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
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
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’ development.as 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).
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 labour.in 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
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
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 consumption.to 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
6Figure 1.1 : Geography and its relation with other subjects FUNDAMENTALS OF PHYSICAL GEOGRAPHY
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.
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.
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
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 beings.in 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
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.
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.
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
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 expands.solar 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
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.
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
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
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.
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
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: www.sci.edu/public.html and www.nasm.edu) 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?
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.
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
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.
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.
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 areas.be 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
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
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
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 batholiths.km. 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
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 sills.to 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.