Module 3 cape unit 1 geography.

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MODULE 3
6th form Geography
Prepared by Oral Johnson
Natural events
The natural events are the physical processes that shape the earth. They are naturally occurring
phenomena such as the eruptions of volcanoes, hurricanes and landslides. These are not new processes
but have been at work throughout the earth’s history. They are now becoming more hazardous because
they are affecting more people and more property. Technological developments are also creating
chemical leaks and nuclear accidents. Disasters are created when the impact of the hazard creates
widespread destruction and distress.
Natural events, hazards and disasters
The natural events are the physical processes that shape the earth. They are naturally occurring
phenomena such as the eruptions of volcanoes, hurricanes and landslides
A natural hazard is a natural event (volcanic eruption, hurricane) that has the potential to cause damage
or when they human life and property.
A natural hazard becomes a natural disaster when it affects human life and property ( i.e. there is a
significant number of fatalities and or overwhelming property damage) for example Hurricane Katrina
was a natural disaster because it killed 1826 people and cause significant amount of property damage
Hazards may be classified according to the causal process
Technological Hazards are those caused by human activity for example, collapse of construction
machinery and mines, acid rain and nuclear leaks. Other examples includes industrial pollution, nuclear
radiation, toxic wastes, dam failures, transport accidents, factory explosions, fires, and chemical spills

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For example the collapse of construction cranes in New York in 2008 was a technological hazard. Larger
disasters like the Chernobyl nuclear leak in Russia in 1986 and the chemical leak in Bhopal, India were
both technological hazards caused by human activity.
At least 107 people died and more than 230 people were injured when a crane collapsed in the Muslim
holy city of Mecca in Saudi Arabia in September 2015
Tianjin disaster (2015)
An explosion in a warehouse containing hazardous chemicals, were so powerful that few of the
recovered bodies have been identified. At least 112 people died in the blast and hundreds have been
hospitalised, 721 people injured
Natural hazards (result of physical process)
Climatic hazards
Climatic conditions, such as hurricanes, high temperatures and low rainfall can be hazardous to
human life and property. Example of climatic hazards includes: Blizzard, droughts, Hailstorms,
lighting, hurricanes, tornadoes, floods (coastal and river), heat waves and cold spells
Hurricane Ivan in 2004 created disastrous in the Caribbean. Heat waves in France resulted in
deaths of many elderly persons in 2007: while drought conditions have occurred in Cuba and
parts of Africa in successive years.
Hurricane Katrina was among the deadliest hurricane to have ever reached the United States.
Date: 23rd – 30th of August 2005)
Deaths: 1,836 people
Damage: $81.2 billion
Effects: Major flooding, many homeless, very little food and water supplies
Tectonic Hazard
A tectonic hazard can be defined as an event occurring due to movement or deformation of the
earth's crust with the potential to cause damage to property and loss of life. Examples include
earthquakes, volcanic hazards and tsunami.

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The Tsunami of Thailand 2006, the Chinese earthquake of 2008, the Haitian earthquake in 2010
and volcanic eruptions in Montserrat after 1996 have had a disastrous impacts on human life
and property. Hundreds of thousands of lives have been lost and there has been billions of
dollars worth of damage to buildings and agriculture.
Geomorphological hazards
Geomorphologic processes such as landslides, avalanches and flooding can also be hazardous.
The classic landslide of Alberta, Canada in 1903, involved an estimated 27 million cu m falling
900m and burying the town of Frank. River flooding takes many lives annually across the world,
For example, in Bangladesh in the delta of the Brahmaputra river.

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Some of the deadliest natural disasters in the world

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Flooding
Flooding occurs when more water remains on the land surface than can be contained in river
channels or removed by surface runoff.
Floods may be classified by the area which is flooded as a result of different processes.
Types of floods
• Riverine Flooding
Riverine floods are one of the most common types. These occur when the river channel cannot
contain the available water and overflows unto adjacent land. Most large rivers flood
periodically as a result of seasonal fluctuations in discharge. The Mississippi, Ganges, Nile and
Hwang-Ho rivers all flood periodically. In the Caribbean the Caroni River (Trinidad), Black River
(Jamaica), Essequibo River (Guyana) flood especially during the hurricane/rainy season
• Flash Floods
Flash floods are local floods of great volume and short duration. Extreme precipitation events
such as severe thunderstorms can deposit large amount of water on the surface in a short
period. The water picks up loose material on dry surfaces and moves rapidly downstream with
little warning. Because they come with little warning, flash floods are the most dangerous to
human lives. Dam failures can also cause flash flooding. Flash floods also occur in deserts and
urban areas.
The key elements in flash flooding are RAINFALL INTENSITY AND DURATION
• Estuarine flooding
An estuary is a partially enclosed coastal body of brackish water with one or more rivers or
streams flowing into it, and with a free connection to the open sea. Estuaries form a transition
zone between river environments and maritime environments. Adjacent low-lying areas are
easily flooded waves pushing water up the bay. As it is confined by the bay, the waves can
reach great heights and flood the land for example, River Severn and the Bay of Fundy, Canada.

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• Coastal flooding
Simply put a coastal flood is when the coast is flooded by the sea. Coastal flooding can be
caused by storm surges from hurricanes, tsunamis from earthquakes and from sea level rise.
Unusually high tides or storm waves can cause water to cover areas above high tide. For
example, storm surges can push water onto the land. A storm surge is a wide dome of water
pushing onshore near to the eye of the hurricane influenced by both the wind and the low
pressure. But the water draining from the land can also cause coastal flooding, for example, if
the water table is very high; or surface run-off is not being taken off quickly. In addition coasts
can be flooded more slowly by sea level changes, for example the Maldives
Causes of Flooding
Flooding occurs when a river’s discharge exceeds its channel’s volume causing the river to
overflow onto the area surrounding the channel known as the floodplain. The increase in
discharge can be triggered by several events
Causes of Flooding
1. Types of precipitation events
Prolonged Rainfall- The most common cause of flooding is prolonged rainfall. If it rains for a
long time, the ground will become saturated and the soil will no longer be able to store water
leading to increased surface runoff. Rainwater will enter the river much faster than it would if
the ground wasn’t saturated leading to higher discharge levels and floods.
Intense Storms- As well as prolonged rainfall, brief periods of heavy rain can also lead to floods.
If there’s a sudden “burst” of heavy rain, the rainwater won’t be able to infiltrate fast enough
and the water will instead enter the river via surface runoff. This leads to a sudden and large
increase in the river’s discharge which can result in a flash flood.

