent the sun’sgravitational pull, are the major causes for the The tidal bulges on wide continentaloccurrence of tides. Another factor is centrifugal shelves, have greater height. When tidal bulgesforce, which is the force that acts to counter hit the mid-oceanic islands they become low.the balance the gravity. Together, the The shape of bays and estuaries along agravitational pull and the centrifugal force are coastline can also magnify the intensity of tides.responsible for creating the two major tidal Funnel-shaped bays greatly change tidalbulges on the earth. On the side of the earth magnitudes. When the tide is channelledfacing the moon, a tidal bulge occurs while on between islands or into bays and estuariesthe opposite side though the gravitational they are called tidal currents.
  • 123. 122 FUNDAMENTALS OF PHYSICAL GEOGRAPHY Spring tides : The position of both the sun and Tides of Bay of Fundy, Canada the moon in relation to the earth has direct The highest tides in the world occur in bearing on tide height. When the sun, the moon the Bay of Fundy in Nova Scotia, Canada. and the earth are in a straight line, the height The tidal bulge is 15 - 16 m. Because of the tide will be higher. These are called spring there are two high tides and two low tides tides and they occur twice a month, one on every day (roughly a 24 hour period); then full moon period and another during new moon a tide must come in within about a six period. hour period. As a rough estimate, the tide rises about 240 cm an hour (1,440 cm Neap tides : Normally, there is a seven day divided by 6 hours). If you have walked interval between the spring tides and neap down a beach with a steep cliff alongside tides. At this time the sun and moon are at (which is common there), make sure you right angles to each other and the forces of the watch the tides. If you walk for about an sun and moon tend to counteract one another. hour and then notice that the tide is The Moon’s attraction, though more than twice coming in, the water will be over your as strong as the sun’s, is diminished by the head before you get back to where you started! counteracting force of the sun’s gravitational pull. Once in a month, when the moon’s orbit isTypes of Tides closest to the earth (perigee), unusually highTides vary in their frequency, direction and and low tides occur. During this time the tidalmovement from place to place and also from range is greater than normal. Two weeks later,time to time. Tides may be grouped into various when the moon is farthest from earth (apogee),types based on their frequency of occurrence the moon’s gravitational force is limited andin one day or 24 hours or based on their height. the tidal ranges are less than their average heights.Tides based on Frequency When the earth is closest to the sun (perihelion), around 3rd January each year,Semi-diurnal tide : The most common tidal tidal ranges are also much greater, withpattern, featuring two high tides and two low unusually high and unusually low tides. Whentides each day. The successive high or low tides the earth is farthest from the sun (aphelion),are approximately of the same height. around 4th July each year, tidal ranges are much less than average.Diurnal tide : There is only one high tide and The time between the high tide and low tide,one low tide during each day. The successive when the water level is falling, is called the ebb.high and low tides are approximately of the The time between the low tide and high tide,same height. when the tide is rising, is called the flow or flood.Mixed tide : Tides having variations in heightare known as mixed tides. These tides generally Importance of Tidesoccur along the west coast of North America Since tides are caused by the earth-moon-sunand on many islands of the Pacific Ocean. positions which are known accurately, the tides can be predicted well in advance. ThisTides based on the Sun, Moon and the Earth helps the navigators and fishermen plan theirPositions activities. Tidal flows are of great importanceThe height of rising water (high tide) varies in navigation. Tidal heights are very important,appreciably depending upon the position of especially harbours near rivers and withinsun and moon with respect to the earth. estuaries having shallow ‘bars’ at the entrance,Spring tides and neap tides come under this which prevent ships and boats from enteringcategory. into the harbour. Tides are also helpful in
  • 124. MOVEMENTS OF OCEAN WATER 123desilting the sediments and in removing Differences in water density affect verticalpolluted water from river estuaries. Tides are mobility of ocean currents. Water with highused to generate electrical power (in Canada, salinity is denser than water with low salinityFrance, Russia, and China). A 3 MW tidal and in the same way cold water is denser thanpower project at Durgaduani in Sunderbans warm water. Denser water tends to sink, whileof West Bengal is under way. relatively lighter water tends to rise. Cold-water ocean currents occur when the cold water atOCEAN CURRENTS the poles sinks and slowly moves towards the equator. Warm-water currents travel out fromOcean currents are like river flow in oceans. the equator along the surface, flowing towardsThey represent a regular volume of water in a the poles to replace the sinking cold water.definite path and direction. Ocean currents areinfluenced by two types of forces namely :(i) primary forces that initiate the movement of Types of Ocean Currentswater; (ii) secondary forces that influence the The ocean currents may be classified based oncurrents to flow. their depth as surface currents and deep water The primary forces that influence the currents : (i) surface currents constitute aboutcurrents are: (i) heating by solar energy; 10 per cent of all the water in the ocean, these(ii) wind; (iii) gravity; (iv) coriolis force. Heating waters are the upper 400 m of the ocean;by solar energy causes the water to expand. (ii) deep water currents make up the other 90That is why, near the equator the ocean water per cent of the ocean water. These waters moveis about 8 cm higher in level than in the middle around the ocean basins due to variations inlatitudes. This causes a very slight gradient the density and gravity. Deep waters sink intoand water tends to flow down the slope. Wind the deep ocean basins at high latitudes, whereblowing on the surface of the ocean pushes the the temperatures are cold enough to cause thewater to move. Friction between the wind and density to increase.the water surface affects the movement of the Ocean currents can also be classifiedwater body in its course. Gravity tends to pull based on temperature : as cold currents andthe water down to pile and create gradient warm currents: (i) cold currents bring coldvariation. The Coriolis force intervenes and water into warm water areas. These currentscauses the water to move to the right in the are usually found on the west coast of thenorthern hemisphere and to the left in the continents in the low and middle latitudessouthern hemisphere. These large accumulations (true in both hemispheres) and on the eastof water and the flow around them are called coast in the higher latitudes in the NorthernGyres. These produce large circular currents Hemisphere; (ii) warm currents bring warmin all the ocean basins. water into cold water areas and are usually observed on the east coast of continents in the Characteristics of Ocean Currents low and middle latitudes (true in both hemispheres). In the northern hemisphere Currents are referred to by their “drift”. they are found on the west coasts of continents Usually, the currents are strongest near in high latitudes. the surface and may attain speeds over five knots. At depths, currents are Major Ocean Currents generally slow with speeds less than 0.5 knots. We refer to the speed of a current Major ocean currents are greatly influenced by as its “drift.” Drift is measured in terms the stresses exerted by the prevailing winds and of knots. The strength of a current refers coriolis force. The oceanic circulation pattern to the speed of the current. A fast current roughly corresponds to the earth’s atmospheric is considered strong. A current is usually circulation pattern. The air circulation over the strongest at the surface and decreases oceans in the middle latitudes is mainly in strength (speed) with depth. Most anticyclonic (more pronounced in the southern hemisphere than in the northern hemisphere). currents have speeds less than or equal The oceanic circulation pattern also to 5 knots. corresponds with the same. At higher latitudes,