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Snowmelt-
Although many floods are triggered directly by precipitation just a few hours after it falls some
floods can be triggered by precipitation that fell many months ago. Precipitation that falls as
snow can remain as snow on the ground until it melts. This mightn’t be until the end of winter,
so potentially several months. When the snow does melt, large volumes of melt water will
enter the river increasing its discharge and triggering floods. These floods are often annual,
occurring every year when snow melts in the spring. In Bangladesh, for example, melting snow
in the Himalayas triggers annual floods in the summer.
2. Drainage Basin characteristics
The likelihood of a river bursting its banks and flooding is determined by factors in the
surrounding landscape, such as steepness of the river valley, the amount of vegetation and the
prevailing rock-type
- Size
If the basin is small it is likely that rainfall will reach the main channel more rapidly than in a
larger basin where the water has much further to travel. Lag time will therefore be shorter in a
smaller basin and the likelihood of flooding will increase.
- Shape
If the basin is circular in shape, the precipitation will enter the river at roughly the same time
because all points in the basin are equidistant from one another. This will produce a high peak
discharge and can lead to flash floods. The lag time is much longer in an elongated drainage
basin.
- Relief

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The relief and steepness of the basin affects how quickly water enters a river and so how likely
a river is to flood. If the river’s valley has steep sides water is likely to reach the river more
quickly than in gently sloping areas. The lag time will be shorter in a smaller basin and the
likelihood of flooding will increase.
- Drainage density
The number of tributaries flowing into a river affects the likelihood of floods. If a river has a lot
of tributaries, the river’s discharge will be much higher because lots of water will be entering it
from its tributaries. After heavy precipitation, the discharge will rise even more and floods are
likely, especially at confluences (where a tributary meets the river) as this is where discharge is
highest. Drainage density refers to the number of surface streams in a given area. The higher
the density the greater the probability of flooding.
- Soil Type
The soil type controls the rate and volume of infiltration. Sandy soils have very large pore
spaces which allows rapid infiltration and they do not encourage flooding. Conversely clays
have much smaller pore spaces, this reduces infiltration but encourages surface runoff and
increase the risk of flooding.
- Rock type ( geology)
The permeability of the rock in a drainage basin is a big factor in flooding. Permeable rocks for
example sandstone discourages surface run off but permits rapid infiltration. This will decrease
the chance of flooding. In contrast impermeable rocks such as granite will restrict infiltration
but encourages surface run off. The river discharge will increase and so is the likelihood of
flooding.
- Vegetation cover
The vegetation cover in a basin will affect flooding. If a basin has very dense vegetation cover,
the vegetation will intercept precipitation and store it, reducing the volume of water entering a
river. Conversely, if a basin is sparsely vegetated then there will be no interception and so more
water will enter a river.
3. Influence of human activity

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More people are now living in towns and cities. Concrete and tarmac, used for roads and
pavements as they are impermeable, precipitation cannot infiltrate so gets into the river much
more quickly. If a river’s drainage basin has been heavily urbanized, a river becomes much more
prone to flooding.
To stop roads and streets from flooding, humans will often build storm drains that collect
rainwater and channel it into a river or stream. The construction of storm drains allows the
rainwater to reach the river more quickly increasing its discharge and the likelihood of flooding.
Large scale deforestation is also taking place in many countries. As aforementioned a vegetated
slope will intercept precipitation reducing the volume of water entering a river. Humans are
now removing trees for different purposes. Deforestation reduces vegetation cover. The
intercepting effect is removed and as a result rapid surface runoff will take place increasing the
river discharge and the likelihood of flooding.
The increasingly frequent and serious flooding in Bangladesh is attributed to the removal of
trees in Nepal and other Himalayan areas.
4. Sea level changes
The boundary between ocean and continents has changed over geologic time. Depending on
the amount of water stored as ice relative to the amount in sea basins, the average sea level
can change. In colder geologic eras, sea level has been lower than present. Currently increased
temperatures of global warming and melting ice caps predict a rise in sea level. This would lead
to flooding of heavily populated coastal areas.
Many of the coastal cities (New York and Florida) across the world are now suffering from the
increase in sea level rise.

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Earth Structure
Structure of the Earth
Only the outermost few kilometres of the earth’s interior structure have been directly
investigated. The structure of its 6370km radius is mainly inferred from the passage of
earthquake waves through it. The crust is the thinnest layer of the Earth. The crust is usually
between 10km and 60km thick. The crust thickness is often referred to as the relative thickness
of an apple skin (when compared to the size of an apple). There are two types of crust, oceanic

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and continental. Continental crust is made up of silica (Si) and aluminium (Al) while oceanic
crust is made up of silica (Si) and Magnesium (Ma). Continental crust is called (SIAL) and
oceanic crust is called (SIMA). Oceanic crust is between 6 and 10 km thick. Continental crust
can be up to 70km thick.
The mantle is the thickest layer of the Earth at 2,900km thick. It makes up nearly 80% of the
volume of the Earth. The mantle itself is divided into 2 layers, the upper and lower mantles.
The mantle is often described as being semi-solid or molten. Here we have magma that flows
slowly due to the convection currents. The rocks in the upper mantle are cool and brittle
enough to break under stress. Rocks in the lower mantle are hot and soft and flow rather than
break. Differences in behaviour separate the upper from the lower mantle.
The upper most part of the mantle and the entire crust makes up the rigid lithosphere. Below
the lithosphere is a more mobile lower layer called the asthenosphere. The two are separated
by the mohorovicic discontinuity.
At the centre of the earth is the core. The outer core is made of liquid iron and nickel. Heat
from the core powers the convection currents in the mantle. The inner core is the hottest part
of the Earth reaching temperatures between 4,000-4,700°C, which are as hot as the surface of
the sun. It contains the centre of the earth which is about 6,378km from the surface. It is made
of solid iron and nickel that are under so much pressure they cannot melt.

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THEORY OF CONTINENTAL DRIFT
In 1912 Alfred Wegener a German Meteorologist proposed the theory of Continental Drift
Wegener hypothesized that about 300 million years ago all the continents were once joined
together in one large super continent called Pangaea. Pangea is a Greek term which means "All-
earth".
Pangaea started to break up into two smaller continents, Laurasia and Gondwanaland. The
northernmost continents made up Laurasia ( Europe, Asia and North America). The
southernmost continents made up Gondwanaland ( Australia, Antarctica, India, South America
and Africa). Since then, the continents have been moving to their current positions
Wenger collected evidence from several sciences to support his theory
1) Geographic fit of the continents
2) Biology ( identical fossils found on continents now separated by ocean)
3) Climatology (evidence of glaciations in areas whose distribution could not be explained by
current climatic conditions)

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4) Geology (similar rocks found on separate continents)
GEOGRAPHIC FIT OF THE CONTINENTS
Wenger believed that the coastlines/edges of the continents appeared to fit together like the
pieces of a jigsaw puzzle. Wegener believed that the coastlines of Eastern South America and
Western Africa fit perfectly together. According to Wegner the apparent fit of the continents
were not some mere coincidence but that they were once together and a part of a larger
landmass (Pangaea).