  • 125. 124 FUNDAMENTALS OF PHYSICAL GEOGRAPHY Fig.14.3 : Major currents in the Pacific, Atlantic and Indian oceanswhere the wind flow is mostly cyclonic, the Effects of Ocean Currentsoceanic circulation follows this pattern. In Ocean currents have a number of direct andregions of pronounced monsoonal flow, the indirect influences on human activities. Westmonsoon winds influence the current coasts of the continents in tropical andmovements. Due to the coriolis force, the warm subtropical latitudes (except close to thecurrents from low latitudes tend to move to the equator) are bordered by cool waters. Theirright in the northern hemisphere and to their average temperatures are relatively low with aleft in the southern hemisphere. narrow diurnal and annual ranges. There is The oceanic circulation transports heat fog, but generally the areas are arid. West coastsfrom one latitude belt to another in a manner of the continents in the middle and highersimilar to the heat transported by the general latitudes are bordered by warm waters whichcirculation of the atmosphere. The cold waters cause a distinct marine climate. They areof the Arctic and Antarctic circles move towards characterised by cool summers and relativelywarmer water in tropical and equatorial mild winters with a narrow annual range ofregions, while the warm waters of the lower temperatures. Warm currents flow parallel tolatitudes move polewards. The major currents the east coasts of the continents in tropical andin the different oceans are shown in Figure14.3. subtropical latitudes. This results in warm and rainy climates. These areas lie in the western Prepare a list of currents which are margins of the subtropical anti-cyclones. The found in Pacific, Atlantic and Indian mixing of warm and cold currents help to Oceans. replenish the oxygen and favour the growth of How is the movement of currents is planktons, the primary food for fish population. influenced by prevailing winds? Give The best fishing grounds of the world exist some examples from Figure14.3. mainly in these mixing zones.
  • 126. MOVEMENTS OF OCEAN WATER 125 EXERCISES 1. Multiple choice questions. (i) Upward and downward movement of ocean water is known as the : (a) tide (c) wave (b) current (d) none of the above (ii) Spring tides are caused : (a) As result of the moon and the sun pulling the earth gravitationally in the same direction. (b) As result of the moon and the sun pulling the earth gravitationally in the opposite direction. (c) Indention in the coast line. (d) None of the above. (iii) The distance between the earth and the moon is minimum when the moon is in : (a) Aphelion (c) Perihelion (b) Perigee (d) Apogee (iv) The earth reaches its perihelion in: (a) October (c) July (b) September (d) January 2. Answer the following questions in about 30 words. (i) What are waves? (ii) Where do waves in the ocean get their energy from? (iii) What are tides? (iv) How are tides caused? (v) How are tides related to navigation? 3. Answer the following questions in about 150 words. (i) How do currents affect the temperature? How does it affect the temperature of coastal areas in the N. W. Europe? (ii) What are the causes of currents? Project Work (i) Visit a lake or a pond and observe the movement of waves. Throw a stone and notice how waves are generated. Draw the diagram of a wave and measure its length, distance and amplitude and record them in your note. (ii) Take a globe and a map showing the currents of the oceans. Discuss why certain currents are warm or cold and why they deflect in certain places and examine the reasons.
  • 127. UNIT VI LIFE ON THE EARTHThis unit deals with• Biosphere — importance of plants and other organisms; ecosystems, bio-geo chemical cycle and ecological balance; biodiversity and conservation
  • 128. CHAPTERLIFE ON THE EARTHB y now you might have realised that all E COLOGY units of this book have acquainted you You have been reading about ecological and with the three major realms of the environmental problems in newspapers andenvironment, that is, the lithosphere, the magazines. Have you ever thought whatatmosphere and the hydrosphere. You know ecology is? The environment as you know, isthat living organisms of the earth, constituting made up of abiotic and biotic components. Itthe biosphere, interact with other environmental would be interesting to understand how therealms. The biosphere includes all the living diversity of life-forms is maintained to bring acomponents of the earth. It consists of all plants kind of balance. This balance is maintained inand animals, including all the micro- a particular proportion so that a healthy interaction between the biotic and the abiotic Life on the earth is found almost components goes on. everywhere. Living organisms are found The interactions of a particular group of from the poles to the equator, from the organisms with abiotic factors within a bottom of the sea to several km in the particular habitat resulting in clearly defined air, from freezing waters to dry valleys, energy flows and material cycles on land, water from under the sea to underground water and air, are called ecological systems. lying below the earth’s surface. The term ecology is derived from the Greekorganisms that live on the planet earth and their word ‘oikos’ meaning ‘house’, combinedinteractions with the surrounding environment. with the word ‘logy’ meaning the ‘scienceMost of the organisms exist on the lithosphere of’ or ‘the study of ’. Literally, ecology isand/or the hydrosphere as well as in the the study of the earth as a ‘household’,atmosphere. There are also many organisms of plants, human beings, animals and micro-organisms. They all live togetherthat move freely from one realm to the other. as interdependent components. A The biosphere and its components are very German zoologist Ernst Haeckel, whosignificant elements of the environment. These used the term as ‘oekologie’ in 1869,elements interact with other components of the became the first person to use the termnatural landscape such as land, water and ‘ecology’. The study of interactionssoil. They are also influenced by the between life forms (biotic) and theatmospheric elements such as the temperature, physical environment (abiotic) is the science of ecology. Hence, ecology can berainfall, moisture and sunlight. The defined as a scientific study of theinteractions of biosphere with land, air and interactions of organisms with theirwater are important to the growth, physical environment and with each other.development and evolution of the organism.