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BIOLOGICAL EVIDENCE
Wegener found evidence from ancient fossils. He found fossils of the same species of extinct
plants and animals in rocks of the same age on continents now widely separated by oceans.
Mesosaurus was an ancient reptile that lived during the early Permian (between 286 and 258
million years ago). It only lived in fresh water. Remains of Mesosaurus are found solely in South
America and Africa. These continents are now separated by the Atlantic Ocean. This reptile
only lived in fresh water and the entire Atlantic ocean is saline (salt water) . It was impossible
for Mesosaurus to swim across the entire Atlantic Ocean as they couldn’t survived in the salt
water.
While Meosaurus roamed the land Africa and South America were connected and they were
able to move freely across the landmasses and after going extinct the landmasses were torn
part carrying some of the fossils to South America and some to Africa. This suggested that
South America and Africa were once joined.
Mesosaurus

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Cynognathus and Lystrosaurus were reptiles that lived on land. Both of these animals were
unable to swim, let alone swim across wide seas.
Cynognathus was as large as a modern wolf and lived during the early Triassic period (250 to
240 million years ago). Remains of Cynognathus were found in SOUTH AMERICA and AFRICA .
As Aforementioned this reptile only lived on land and were unable to swim. Therefore It would
have been IMPOSSIBLE for Cynognathus to swim between the continents. Wegener again
proposed that the continents were together and the reptiles were able to roam freely across
the landmasses but that the lands had moved apart after they were dead and fossilized. So
fossil evidence are now in these continents. This again suggested that Africa and South America
were once joined.
Cynognathus
Fossil of mesosaurus

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Lystrosaurus was dominant on land in the early Triassic, 250 million years ago. It grew to
approximately one metre in length, with a stocky build like a pig.
Remains of Lystrosaurus were found only in ANTARTICA, INDIA and AFRICA. This reptile only
lived on land and like Cynognathus they were unable to swim. These three landmasses are now
separated by the Indian ocean and the Antarctic Ocean. It would have been impossible for
Lystrosaurus to swim across these large oceans. Wegener again proposed that the continents
were together and the reptiles were able to roam freely across the landmasses but that the
lands had moved apart after they were dead and fossilized. So fossil evidence are now in these
continents
Glossopteris was a fernlike plant that lived during the Early Permian (299 million years ago). The
discovery of fossils from the plant glossopteris show that its seeds were too heavy to be blown
by wind and would not be able to survive in salt water, therefore they would not be able to
cross oceans.
Fossils of glossopteris have been found on all five Gondwanaland continents (South America,
India, Africa, Australia and Antarctica). This supports the theory of continental drift as it would
have been impossible for this plant to get to these continents the way they currently are as it
could not cross oceans.

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DIAGRAM SHOWING COMBINED DISTRIBUTION OF FOSSILE EVIDENCE

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Glaciation/ Evidence of Glaciers
Wegener also looked at evidence from ancient glaciers. Glaciers are found in very cold climates
near the poles. Glaciers are giant rivers of ice that moves. When they move over rocks they
leave evidence in the form of scratches called glacial striations.
Glacial evidence (glacial striations) have been found in Africa, South America, India, Australia
and Antarctica. All of the continents above except Antarctica are currently located near the
equator in subtropical to tropical climates. These climate could not support the formation of
glaciers. The climate of South Africa or South America is too mild today for glaciers to form.
Wegener concluded that when Pangaea existed South America, South Africa, India, Antarctica
and Australia were closer to the south pole where the climate would have supported the
formation of glaciers and have since drifted apart to their current locations.
Glacial striations

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GEOLOGICAL EVIDENCE
Wegener discovered that identical rocks could be found on both sides of the Atlantic Ocean.
These rocks were the same type and the same age. Wegener understood that the rocks had
formed side-by-side and that the land has since moved apart.
When the geology of eastern South America and West Africa was mapped it revealed that
ancient rock outcrops (cratons) or crystalline basement rocks over 2,000 million years old were
continuous from one continent to the other

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Wegener also matched up mountain ranges that had the same rock types, structures, and ages,
but that are now on opposite sides of the Atlantic Ocean. The Appalachians of the eastern
United States and for example, are just like mountain ranges in eastern Greenland, Ireland,
Great Britain, and Norway.
Appalachian Mountains and their equivalent age mountain ranges of Great Britain are currently
separated by the Atlantic Ocean, they form an essentially continuous mountain range when the
continents are positioned next to each other.
Wegener concluded that they formed as a single mountain range that was separated as the
continents drifted.

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Wegener, however, could not explain how continents could move, that is the
mechanism/energy which could fuel these movements. His theory was rejected as ridiculous.
Now that plate tectonics has provided the mechanism of convection currents in the molten
magma of the mantle, it is widely accepted that continents move and in fact, these movements
are measured and widely predicted

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THEORY OF PLATE TECTONICS
The lithosphere (the earth’s crust and the rigid upper part of the mantle) is broken up into
sections called plates. A crustal plate is therefore a large rigid portion of the earth’s lithosphere.
The plates which are rigid, float like rafts on the underlying semi-molten mantle (the
asthenosphere) and are moved by convection currents.
There are two types of plates: continental and oceanic. However, these terms do not refer to
actual continents and oceans but to different types of crust or rock. Oceanic crust is denser
than continental crust. Most of the plates consists of both oceanic and continental lithosphere.
The large pacific plate is almost entirely oceanic.
There are seven major plates (African, Eurasian, North American, South American, Pacific, Indo-
Australian and Antarctica).
There are several smaller or minor plates (Nazca, Cocos, Caribbean, Scotia, Arabian, Philippine
and Juan de Fuca)
The plates meet at different types of boundaries or margins

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WHY THE PLATES MOVE
Magma (semi-molten rock) near the outer core is heated.
As the magma warms it expands and becomes less dense.
The less dense magma then starts to rise towards the crust
As the magma nears the crust it begins to cool.
The cooling magma becomes denser and begins to sink
The rising and falling magma creates circular currents with the mantle
these currents create friction with the crust above and causes it to move.
The process is known as convection currents
Where the movement is upwards plates are forced apart and new crust is formed. Where the
movement is downward plates are brought together and plate material may be destroyed. Plate
movement is usually no more than a few centimetres a year

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EVIDENCE FOR PLATE TECTONICS
Since Wegener’s theory of continental drift was rejected, advances in technology and science
have accumulated evidence to support plate tectonics
 Global Positioning System (GPS) is now used to measure and record movements of
plates and movements along faults.
 The youthfulness of ocean basins that are continuously being formed and destroyed
 Earthquake epicentres outline the edges of tectonic plates
 More recent discoveries of areas of plate destruction
The discovery and study of the Mid-Atlantic Ridge
While investigating islands in the Atlantic in 1948, Maurice Ewing noted the presence of a
continuous mountain range extending the whole length of the ocean bed. This mountain range,
named the Mid-Atlantic Ridge, is about 1000km wide and rises to 2500m in height. Ewing also

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noted that the rocks of this range were volcanic and recent in origin-not ancient as previously
assumed was the case in mid-oceans.
Sea floor spreading
In 1962, Harry Hess studied the age of rocks from the middle of the Atlantic outwards to the
coast of North America. He confirmed that the newest rocks were in the centre of the ocean,
and that the oldest rocks were those nearest to the USA and the Caribbean. He also suggested
that the Atlantic could be widening by up to 5cm a year.
Studies of palaeomagnetism in the 1950s
During underwater volcanic eruptions basaltic lava is intruded into the crust and cools. During
the cooling processes, individual minerals, especially iron, align themselves along the earth’s
magnetic field, i.e. in the direction of the magnetic pole. Recent refinements in dating
techniques enable the time at which rocks were formed to be accurately calculated. It was
known before the 1950s that the Earth’s magnetic pole varied a little from year to year, but
only then was it discovered that the magnetic field reverses periodically, i.e. the magnetic pole
is in the south for a period of time and then in the north for a further period of time and then in
the north for a further period, and so on. It is claimed that there have been 171 reversal over 76
million years. If formed when the magnetic pole was in the north, new basalt would be aligned
to the north. After a reversal in the magnetic poles, newer lava would be oriented to the south.
After a further reversal the alignment would again be to the north. Subsequent investigations
have shown that these alterations in alignment are almost symmetrical in rocks on either side
of the Mid-Atlantic Ridge.

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As a result of the convection cells generated by heat from the center of the earth, plates may
move towards, away from or sideways along adjacent plates. It is at plate boundaries most of
the world’s major landforms occur, and where earthquake, volcanic and mountain building
zones are located. However before trying to account for the formation of these landforms
several points should be noted.
1) Due to its relatively low density, continental crust does not sink and so is permanent;
being denser oceanic crust can sink. Oceanic crust is being formed and destroyed
continuously.
2) Continental plates, such as Eurasian plate, may consist of both continental and oceanic
crust.
3) Plates cannot overlap. This means that either they must be pushed upwards on impact
to form mountains or one plate must be forced downwards into the mantle and
destroyed.
4) No ‘gaps’ may occur on the earth’s surface so, if two plates are moving apart, new
oceanic crust originating from the mantle must be formed.
5) The earth is neither expanding nor increasing in size. Thus when new oceanic crust is
being formed in one place, older oceanic crust must be being destroyed in another.

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6) Plate movement is slow and usually continuous. Sudden movements are detected as
earthquakes.
7) Most significant landforms (fold mountains, volcanoes, island arc, deep sea trenches
and batholith intrusions are found at plate boundaries. Very little change occurs in plate
centers.
Types of plate boundaries
Transform plate boundaries
Transform plate boundaries are also called conservative plate boundaries. Conservative
margins occur where two plates move parallel to each other. As the plates move past one
another they don't do so smoothly, rather, they snag and grind. Sometimes the plate ‘stick or
lock together’ allowing energy pressure to build up. When the plates move again this built up
energy/pressure is released as earthquakes. The margins between the plates is said to be
conservative because crustal rocks are being neither created nor destroyed. The boundary
between the two plates are characterised by pronounced transform faults. The San Andres fault
is the most notorious of several hundred known transform faults in California.

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There is a transform plate boundary in the Caribbean as the North American and Caribbean
plates slides or grind past each other. The Haiti earthquake in 2010 was a result of the
movement between these two plates.

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Transform plate
boundary in the
Caribbean. Caribbean
plate moving in an
easterly direction and
north American plate
moving in a westerly
direction

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Convergent/ destructive plate boundaries
Convergent margins mark areas where plates are coming together and crust is destroyed. The
process and landforms produced at convergent margins vary according to whether the
converging plates are carrying continents or oceans. Convergence can take place between a
continental and oceanic plate, two oceanic plates and two continental plates. The collision
results in either subduction or upheaval
Oceanic- continental convergence
This is where an oceanic and continental plate collides. Being the denser of the two, the oceanic
plate plunges into the mantle to form a subduction zone with its associated deep sea trench.
As the oceanic plate descends, the increase in pressure can trigger off major earthquakes (
wadati-benioff zone) . The heat produced by friction helps to convert the disappearing crust
back into magma. Being less dense than the mantle, the newly formed magma will try to rise
through the continental crust. Where the magma does reach the surface volcanoes will occur.
These volcanoes are likely to form either a long chain of fold mountains e.g. the Andes.
Estimates claim that 80% of the world’s present active volcanoes are located above subduction
zones. As the rising magma t destructive margin is more acidic than the lava of constructive
margins, it is more viscous and flows less easily. It may solidify within the mountain mass to
form large intrusive features called batholiths.

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The Pacific Ocean, which extends over five oceanic plates. The pacific plate, the largest of the
oceanic plates, and the Philippines plate move north-west to collide with eastern Asia. In
contrast, the smaller Nazca, Cocos and Juan de Fuca plates travel eastwards towards South
America, Central America and North America respectively.
Below will describe what happen as the Nazca and South America plate collides
The smaller Nazca plate is oceanic while the South American plate is continental. When the
Nazca plate collides with the South American plate. Being denser the Nazca plate subducts into
the mantle forming a subduction zone. A deep sea trench known as the Peru- Chile trench is
also formed as the oceanic plates subduct into the mantle. Earthquakes often happen as
pressure is built up as the plate subducts. As the Nazca plate subducts heat produce by friction
causes the plate to melt into magma. The newly formed magma is less dense and rises through
the South American plate to form a long chain of volcanoes (The Andes). The highest active

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volcano in the world, Ojos del Salado, which stands at 6, 900m, is found here. Cotopaxi and
Acongua are also located in the Andes.

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Oceanic Oceanic Convergence
When oceanic plates meet, one subducts and is assimilated into the mantle. The older, heavier
plate which is denser, will plunge beneath the younger. Older rigid plates will also subduct at a
sharper angle than younger. . As the oceanic plate descends, the increase in pressure can
trigger off major earthquakes ( wadati-benioff zone) . The heat produced by friction helps to
convert the disappearing crust back into magma. Being less dense than the mantle, the newly
formed magma will try to rise through the OCEANIC CRUST. This magma then rises to the
surface produce a chain of volcanic islands or an island arc e.g. the Eastern Caribbean islands.
A deep sea ocean trench is also form. Trenches are the deepest part of the ocean floor.
Trenches generally run parallel to volcanic island arcs. The Mariana Islands, an archipelago of 15
volcanic islands, lie to the west of the Marianas trench. Marianas trench marks the area where
the pacific plates plunges beneath Philippines plate. The challenger deep at the southern end of
this trench is 11,000m deep.
As the plate subducts, the overriding plate scrape sediments as well as projecting portions of
ocean floor off the upper crust of the lower plate. This creates a zone of deformed rocks that
attach itself to the overriding plate in a process called obduction. This zone is known as
accretionary prism or wedge. Barbados to the east of the Lesser Antilles is an emergent part of
the accretianory prism.