  • 129. 128 FUNDAMENTALS OF PHYSICAL GEOGRAPHYA habitat in the ecological sense is the totality ecosystem includes lakes, ponds, streams,of the physical and chemical factors that marshes and bogs.constitute the general environment. A system Structure and Functions of Ecosystemsconsisting of biotic and abiotic components isknown as ecosystem. All these components in The structure of an ecosystem involves aecosystem are inter related and interact with description of the available plant and animaleach other. Different types of ecosystems exist species. From a structural point of view, allwith varying ranges of environmental ecosystems consist of abiotic and biotic factors.conditions where various plants and animal Abiotic factors include rainfall, temperature,species have got adapted through evolution. sunlight, atmospheric humidity, soilThis phenomenon is known as ecological conditions, inorganic substances (carbonadaptation. dioxide, water, nitrogen, calcium, phosphorus, potassium, etc.). Biotic factors include theTypes of Ecosystems producers, (primary, secondary, tertiary) theEcosystems are of two major types: terrestrial consumers and the decomposers. Theand aquatic. Terrestrial ecosystem can be producers include all the green plants, whichfurther be classified into ‘biomes’. A biome is a manufacture their own food throughplant and animal community that covers a photosynthesis. The primary consumerslarge geographical area. The boundaries of include herbivorous animals like deer, goats,different biomes on land are determined mainly mice and all plant-eating animals. Theby climate. Therefore, a biome can be defined carnivores include all the flesh-eating animalsas the total assemblage of plant and animal like snakes, tigers and lions. Certain carnivoresspecies interacting within specific conditions. that feed also on carnivores are known as topThese include rainfall, temperature, humidity carnivores like hawks and mongooses.and soil conditions. Some of the major biomes Decomposers are those that feed on deadof the world are: forest, grassland, desert and organisms (for example, scavengers liketundra biomes. Aquatic ecosystems can be vultures and crows), and further breakingclassed as marine and freshwater ecosystems. down of the dead matter by other decomposingMarine ecosystem includes the oceans, coastal agents like bacteria and various micro-estuaries and coral reefs. Freshwater organisms. Figure 15.1 : Structure and functions of ecosystems
  • 130. LIFE ON THE EARTH 129 The producers are consumed by the aquatic and altitudinal biomes. Some featuresprimary consumers whereas the primary of these biomes are given in Table 15.1.consumers are, in turn, being eaten by thesecondary consumers. Further, the secondary Biogeochemical Cyclesconsumers are consumed by the tertiary The sun is the basic source of energy on whichconsumers. The decomposers feed on the dead all life depends. This energy initiates lifeat each and every level. They change them into processes in the biosphere throughvarious substances such as nutrients, organic photosynthesis, the main source of food andand inorganic salts essential for soil fertility. energy for green plants. During photosynthesis,Organisms of an ecosystem are linked together carbon dioxide is converted into organicthrough a foodchain (Figure 15.1). For compounds and oxygen. Out of the total solarexample, a plant eating beetle feeding on a insolation that reaches the earth’s surface, onlypaddy stalk is eaten by a frog, which is, in turn, a very small fraction (0.1 per cent) is fixed ineaten by a snake, which is then consumed by photosynthesis. More than half is used for planta hawk. This sequence of eating and being respiration and the remaining part iseaten and the resultant transfer of energy from temporarily stored or is shifted to otherone level to another is known as the food-chain. portions of the plant.Transfer of energy that occurs during the Life on earth consists of a great variety ofprocess of a foodchain from one level to living organisms. These living organisms existanother is known as flow of energy. However, and survive in a diversity of associations. Suchfood-chains are not isolated from one another. survival involves the presence of systemic flowsFor example, a mouse feeding on grain may such as flows of energy, water and eaten by different secondary consumers These flows show variations in different parts(carnivores) and these carnivores may be eaten of the world, in different seasons of the yearby other different tertiary consumers (top and under varying local circumstances. Studiescarnivores). In such situations, each of the have shown that for the last one billion years,carnivores may consume more than one type the atmosphere and hydrosphere have beenof prey. As a result, the food- chains get composed of approximately the same balanceinterlocked with one another. This inter- of chemical components. This balance of theconnecting network of species is known as food chemical elements is maintained by a cyclicweb. Generally, two types of food-chains are passage through the tissues of plants andrecognised: grazing food-chain and detritus animals. The cycle starts by absorbing thefood-chain. In a grazing food-chain, the first chemical elements by the organism and islevel starts with plants as producers and ends returned to the air, water and soil throughwith carnivores as consumers as the last level, decomposition. These cycles are largelywith the herbivores being at the intermediate energised by solar insolation. These cycliclevel. There is a loss of energy at each level movements of chemical elements of thewhich may be through respiration, excretion biosphere between the organism and theor decomposition. The levels involved in a food- environment are referred to as biogeochemicalchain range between three to five and energy cycles. Bio refers to living organisms and geois lost at each level. A detritus food-chain is to rocks, soil, air and water of the earth.based on autotrophs energy capture initiated There are two types of biogeochemicalby grazing animals and involves the cycles : the gaseous and the sedimentary cycle.decomposition or breaking down of organic In the gaseous cycle, the main reservoir ofwastes and dead matter derived from the nutrients is the atmosphere and the ocean. Ingrazing food-chain. the sedimentary cycle, the main reservoir is the soil and the sedimentary and other rocks ofTypes of Biomes the earth’s crust.In the earlier paragraphs, you have learnt the The Water Cyclemeaning of the term ‘biome’. Let us now try toidentify the major biomes of the world. There All living organisms, the atmosphere and theare five major biomes — forest, desert, grassland, lithosphere maintain between them a