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The Eastern Caribbean Islands

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The South American Plate is moving westwards due to sea floor spreading at the Mid Atlantic
Ridge. Where it meets the Caribbean Plate, it descends (subducts) beneath it. This is because
the oceanic lithosphere of the South American Plate is denser than that of the Caribbean Plate.
The subduction causes low density ocean floor sediment to be scraped off the surface of the
South American Plate and thrust onto the Caribbean Islands as accretionary wedges, in a
process called obduction. Barbados to the east of the Lesser Antilles is an emergent part of the
accretianory prism. The line of subduction is marked by the deep sea Puerto Rico Trench
As the South American Plate descends, it drags against the overlying plate, causing both to
fracture and deform. This results in frequent shallow focus earthquakes that get deeper as the
ocean plate descends further, defining a zone of earthquake foci known as a Benioff zone. As
the South American plate continues to descend into the mantle, it starts to melt. The newly
formed magma is less dense and rises through the oceanic crust to form the Eastern Caribbean
Volcanic Island Arc. The Caribbean volcanic islands form a curved linear chain or ‘volcanic island
arc’ parallel and to the west of the Puerto Rico Trench.

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Continent – continent convergence
When two plates converge there is no subduction as both plates are of the same density.
Instead the rocks crumple and buckle. The plates push into each other creating crustal
thickening, folding and overriding some of the most complex structures in the world. The
continental crust is around 75 km thick. The collision of the Indian and Eurasian plates have
thrown up the Himalayas and the Tibetan plateau. The Himalayas now rise to 8,854m. The
Indian plate is moving northwards at a rate of about 5cm annually producing earthquakes that
affect several countries in the region such as India, Pakistan and China. The collision of the
Africa and European plates formed the folded Alps of Southern Europe.

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Divergent plate boundaries
Constructive plate margins occur where two plates diverge, or move away from each other and
new crust is created at the boundary. This process, known as seafloor spreading, occurs in the
Mid-Atlantic where the North and South American plates are being pulled apart from the
Eurasian and African plates. As the plates diverge, molten rock or magma from the mantle to fill
any possible gaps between them and, in doing so, creates new oceanic crust. The magma
initially forms submarine volcanoes which may in time grow above sea level, e.g. Surtsey and
Iceland on the Mid Atlantic ridge. Eastern Island on the each pacific rise is another example. The
Atlantic Ocean did not exist some 150 million years ago and is still widening 2-5cm annually.

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The largest visible product of constructive divergent plates is Iceland where one-third of the
lava emitted onto the Earth’s surface in the last 500 years can be found.
Another divergent boundary has developed on the African continental plate mark by the
African Rift Valley. Stretching of the earth’s crust under forces of tension causes it to crack
producing a dramatic area of steep escarpment and valleys. In Africa the rift valley extends for
4000km from Mozambique to the red sea. In places its sides are over 600m in height while its
width varies between 10 and 50km. The western rift valley contains the world’s second
deepest lake, Lake Tanganyika. Mount Kilimanjaro, Africa’s highest mountain is found in the
eastern rift valley. Ultimately if the spreading continues the continent would be broken into
two parts and a new ocean created.

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Earthquakes
Earthquakes can be created by undersea landslides, volcanic eruptions and the explosion of
bombs. Most are explained on the basis of plate tectonics. They are caused by movements
along a fault or fracture in the earth.
Plates move away, towards and past each other along fractures or faults. Where the plates
move apart there is tension rupturing rocks and producing small earthquakes. Where the plates
slide pass each other, the rough edges lock, pressure builds up and energy is released when the
stresses are overcome.
Subduction and continental collision release large amounts of concentrated energy which
results in some of the world’s largest earthquake. The rupture produces shock waves which are
felt as earthquakes .The point below at the surface where the pressure is released is known as
the focus or the point where the wave originates. It is also called the hypocenter. The point
directly above the focus on the surface is the epicenter .The epicenter usually experiences the
greatest shock or seismic waves with decreasing intensity in concentric circles away from it.

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When the epicenter of an earthquake is undersea, the energy passes through the water
creating fast-moving waves travelling over great distances called tsunamis. In Thailand 2004,
there was displacement along thousand of kilometers of an underwater plate margin, the great
release of energy created tsunamis reaching right across the Indian Ocean to Africa hours later,
Tsunamis are extremely hazardous to human life and property in densely settled coastal areas.
Seismic waves
Earthquakes consist of waves of different speeds and lengths created by energy released at
points in or on the earth’s surface . Seismic waves are recorded on very sensitive instruments
called seismographs. The energy released in seismic waves may either pass through the entire
body of the earth or along the surface only. These are body and surface waves

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Seismograph
Body waves are faster and are subdivided into:
P waves
The first kind of body wave is the P wave or primary wave. This is the fastest seismic wave and
the first to detect on a seismomgraph. They have a push and pull motion ( compression and
move through both solid and liquid. They move the ground back and forth along the direction
of wave travel or It pushes and pulls the rock it moves through. P waves are also called
longitudinal, compressional or push-pull wave

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P wave on a slinky, back and forth movement
P waves shake the ground back and forth in the direction that the wave is moving.
S waves
S waves, or secondary waves, are the waves directly following the P waves. They travel about
1.7 times slower than P waves. S waves will not travel through liquids like water, molten rock,
or the Earth's outer core. S waves ONLY travel through SOLID or rock only S. S waves are also

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called TRANSVERSE waves. They are transverse waves because they vibrate the ground in a
direction that is at right angles/perpendicular to the direction of movement.
Vibrations are perpendicular to the direction the wave is travelling
Up and down motion (perpendicular)
Surface waves
Love wave
The first kind of surface wave is called a Love wave, named after A.E.H. Love in 1911. It's the
fastest surface wave and moves the ground from side-to-side. They are also transverse waves

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causing movement from side to side on a horizontal plane. Love waves produce entirely
horizontal motion. They do not move through air or water.
Perpendicular (up and down motion) but in a horizontal manner
Rayleigh Wave
Rayleigh waves have an elliptical, counter clockwise motion and are very similar to water
waves. It rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls,
it moves the ground up and down, and side-to-side in the same direction that the wave is
moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be
much larger than the other waves. They are Slowest of all waves.