  • 131. 130 FUNDAMENTALS OF PHYSICAL GEOGRAPHY Table 15.1 : World Biomes ClimaticBiomes Subtypes Regions Soil Flora and Fauna Characteristics Forest A. Tropical A1. 10° N-S A1. Temp. 20-25°C, A1. Acidic, A1. M u l t i - l a y e r e d 1. Equitorial A2. 10° - 25° N-S evenly distributed poor in canopy tall and 2. Deciduous B. Eastern North A2. Temp. 25-30°C, nutrients large trees B. Temperate America, N.E. Rainfall, ave. ann. A2. Rich in A2. Less dense, trees C. Boreal Asia, Western 1,000mm, seasonal nutrients of medium height; and Central B. Temp. 20-30° C, B. Fertile, many varieties co- Europe Rainfall evenly en-riched exis t. Insects, C. Broad belt of distributed 750- with bats, birds and Eurasia and 1,500mm, Well- decaying mammals are North America, defined seasons litter common species parts of and distinct winter. C. Acidic and in both Siberia, C. Short moist moder- poor in B. Moderately dense Alaska, ately warm nutrients, broad leaved trees. Canada and summers and long thin soil With less diversity Scandinavia cold dry winter; cover of plant species. very low Oack, Beach, temperatures. Maple etc. are Precipitation mostly some common snowfall species. Squirrels, 400 -1,000mm rabbits, skunks, birds, black bears, mountain lions etc. C. Evergreen conifers like pine, fur and spruce etc. Wood peckers, hawks, bears, wolves, deer, hares and bats are common animals Desert A. Hot and Dry A. S a h a r a , A. Temp. 20 - 45°C. Rich in A-C. Scanty vege- desert Kalahari, B. 21 - 38°C. nutrients with tation; few large B. Semi arid Marusthali, C. 15 - 35°C. little or no mammals, desert Rub-el-Khali D. 2 - 25°C organic matter insects, reptiles C. Coastal B. Marginal areas A-D Rainfall is less than and birds desert of hot deserts 50 mm D. Rabbits, rats, D. Cold desert C. Atacama antelopes D. Tundra climatic and ground regions squirrelsGrassland A. Tropical A. Large areas A. Warm hot A. Porous with A. Grasses; trees Savannah of A f r i c a , climates, Rainfall thin layer of and large shrubs B. Temperate Australia, 500-1,250 mm humus. absent; giraffes Steppe South B. Hot summers and B. Thin floccu- zebras, buffalos, America and cold winter. lated soil, leopards, hyenas, India Rainfall 500 - rich in bases elephants, mice, B. P a r t s of 900 mm moles, snakes Eurasia and and worms etc., North America are common animals B. Grasses; occ- asional trees such as cotton- woods, oaks and willows; gazelles, zebras, rhin-
  • 132. LIFE ON THE EARTH 131 oceros, wild horses, lions, varieties of birds, worms, snakes etc., are common animals Aquatic A. Freshwater A. Lakes, streams, A-B Temperatures vary A. Water, swamps Algal and other aquatic B. Marine rivers and widely with cooler air and marshes and marine plant wetlands temperatures and communities with B. Oceans, coral high humidity B.Water, tidal varieties of water reefs, lagoons swamps and dwelling animals and estuaries marshesAltitudinal ——— Slopes of high Temperature and Regolith over Deciduous to tundra mountain ranges precipitation vary slopes vegetation varying like the Himalayas, depending upon according to altitude the Andes and the latitudinal zone Rockiescirculation of water in solid, liquid or gaseous dioxide and are returned to the atmosphereform referred to as the water or hydrologic cycle (Figure 15.2).(Chapter 13 of this book).The Carbon CycleCarbon is one of the basic elements of all livingorganisms. It forms the basic constituent ofall the organic compounds. The biospherecontains over half a million carbon compoundsin them. The carbon cycle is mainly theconversion of carbon dioxide. This conversionis initiated by the fixation of carbon dioxidefrom the atmosphere through photosynthesis.Such conversion results in the production ofcarbohydrate, glucose that may be convertedto other organic compounds such as sucrose,starch, cellulose, etc. Here, some of thecarbohydrates are utilised directly by the plant Figure 15.2 : Carbon Cycleitself. During this process, more carbon dioxide The Oxygen Cycleis generated and is released through its leavesor roots during the day. The remaining Oxygen is the main by-product ofcarbohydrates not being utilised by the plant photosynthesis. It is involved in the oxidationbecome part of the plant tissue. Plant tissues of carbohydrates with the release of energy,are either being eaten by the herbivorous carbon dioxide and water. The cycling ofanimals or get decomposed by the micro- oxygen is a highly complex process. Oxygenorganisms. The herbivores convert some of the occurs in a number of chemical forms andconsumed carbohydrates into carbon dioxide combinations. It combines with nitrogen tofor release into the air through respiration. The form nitrates and with many other mineralsmicro-organisms decompose the remaining and elements to form various oxides such ascarbohydrates after the animal dies. The the iron oxide, aluminium oxide and others.carbohydrates that are decomposed by the Much of oxygen is produced from themicro-organisms then get oxidised into carbon decomposition of water molecules by sunlight
  • 133. 132 FUNDAMENTALS OF PHYSICAL GEOGRAPHYduring photosynthesis and is released in the Other Mineral Cyclesatmosphere through transpiration and Other than carbon, oxygen, nitrogen andrespiration processes of plants. hydrogen being the principal geochemical components of the biosphere, many otherThe Nitrogen Cycle minerals also occur as critical nutrients forNitrogen is a major constituent of the plant and animal life. These mineral elementsatmosphere comprising about seventy-nine required by living organisms are obtainedper cent of the atmospheric gases. It is also an initially from inorganic sources such asessential constituent of different organic phosphorus, sulphur, calcium and potassium.compounds such as the amino acids, nucleic They usually occur as salts dissolved in soilacids, proteins, vitamins and pigments. Only water or lakes, streams and seas. Mineral saltsa few types of organisms like certain species of come directly from the earth’s crust bysoil bacteria and blue green algae are capable weathering where the soluble salts enter theof utilising it directly in its gaseous form. water cycle, eventually reaching the sea. OtherGenerally, nitrogen is usable only after it is salts are returned to the earth’s surface throughfixed. Ninety per cent of fixed nitrogen is sedimentation, and after weathering, they againbiological. The principal source of free nitrogen enter the cycle. All living organisms fulfill theiris the action of soil micro-organisms and mineral requirements from mineral solutionsassociated plant roots on atmospheric nitrogen in their environments. Other animals receivefound in pore spaces of the soil. Nitrogen can their mineral needs from the plants and animalsalso be fixed in the atmosphere by lightning and they consume. After the death of livingcosmic radiation. In the oceans, some marine organisms, the minerals are returned to the soilanimals can fix it. After atmospheric nitrogen and water through decomposition and flow.has been fixed into an available form, greenplants can assimilate it. Herbivorous animals Ecological Balancefeeding on plants, in turn, consume some of it. Ecological balance is a state of dynamicDead plants and animals, excretion of equilibrium within a community of organismsnitrogenous wastes are converted into nitrites in a habitat or ecosystem. It can happen whenby the action of bacteria present in the soil. the diversity of the living organisms remainsSome bacteria can even convert nitrites into relatively stable. Gradual changes do takenitrates that can be used again by green plants. place but that happens only through naturalThere are still other types of bacteria capable succession. It can also be explained as a stableof converting nitrates into free nitrogen, a balance in the numbers of each species in anprocess known as denitrification (Figure 15.3). ecosystem. This occurs through competition and cooperation between different organisms where population remains stable. This balance is brought about by the fact that certain species compete with one another determined by the environment in which they grow. This balance is also attained by the fact that some species depend on others for their food and sustenance. Such accounts are encountered in vast grasslands where the herbivorous animals (deer, zebras, buffaloes, etc.) are found in plenty. On the other hand, the carnivorous animals (tigers, lions, etc.) that are not usually in large numbers, hunt and feed on the herbivores, thereby controlling their population. In the plants, any disturbance in the native forests such as clearing the forest Figure 15.3 : Nitrogen Cycle for shifting cultivation usually brings about a
  • 134. LIFE ON THE EARTH 133change in the species distribution. This change the ecosystem. This has destroyed its originalityis due to competition where the secondary and has caused adverse effects to the generalforest species such as grasses, bamboos or environment. Ecological imbalances havepines overtakes the native species changing brought many natural calamities likethe original forest structure. This is called floods, landslides, diseases, erratic climaticsuccession. occurrences, etc. Ecological balance may be disturbed due There is a very close relationship betweento the introduction of new species, natural the plant and animal communities withinhazards or human causes. Human interference particular habitats. Diversity of life in ahas affected the balance of plant communities particular area can be employed as anleading to disturbances in the ecosystems. indicator of the habitat factor. ProperSuch disturbances bring about numerous knowledge and understanding of such factorssecondary successions. Human pressure on provide a strong base for protecting andthe earth’s resources has put a heavy toll on conserving the ecosystems. EXERCISES 1. Multiple choice questions. (i) Which one of the following is included in biosphere? (a) only plants (c) only animals (b) all living and non-living organisms (d) all living organisms (ii) Tropical grasslands are also known as : (a) the prairies (c) the steppes (b) the savannas (d) none of the above (iii) Oxygen combines with iron found in the rocks to form : (a) iron carbonate (c) iron oxides (b) iron nitrites (d) iron sulphate (iv) During photosynthesis, carbon dioxide combines with water in the presence of sunlight to form : (a) proteins (c) carbohydrates (b) amino acids (d) vitamins 2. Answer the following questions in about 30 words. (i) What do you understand by the term ‘ecology’? (ii) What is an ecological system? Identify the major types of ecosystems in the world. (iii) What is a food-chain? Give one example of a grazing food-chain identifying the various levels. (iv) What do you understand by the term ‘food web’? Give examples. (v) What is a biome?
  • 135. 134 FUNDAMENTALS OF PHYSICAL GEOGRAPHY 3. Answer the following questions in about 150 words. (i) What are bio-geochemical cycles? Explain how nitrogen is fixed in the atmosphere. (ii) What is an ecological balance? Discuss the important measures needed to prevent ecological imbalances. Project Work (i) Show the distribution of the different biomes on the outline map of the world with a note highlighting the important characteristics of each biome. (ii) Make a note of trees, shrubs and perennial plants in your school campus and devote half a day to observe the types of birds which come to the garden. Can you describe the diversity of birds?
  • 136. CHAPTERBIODIVERSITY AND CONSERVATIONY ou have already learnt about the the earth are today extinct. Biodiversity is not geomorphic processes particularly found evenly on the earth. It is consistently weathering and depth of weathering richer in the tropics. As one approaches themantle in different climatic zones. See the polar regions, one finds larger and largerFigure 6.2 in Chapter 6 in order to recapitulate. populations of fewer and fewer species.You should know that this weathering mantle Biodiversity itself is a combination of twois the basis for the diversity of vegetation and words, Bio (life) and diversity (variety). Inhence, the biodiversity. The basic cause for simple terms, biodiversity is the number andsuch weathering variations and resultant variety of organisms found within a specifiedbiodiversity is the input of solar energy and geographic region. It refers to the varieties ofwater. No wonder that the areas that are rich plants, animals and micro-organisms, thein these inputs are the areas of wide spectrum genes they contain and the ecosystems theyof biodiversity. form. It relates to the variability among living organisms on the earth, including the Biodiversity as we have today is the result variability within and between the species and of 2.5-3.5 billion years of evolution. Before that within and between the ecosystems. the advent of humans, our earth Biodiversity is our living wealth. It is a result supported more biodiversity than in any of hundreds of millions of years of evolutionary other period. Since, the emergence of history. humans, however, biodiversity has begun a rapid decline, with one species after Biodiversity can be discussed at three another bearing the brunt of extinction levels : (i) Genetic diversity; (ii) Species diversity; due to overuse. The number of species (iii) Ecosystem diversity. globally vary from 2 million to 100 million, with 10 million being the best estimate. Genetic Diversity New species are regularly discovered most of which are yet to be classified (an Genes are the basic building blocks of various estimate states that about 40 per cent of life forms. Genetic biodiversity refers to the fresh water fishes from South America variation of genes within species. Groups of are not classified yet). Tropical forests are individual organisms having certain very rich in bio-diversity. similarities in their physical characteristics are called species. Human beings genetically Biodiversity is a system in constant belong to the homo sapiens group and alsoevolution, from a view point of species, as well differ in their characteristics such as height,as from view point of an individual organism. colour, physical appearance, etc., considerably.The average half-life of a species is estimated This is due to genetic diversity. This geneticat between one and four million years, and 99 diversity is essential for a healthy breeding ofper cent of the species that have ever lived on population of species.