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These surface waves are responsible for the most damage. Each creates different stresses and
hazards as they pass through the built environment. The main shock of an earthquake may be
preceded and followed by smaller shocks. There are foreshocks and aftershocks, and
aftershocks may continue for years
Arrival of p, s wave and surface wave on a typical seisomgraph. P wave first, S wave second and
Surface Waves ( last)
Factors that influence the amount of damage caused by an earthquake
Location/population density: An earthquake that hits in a populated area is more likely to do
damage than one that hits an unpopulated area. Some of the most dangerous earthquakes
occurred in densely populated areas and this explains why there are often so much casualties
and infrastructural damage.
Magnitude: Scientists assign a number to represent the amount of seismic energy released by

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an earthquake. The Richter magnitude scale, as it is known, is logarithmic, so each step up
represents an increase in energy of a factor of 10. The more energy in an earthquake, the more
destructive it can be. The higher the magnitude of an earthquake the more damage it is likely to
cause. A magnitude 9 earthquake will cause more damage than a magnitude 7
Depth: Earthquakes can happen anywhere from at the surface to 700 kilometres below. In
general, deeper earthquakes are less damaging because their energy dissipates before it
reaches the surface. These foci of natural earthquakes are found at different depths below the
ground surface. There are three levels, Shallow at 0-70 km below, Intermediate 70-300 km and
Deep foci at 300 km or below. Shallow earthquakes create the most damage and 3/4 of total
energy released of earthquakes in the world.
Distance from the epicentre: The epicentre is the point at the surface right above where the
earthquake originates and is usually the place where the earthquake's intensity is the
greatest. The damage of an earthquake decreases with increasing distance from the epicentre.
A village that is 200km from the epicentre will receive less damage than one that is at the
epicentre
Measuring earthquakes
There are two different scales for classifying earthquakes: The Richter Scale and the Mercalli
scale.
The Richter scale is used to measure the magnitude of an earthquake or the amount energy
released by the earthquake. It was developed by Charles Richter in 1934. It is a logarithmic
scale from 0-9 where a magnitude of 5 is ten times greater than a magnitude of 4 earthquake. It
increases 10 times with each arithmetic increase. Again a magnitude 7 earthquake would be
100 times greater than a magnitude 5. It is recently evolved into an open ended scale because
we have earthquakes exceeding 9.

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The strongest ever recorded earthquake was a level 9.5 off the coast of Chile in 1960.

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The Mercalli scale from I to XII
The Mercalli scale measures the damage caused by an earthquake. It rates each earthquake
from I to XII (1 to 12) depending on how much damage is done. The higher up the scale the
more damage is experienced by people and building structures. The Mercalli scale is considered
less precise than the Richter scale, as it depends on factors such as perception of the observer
and age/structure of the buildings.
There is a relationship between the two scales in that the greater the magnitude and energy
released, the greater is the likelihood of destruction.

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Faulting
As plates move the rocks fracture or crack. As stress builds up in the rocks on both sides of the
fracture movement take place and a crack is transformed into fault.
Geologists describe fault plane with two measurements- the strike and the dip. Striker refers to
the direction of the fault on the earth’s surface. The dip measures the direction and the angle
of the fault plane. The direction of the dip is perpendicular to the strike.
A fault is a break in earth’s crust where broken blocks of rock move with respect to one
another.
The two sides of a fault are the hanging wall and the footwall. When the rocks on both sides of
the fault change their vertical position a dip slip fault is formed. When the rocks move
horizontally, a strike slip fault is formed.

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Normal faults and Reverse faults are both dip slip faults since they both change their vertical
positions .There are NO hanging and foot walls in strike slip faults such as the San Andreas
Fault.
Normal Fault
Normal Faults are created by force of tension and are typical of faults at spreading centres.
Movement takes place along fault plane. In a normal fault the hanging wall moves down in
relation to the footwall
The resulting steep-sided ledge or cliff created at the top of the footwall block is called a fault
scarp or escarpment

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Normal faults are not often found singly in a landscape; typically, they occur in multiple
arrangements, often in pairs of parallel faults.
When movement takes place between parallel normal faults whose fault planes are angled
downward toward each other a crustal block may drop down between them. This down-
dropped block, which forms a valley between the opposing footwall blocks, is called a graben.
The uplifted blocks are called horsts. In the US interior west, the Basin and Range province is
an example of aligned pairs of normal faults and a distinctive horst and graben landscape.

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Reverse fault
Reverse faulting occurs are formed from compressional forces. In reverse faulting the Hanging
wall moves up in relation to the footwall. There is reverse faulting at convergence zones, where
subduction or collision occurs. A fault scarp is created, but it takes the form of an overhanging
ledge.
A reverse fault is called a thrust fault if the dip of the fault plane is small less than 45 degrees

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Strike Slip fault
Strike slip faults are faults where the relative motion on the fault has taken place along a
horizontal direction. Such faults result from shear stresses acting in the crust. Strike slip faults
can be of two varieties. Strike-slip faults are classified according to the direction of motion of
the blocks on either side of the fault.
They may be right lateral or left lateral. To an observer standing on one side of the fault and
looking across the fault, if the block on the other side has moved to the left we say that the
fault is a left-lateral strike slip fault or sinistral.
If the block on the other side has moved to the right we say that the fault is a right-lateral
strike slip fault or dextral. The famous San-Andreas fault in California is an example of a right
lateral strike slip fault that stretches over 1200 km.

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No matter which block your standing on the direction the opposing block is moving will be the
same
Volcanoes
Vulcanicity refers to all the landforms derived from magma cooling within or on the earth’s
crust. Volcanic activity is associated with most plate margins although specific form is related to
the type of vent and lava. When magma cools and solidifies within the crust it forms intrusive
features such as batholiths and dykes. When lava is poured out on to the earth’s surface it form
extrusive volcanic features such as lava plateau, shield and composite volcanoes.
Distribution of volcanic activity
Volcanic activity can be traced around the world to areas of tectonic activity such as plate
margins. They can occur at convergent (excluding the collision all boundary) plate boundaries,

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divergent plate boundaries or hotspots. At divergent plate boundary when two plates separate
magma is forced up from the mantle to form under water volcanic mountain ranges such as
Mid Atlantic ridge and islands such volcanic islands such as Iceland.
At convergent plate boundaries - where two plates collide into each other volcanoes are also
formed. Volcanoes form in oceanic - continental and oceanic- oceanic convergent plate
boundaries. Volcanoes are not formed at a continental continental collision. The oceanic plate
subducts beneath the continental plate, as it moves its way through mantle the crust melts to
form a new magma. The newly formed magma is less dense (lighter) than the mantle. It is then
force to rise. When the new magma reaches the surface it forms a volcano. So volcanoes
coincides with convergent plate boundary and divergent plate boundaries. Volcanoes also form
over ‘hot spots’ such as those forming the Hawaiian Islands.
The ring of fire of the Pacific refers to the encircling or circum-pacific volcanic activity at all
margins of this large ocean. Volcanoes stretch from Aconcagua and Cotopaxi in South America;
Popocatepeti and Mt St Helens in North America; through the Aleutians down the western
pacific in the Kuriles, Japan, the Philippines, Fiji and New Zealand. Other areas of active
vulcanicity include the Eastern Caribbean, East Africa, Indonesia and the Mediterranean
Europe. Volcanoes are also found in Hawaii.