  • 137. 136 FUNDAMENTALS OF PHYSICAL GEOGRAPHYSpecies Diversity ecosystem evolves and sustains without any reason. That means, every organism, besidesThis refers to the variety of species. It relates to extracting its needs, also contributes somethingthe number of species in a defined area. The of useful to other organisms. Can you think ofdiversity of species can be measured through the way we, humans contribute to theits richness, abundance and types. Some areas sustenance of ecosystems. Species captureare more rich in species than others. Areas rich and store energy, produce and decomposein species diversity are called hotspots of organic materials, help to cycle water anddiversity (Figure 16.5). nutrients throughout the ecosystem, fix atmospheric gases and help regulate theEcosystem Diversity climate. These functions are important forYou have studied about the ecosystem in the ecosystem function and human survival. Theearlier chapter. The broad differences between more diverse an ecosystem, better are theecosystem types and the diversity of habitats chances for the species to survive throughand ecological processes occurring within each adversities and attacks, and consequently, isecosystem type constitute the ecosystem more productive. Hence, the loss of speciesdiversity. The ‘boundaries’ of communities would decrease the ability of the system to(associations of species) and ecosystems are not maintain itself. Just like a species with a highvery rigidly defined. Thus, the demarcation of genetic diversity, an ecosystem with highecosystem boundaries is difficult and complex. biodiversity may have a greater chance of adapting to environmental change. In other words, the more the variety of species in an ecosystem, the more stable the ecosystem is likely to be. Economic Role of Biodiversity For all humans, biodiversity is an important resource in their day-to-day life. One important part of biodiversity is ‘crop diversity’, which is also called agro-biodiversity. Biodiversity is seen as a reservoir of resources to be drawn upon for the manufacture of food, pharmaceutical, and cosmetic products. This Figure 16.1 : Grasslands and sholas in Indira Gandhi National Park, Annamalai, Western Ghats — an concept of biological resources is responsible example of ecosystem diversity for the deterioration of biodiversity. At the same time, it is also the origin of new conflicts dealingImportance of Biodiversity with rules of division and appropriation of natural resources. Some of the importantBiodiversity has contributed in many ways to economic commodities that biodiversitythe development of human culture and, in supplies to humankind are: food crops,turn, human communities have played a major livestock, forestry, fish, medicinal resources,role in shaping the diversity of nature at the etc.genetic, species and ecological levels.Biodiversity plays the following roles: Scientific Role of Biodiversityecological, economic and scientific. Biodiversity is important because each speciesEcological Role of Biodiversity can give us some clue as to how life evolved and will continue to evolve. Biodiversity alsoSpecies of many kinds perform some function helps in understanding how life functions andor the other in an ecosystem. Nothing in an the role of each species in sustaining
  • 138. BIODIVERSITY AND CONSERVATION 137ecosystems of which we are also a species. This The International Union of Conservation offact must be drawn upon every one of us so Nature and Natural Resources (IUCN) hasthat we live and let other species also live their classified the threatened species of plants andlives. animals into three categories for the purpose It is our ethical responsibility to consider of their conservation.that each and every species along with us havean intrinsic right to exist. Hence, it is morally Endangered Specieswrong to voluntarily cause the extinction of any It includes those species which are in dangerspecies. The level of biodiversity is a good of extinction. The IUCN publishes informationindicator of the state of our relationships with about endangered species world-wide as theother living species. In fact, the concept of Red List of threatened species.biodiversity is an integral part of many humancultures.LOSS OF BIODIVERSITYSince the last few decades, growth in humanpopulation has increased the rate ofconsumption of natural resources. It hasaccelerated the loss of species and habitationin different parts of the world. Tropical regionswhich occupy only about one-fourth of thetotal area of the world, contain about three-fourth of the world human population. Over-exploitation of resources and deforestationhave become rampant to fulfil the needs of largepopulation. As these tropical rain forestscontain 50 per cent of the species on the earth, Figure 16.2 : Red Panda — an endangered speciesdestruction of natural habitats have proveddisastrous for the entire biosphere. Natural calamities such as earthquakes,floods, volcanic eruptions, forest fires,droughts, etc. cause damage to the flora andfauna of the earth, bringing change thebiodiversity of respective affected regions.Pesticides and other pollutants such ashydrocarbons and toxic heavy metals destroythe weak and sensitive species. Species whichare not the natural inhabitants of the localhabitat but are introduced into the system, arecalled exotic species. There are manyexamples when a natural biotic community of Figure 16.3 : Zenkeria Sebastinei — a critically endangered grass in Agasthiyamalai peak (India)the ecosystem suffered extensive damagebecause of the introduction of exotic species. Vulnerable SpeciesDuring the last few decades, some animals liketigers, elephants, rhinoceros, crocodiles, minks This includes the species which are likely toand birds were hunted mercilessly by poachers be in danger of extinction in near future if thefor their horn, tusks, hides, etc. It has resulted factors threatening to their extinction the rendering of certain types of organisms Survival of these species is not assured as theiras endangered category. population has reduced greatly.
  • 139. 138 FUNDAMENTALS OF PHYSICAL GEOGRAPHYRare Species (i) Efforts should be made to preserve the species that are endangered.Population of these species is very small in the (ii) Prevention of extinction requires properworld; they are confined to limited areas or planning and management.thinly scattered over a wider area. (iii) Varieties of food crops, forage plants, timber trees, livestock, animals and their wild relatives should be preserved; (iv) Each country should identify habitats of wild relatives and ensure their protection. (v) Habitats where species feed, breed, rest and nurse their young should be safeguarded and protected. (vi) International trade in wild plants and animals be regulated. To protect, preserve and propagate the variety of species within natural boundaries, the Government of India passed the Wild Life (Protection) Act, 1972, under which nationalFigure 16.4 : Humbodtia decurrens Bedd — highly rare parks and sanctuaries were established and endemic tree of Southern Western Ghats (India) biosphere reserves declared. Details of these biosphere reserves are given in the book India:CONSERVATION OF BIODIVERSITY Physical Environment (NCERT, 2006).Biodiversity is important for human existence. There are some countries which areAll forms of life are so closely interlinked that situated in the tropical region; they possess adisturbance in one gives rise to imbalance in large number of the world’s species diversity.the others. If species of plants and animals They are called mega diversity centres. Therebecome endangered, they cause degradation are 12 such countries, namely Mexico,in the environment, which may threaten Columbia, Ecuador, Peru, Brazil, Zaire,human being’s own existence. Madagascar, China, India, Malaysia, There is an urgent need to educate people to Indonesia and Australia in which these centresadopt environment-friendly practices and are located (Figure 16.5). In order toreorient their activities in such a way that our concentrate resources on those areas that aredevelopment is harmonious with other life forms most vulnerable, the International Union forand is sustainable. There is an increasing the Conservation of Nature and Naturalconsciousness of the fact that such conservation Resources (IUCN) has identified certain areaswith sustainable use is possible only with the as biodiversity hotspots. Hotspots are definedinvolvement and cooperation of local according to their vegetation. Plants arecommunities and individuals. For this, the important because these determine thedevelopment of institutional structures at local primary productivity of an ecosystem. Most,levels is necessary. The critical problem is not but not all, of the hotspots rely on species-merely the conservation of species nor the habitat rich ecosystems for food, firewood, cropland,but the continuation of process of conservation. and income from timber. In Madagascar, for The Government of India along with 155 example, about 85 per cent of the plants andother nations have signed the Convention of animals are not only found nowhere else inBiodiversity at the Earth Summit held at Riode the world, but its people are also among theJaneiro, Brazil in June 1992. The world world’s poorest and rely on slash and burnconservation strategy has suggested the agriculture for subsistence farming. Otherfollowing steps for biodiversity conservation: hotspots in wealthy countries are facing
  • 140. BIODIVERSITY AND CONSERVATION 139 Figure 16.5 : Ecological ‘hotspots’ in the worlddifferent types of pressures. The islands of that are threatened by introduced species andHawaii have many unique plants and animals land development. EXERCISES 1. Multiple choice questions. (i) Conservation of biodiversity is important for : (a) Animals (c) Plants (b) Animals and plants (d) All organisms (ii) Threatened species are those which : (a) threaten others (b) Lion and tiger (c) are abundant in number (d) are suffering from the danger of extinction (iii) National parks and sanctuaries are established for the purpose of : (a) Recreation (c) Pets (b) Hunting (d) Conservation
  • 141. 140 FUNDAMENTALS OF PHYSICAL GEOGRAPHY (iv) Biodiversity is richer in : (a) Tropical Regions (c) Temperate Regions (b) Polar Regions (d) Oceans (v) In which one of the following countries, the ‘Earth Summit’ was held? (a) the UK (c) Brazil (b) Mexico (d) China 2. Answer the following questions in about 30 words. (i) What is biodiversity? (ii) What are the different levels of biodiversity? (iii) What do you understand by ‘hotspots’? (iv) Discuss briefly the importance of animals to human kind. (v) What do you understand by ‘exotic species’? 3. Answer the following questions in about 150 words. (i) What are the roles played by biodiversity in the shaping of nature? (ii) What are the major factors that are responsible for the loss of biodiversity? What steps are needed to prevent them? Project Work Collect the names of national parks, sanctuaries and biosphere reserves of the state where your school is located and show their location on the map of India.
  • 142. GLOSSARYAbiotic : Non-living thing. Usually refers to the physical and chemical componentsof an organism’s environment.Adiabatic Lapse Rate : The rate of change of temperature by an ascending ordescending airmass. If no other non-adiabatic processes (i.e. no heat enters orleaves the system) occur (like condensation, evaporation and radiation), expansioncauses the parcel of air to cool at a set rate of 0.98° per 100 m. The opposite occurswhen a parcel of air descends in the atmosphere. The air in a descending parcelbecomes compressed. Compression causes the temperature within the parcel toincrease at a rate of 0.98° per 100 m.Air Mass : A body of air whose temperature and humidity characteristics, acquiredin source region, remain relatively constant over a horizontal distance of hundredsto thousands of km. Air masses develop their climatic characteristics by remainingstationary over a source region for a number of days. Air masses are classifiedaccording to their temperature and humidity characteristics.Aphelion : It is the point in the Earth’s orbit when it is farthest from the sun (152.5million km). Aphelion occurs on the 3rd or 4th of July.Asthenosphere : Zone in the Earth’s mantle that exhibits plastic properties. Locatedbelow the lithosphere at between 100 and 200 km.Atmospheric Pressure : Weight of the atmosphere on a surface. At sea-level, theaverage atmospheric pressure is 1013.25 mb. Pressure is measured by a devicecalled a barometer.Aurora : Multicoloured lights that appear in the upper atmosphere (ionosphere)over the polar regions and visible from locations in the middle and high latitudes.Caused by the interaction of solar wind with oxygen and nitrogen gas in theatmosphere. Aurora in the Northern Hemisphere are called aurora borealis andaurora australis in the Southern Hemisphere.Bajada : Consecutive series of alluvial fans forming along the edge of a linear mountainrange. Surface of this feature undulates in a rolling fashion as one moves from thecentre of one alluvial fan to another. Normally occur in arid climates.Batholith : A large mass of subsurface intrusive igneous rock that has its originsfrom mantle magma.Big Bang : Theory about the origin of universe. It suggests that about 15 billionyears ago all of the matter and energy in the Universe was concentrated into anarea smaller than an atom. At this instant, matter, energy, space and time werenot existant. Then suddenly with a bang, the Universe began to expand at anincredible rate and matter, energy, space and time came into being. As the Universeexpanded, matter began to coalesce into gas clouds, and then stars and planets.Some scientists believe that this expansion is finite and will one day cease. Afterthis point in time, the Universe will begin to collapse until a Big Crunch occurs.Biodiversity : The diversity of different species (species diversity), genetic variabilityamong individuals within each species (genetic diversity), and variety of ecosystems(ecosystem diversity).Biomass : The weight of living tissues usually measured per unit area over a
  • 143. 142 FUNDAMENTALS OF PHYSICAL GEOGRAPHY particular time interval. Can include the dead parts of organisms like bark, hair, and nails. Biome : Largest recognisable assemblage of animals and plants on the Earth. The distribution of the biomes is controlled mainly by climate. Calcification : A dry environment soil-forming process that results in the accumulation of calcium carbonate in surface soil layers. Caldera Volcano : Explosive type of volcano that leaves a large circular depression. Some of these depressions can be as large as 40 km in diameter. These volcanoes form when wet granitic magma quickly rises to the surface of the Earth. Chlorofluorocarbons (CFCs) : Is an artificially created gas that has become concentrated in the Earth’s atmosphere. This very strong greenhouse gas is released from aerosol sprays, refrigerants, and the production of fumes. Cirrocumulus Clouds : Patchy white high altitude cloud composed of ice crystals. Found in an altitude range from 5,000 - 18,000 m. Cirrostratus Clouds : High altitude sheet like clouds composed of ice crystals. These thin clouds often cover the entire sky. Found in an altitude range from 5,000 - 18,000 m. Cold Front : A transition zone in the atmosphere where an advancing cold air mass displaces a warm air mass. Continental Crust : Granitic portion of the Earth’s crust that makes up the continents. Thickness of the continental crust varies between 20 - 75 km. See sial layer. Coriolis Force : An apparent force due to the Earth’s rotation. Causes moving objects to be deflected to the right in the Northern Hemisphere and to the left in the Southern hemisphere. Coriolis force does not exist on the equator. This force is responsible for the direction of flow in meteorological phenomena like mid-latitude cyclones, hurricanes, and anticyclones. Cumulus Cloud : Large clouds with relatively flat bases. These are found in an altitude range from 300 - 2,000 m. Cumulonimbus Cloud : A well developed vertical cloud that often has top shaped like an anvil. These clouds can extend in altitude from a few hundred m above the surface to more than 12,000 m. Desert Pavement : A veneer of coarse particles left on the ground after the erosion of finer particles by wind. Earthquake : A sudden motion or shaking in the Earth. The motion is caused by the quick release of slowly accumulated energy in the form of seismic waves. Earthquake Focus : Point of stress release in an earthquake (also known as hypocentre). Ebb Tide : Time during the tidal period when the water level in the sea is falling. Ecosystem : A system consisting of biotic and abiotic components. Both these groups are interrelated and interacting. El Nino : The name given to the occasional development of warm ocean surface waters along the coast of Ecuador and Peru. Recently this phenomenon has been used for forecasting of climatic conditions in different parts of the world. The El Nino normally occurs around Christmas and lasts usually for a few weeks to a few months.