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Intra plate Volcanism / Hotspots

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Some volcanoes do not occur at plate boundaries. These volcanoes are formed in the middle (interior) of
plates over ‘hotspots’
These are fixed points in the mantle that generate intense heat (in a mantle plume). Small, long lasting,
exceptionally hot areas of magma exist under the Earth’s surface which in turn sustains long-lasting
volcanic activity.
The hotspot is STATIONARY ‘ does not move’ As the crustal plate moves over the stationary hotspot,
new volcanoes are formed. Hotspots are associated with chains of islands. Examples are the Samoa
Islands and Hawaii. As the plate moves the volcano will progressively become dormant and extinct and
the volcano will be eroded by the sea. This is because as the volcano move off the hotspot they lose
their source of magma
The Hawaiian Islands were formed by such a hot spot occurring in the middle of the Pacific Plate. While
the hot spot itself is fixed or remain stationary, the pacific plate is moving in a north westerly direction.
So, as the plate moved over the hot spot, the string of islands that make up the Hawaiian Island chain
were formed.The island of Hawaii is now over the hotspot. The volcanoes are often very wide, with
gently sloping sides comprising many thin (1 to 5 metres thick) basaltic lava flows. These are referred to
as 'shield volcanoes'. Kilauea and Mauna Loa on Big Island are currently active examples.

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Characteristics of volcanoes
Types of Magma
There are three main types of magma- basalt, andesite and rhyolite.
They behave differently because their compositions are different. They have different
temperatures, water content and viscosities

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Viscosity of Magmas
• Viscosity is the resistance to flow (thickness). The lower the viscosity the more fluid the
liquid. Water has low viscosity. Viscosity depends primarily on the composition of the
magma, and temperature.
• Higher SiO2 (silica) content magmas have higher viscosity than lower SiO2 content
magmas (viscosity increases with increasing SiO2 concentration in the magma).
• Lower temperature magmas have higher viscosity than higher temperature magmas
(viscosity decreases with increasing temperature of the magma). Therefore basalt have
the highest temperature and lowest viscosity and the easiest flow. Andesite occupies an
intermediate position, rhyolite have the lowest temperatures and the highest viscosity
• Thus, basaltic magmas tend to be fairly fluid (low viscosity). Andesite have a higher
viscosity than basaltic magma, Rhyolitic magmas tend to have even higher viscosity. The

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higher the viscosity the more powerful the eruption. Basaltic magmas are non explosive
while rhyolite is explosive.
• At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but the gas
forms a separate vapor phase when pressure is decreased as magma rises toward the
surface of the Earth
• This is similar to carbonated beverages which are bottled at high pressure. The high
pressure keeps the gas in solution in the liquid, but when pressure is decreased, like
when you open the can or bottle, the gas comes out of solution and forms a separate
gas phase that you see as bubbles
• Gas gives magmas their explosive character, because volume of gas expands as pressure
is reduced
• The composition of the gases in magma are:
Mostly H2O (water vapor) & some CO2 (carbon dioxide)
Minor amounts of Sulfur, Chlorine, and Fluorine gases
• The amount of gas in a magma is also related to the chemical composition of the
magma. Rhyolitic magmas usually have higher gas contents than basaltic magmas.
Gases in Magma
• In general, magmas that are generated deep within the Earth begin to rise because they
are less dense than the surrounding solid rocks.
• As they rise they may encounter a depth or pressure where the dissolved gas no longer
can be held in solution in the magma, and the gas begins to form a separate phase (i.e. it
makes bubbles just like in a bottle of carbonated beverage when the pressure is
reduced).
• When a gas bubble forms, it will also continue to grow in size as pressure is reduced and
more of the gas comes out of solution. In other words, the gas bubbles begin to expand

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• If the magma has low viscosity like basalt, then the gas can expand relatively
easily. When the magma reaches the Earth's surface, the gas bubble will simply burst,
the gas will easily expand to and a non-explosive eruption will occur, usually as a lava
flow
• If the magma has a high viscosity, then the gas will not be able to expand very easily,
and thus, pressure will build up inside of the gas bubble(s). When this magma reaches
the surface, the gas bubbles will have a high pressure inside, which will cause them to
burst explosively on reaching atmospheric pressure. This will cause an explosive
volcanic eruption.
Types of eruptions
There are predominantly two types of eruption: Effusive eruptions/ non explosive or Explosive
eruptions.
Effusive eruptions
Effusive eruptions are the relatively gentle ones that produce enormous volumes of lava
annually on the sea floor and in places such as Hawaii and Iceland. These direct eruptions from
the asthenosphere produce a low viscosity magma that is very fluid and cools to form, a dark
basaltic rock. Gases readily escape from this magma because of its low viscosity, causing a very
gentle effusive eruption that pours out on to the surface, with relatively small explosions and
little pyroclastics. Effusive eruptions may come from a single vent or linear opening called
fissures. Effusive eruptions are typically located at constructive plate boundaries and hotspots.
Because the magma is of low viscosity (thin and runny) the lava will travel very far before it
cools and solidifies. When this happens around a central vent the result landforms have a
distinctive shape of gently sloping sides. If the magma is emitted from linear cracks or fissures
the lava spreads on the surrounding landscape to form lava plateaus.

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Explosive eruptions
Volcanic activity from subduction zones produces well known explosive volcanoes. Magma
produced by the melting subducted oceanic plate and other materials is thicker (more viscous)
than magma from effusive volcanoes; it is 50-75% silica and high in aluminium. Consequently, it
tends to block the magma conduit inside the volcano; the blockage traps and compressed
gases, causing pressure to build and creating conditions for a possible explosive eruption.
Unlike the volcanoes in Hawaii, where tourists gather to watch the relatively calm effusive
eruptions, these explosive eruptions do not invite close inspection and can explode with little
warning. Because the magma is of high viscosity the lava doesn’t travel far and form steep
volcanic landforms.