  • 144. GLOSSARY 143 Epicentre : A place on the surface of the earth located at the shortest distance from the focus of the earthquake, the point at which the seismic energy gets released. Global Warming : Warming of the Earth’s average global temperature because of an increase in the concentration of greenhouse gases. Geomagnetism : A property of magnetically susceptible minerals to get aligned to the earth’s magnetic field during the period of rock formation. Geostrophic Wind : Horizontal wind in the upper atmosphere that moves parallel to isobars. Results from a balance between pressure gradient force and Coriolis force. Greenhouse Effect : The greenhouse effect causes the atmosphere to trap more heat energy at the Earth’s surface and within the atmosphere by absorbing and re-emitting longwave energy. Greenhouse Gases : Gases responsible for the greenhouse effect. These gases include: carbon dioxide (CO2); methane (CH4); nitrous oxide (N2O); chlorofluorocarbons (CFC); and tropospheric ozone (O3). Habitat : Location where a plant or animal lives. Hail : It is a type of precipitation received in the form of ice pellets or hail stones. The size of hailstones can be between 5 and 190 mm in diameter. Halocline : The dinstinct zone in the ocean below which the salinity increases sharply. Hydration : A form of chemical weathering that involves the rigid attachment of H+ and OH- ions to the atoms and molecules of a mineral. Hydrolysis : Chemical weathering process that involves the reaction between mineral ions and the ions of water (OH- and H+), and results in the decomposition of the rock surface by forming new compounds. Infiltration : A portion of the precipitation which reaches the earth surface seeps into the ground in the permeable strata. This process is known as infiltration. Insolation : Incoming solar radiation in short wave form. Inter Tropical Convergence Zone (ITCZ) : Zone of low atmospheric pressure and ascending air located at or near the equator. Rising air currents are due to global wind convergence and convection from thermal heating. Katabatic Wind : Any wind blowing down the slope of a mountain. Land Breeze : Local thermal circulation pattern found at the interface between land and water. In this circulation system, surface winds blow from land to water during the night. La Nina : Condition opposite of an El Nino. In a La Nina, the tropical Pacific trade winds become very strong and an abnormal accumulation of cold water occurs in the central and eastern Pacific Ocean. Latent Heat : It is the energy required to change a substance to a higher state of matter (solid > liquid > gas). This same energy is released from the substance when the change of state is reversed (gas > liquid > solid). Neap Tide : Tide that occurs every 14 - 15 days and coincides with the first and last quarter of the moon. This tide has a small tidal range because the gravitational forces of the moon and sun are perpendicular to each other.
  • 145. 144 FUNDAMENTALS OF PHYSICAL GEOGRAPHY Nimbostratus Clouds : Dark, gray low altitude cloud that produces continuous precipitation in the form of rain or snow. Found in an altitude range from the surface to 3,000 m. Occluded Front : A transition zone in the atmosphere where an advancing cold air mass sandwiches a warm air mass between another cold air mass pushing the warm air into the upper atmosphere. Ozone : Tri-atomic oxygen that exists in the earth’s atmosphere as a gas. Ozone is highest in concentration in the stratosphere (10-50 km above the earth’s surface) where it absorbs the sun’s ultraviolet radiation. Stratospheric ozone is produced naturally and helps to protect life from the harmful effects of solar ultraviolet radiation. Ozone Hole : It is a sharp seasonal decrease in stratospheric ozone concentration that occurs over Antarctica in the spring. First detected in the late 1970s, the ozone hole continues to appear as a result of complex chemical reaction in the atmosphere that involves CFCs. Palaeomagnetism : The alignment in terms of inclination from horizon acquired by magnetically susceptible minerals in the rock during the period of their formation. Photosynthesis : It is the chemical process where plants and some bacteria can capture and organically fix the energy of the sun. Plate Tectonics : Theory suggesting that the earth’s surface is composed of a number of oceanic and continental plates. Driven by convection currents in the mantle, these plates have the ability to slowly move across the earth’s plastic asthenosphere. Precipitation : Showering of the raindrops, snow or hailstones from the clouds onto the surface of the earth. Rainfall, snowfall, cloud burst and hailstones are forms of precipitation. Runoff : It is the flow of water over land through different channels. Solar Wind : Mass of ionised gas emitted to space by the sun. Plays a role in the formation of auroras. Subsurface flow : It is the movement of water below the surface of the earth. After infiltration, the subsurface water returns to the surface through seepage into the streams or eventually goes into the ocean. The subsurface water flow is influenced by land slope, rainfall, intensity of groundwater extraction, etc. Thermocline : Boundary in a body of water where the greatest vertical change in temperature occurs. This boundary is usually the transition zone between the layer of warm water near the surface that is mixed and the cold deep water layer.