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Intrusive volcanic features
Only a small amount of the magma that moves up from the mantle and through the crust
reaches the surface. Most magma cooks and solidifies ( hardens ) before it Rwanda the surface .
As the magma moves upwards it forces its way into lines of weakness in the rock. Bedding
planes, joints and faults are all lines of weakness followed by the magma. Once magma gets
into the crack in the crust the huge force behind it can cause the crack to widen. Although
intrusive volcano features are fomented underground, the maybe be exposed million of years
later the rocks at the surface are eroded. Because volcanic rocks are hard they are often more
resistant to erosion than the rocks around them and they stand out in the landscape as higher
ground.
A dyke is formed when magma moving upwards towards the surface cooks and solidifies. The
magma cuts across the bedding planes of sedimentary rock. Sometimes a large number of
dykes called a dyke swarm can occur together in an area. An example is the moule a chique
headland found at the southern tip of St Lucia. The material which forms the dyke cooks slowly
although those parts that come in contact with the surrounding rock will cool more rapidly to
produce a chilled margin. Most of the dykes are more resistant to erosion that the surrounding
sandstones and so when they cross the islands beaches they stand up like groynes. Although
averaging 3m, these dykes vary from 1 to 15m in width
A sill is formed when the igneous rock is intruded along the bedding planes between the
existing sedimentary rocks. The magma cool and contracts by this time the resultant joints will
be vertical and their hexagonal shapes can be seen when the Landform is later exposed on
headlands such as that at Drumadoon on the west coast of Arran. The sill here is 50m thick
Laccolith is the result of large amounts of magma moving between bedding planes and causing
overlying rock strata to arch upwards.
Batholith is much larger than the other intrusive volcanic features. It forms when a giant
underground reservoir of magma cools and hardens. Batholiths can be several hundred
kilometers in diameter. A batholith may form the root of a mountain. Perhaps the best known

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example in the Caribbean is the Tobago batholiths. It crosses the whole island from west to east
and is about half the size of the island.
Plug is a vertical column of volcanic rock which is formed in the vent of a volcano when the
magma present and cools.

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Extrusive volcanic Features
When lava is poured out onto the surface of the earth it forms extrusive volcanic features.
Some of these features are cinder cones, composite cones, shield volcanoes, caldera and
volcanic dome.
Cinder cones are the simplest types of volcano. A cinder cone is a small cone-shaped hill
usually less than 450m (1500 ft) high, formed from cinders that accumulate during moderately
explosive eruptions. They are built from pieces of lava and tephra that have been ejected from
a single volcanic vent. As the moderately explosive lava is blown into the air, it breaks into small
fragments that solidify and fall as cinders around the vent to form a cone. Most cinder cones
have a bowl-shaped crater at the summit. One of the most famous cinder cones, Paricutin,
grew in the middle of a cornfield in Mexico in 1943.
Paracutin in Mexico
Lava plateaus/flood basalts/

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Lava plateaus are formed when the magma pours out of long fissures covering large areas with
very fluid basaltic lavas. Because the magma is basaltic and is very fluid lava flows considerable
distances before solidifying. Magma erupt along lines of fissures rather than a central vent
spilling liquid lava in successive layers. In Iceland, active fissures are spread throughout the
plateau landscape. The eruption style is Icelandic, the most peaceful eruptions. The Columbian
Plateau of the north-western United States, some 2 to 3 km thick, is the result of the eruption
of plateau basalts, or flood basalts. More than double the size of the Colombian Plateau is the
Deccan Traps, which dominates West-Central India.
The Columbia Plateau is uniformly covered with basaltic lava flows and spans an area of about
100,000 square miles in Idaho, Washington, and Oregon.
Shield volcanoes
An effusive eruption may come from a single vent. When low viscosity basaltic magma comes
from a single vent, because it thin and runny magma flows considerable distances before it
solidifies. The sides are gently sloping. The lower slopes are gentle, middle slopes steeper and
summit flattened. The shape is similar to in outline to a shield of armor laying face up on the
ground and therefore is called a shield volcano.

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After successive eruptions the height of the volcano increases. They are characterized by low
gas contents. Shield volcanoes are a common product of hotspot volcanism. Mauna Loa, Mauna
Kea and Kileaua are all shield volcanoes in Hawaii. Oceanic shield volcanoes such as those in
the Hawaiian Islands can rise as much as 8000 m above the surrounding sea floor
Mauna Loa in Hawaii

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Stratovolcanoes (composite)
Composite volcanoes are characterized by eruptions of lava that is more viscous (resistant to
flow) and higher in gas content. Composite volcanoes tend to have steep sides as the lava is
more viscous and does not flow far before cooling and solidifying. They are sometimes called
strato volcanoes because they are built up in alternating layers of ash, rock and lava. If a single
summit vent erupts repeatedly, a remarkable symmetry may develop as the mountain grow in
size.
They are often found at subduction-related arcs. When magma is slightly cooler it is thick and
sticky, or viscous, which makes it harder for gas bubbles to expand and escape and the
eruptions are usually explosive. They are form from plinian and vulcanian eruptions. Well-
known examples of stratovolcanoes are Mount St. Helens in the United States, Mount Mayo in
Philippines and Mount Fuji in Japan (The tallest mountain in Japan, Mount Fuji towers 3,776
meters (12,380 feet)

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Mount Mayo in Phillipnes
Volcanic domes, also referred to as lava domes, commonly occur within the craters or on the
sides of large stratovolcanoes. Volcanic domes are rounded, steep-sided mounds built by lava
too viscous to flow any great distance. A dome grows largely by expansion from within. As a
dome swells with hot magma inside, its outer surface cools and hardens, and then shatters,
spilling loose fragments down its sides. This viscous lava piles over and around its volcanic vent.
Mount St. Helens has several well-defined lava domes inside the crater.

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A steep sided rounded extrusion of highly viscous lave squeezed out from a volcano forming a
dome shaped or bulbous mass and around the vent.
Calderas
A caldera is a bowl-shaped depression. They usually exceed 1 km in diameter and fill with rain
water or snow melt to form a crater lake
Calderas form when a large magma chamber is emptied by a volcanic eruption. The
unsupported rock that forms the roof of the magma chamber then collapses to form a large
crater. This causes the side of the crater to subside, thus widening the opening to several
kilometres in diameter. In the cases of both Thera ( Santorini) and Krakatoa, the enlarged
craters of calderas have been flooded and later eruptions have formed smaller cones within

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the resultant lagoons.
Crater lake in Oregon

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Minor extrusive features
These are often associated with, but are exclusive to areas of declining of volcanic activity. They
include solfataras, fumaroles, geysers and mud volcanoes .
Mud volcanoes hot water mixes with mud and surface deposits
Solfataras created when gases , mainly sulphuric a, escape onto the surface
Geyser water in the lower crust is heated by rocks and turns to steam ; pressure increases and
the steam and water explode onto the surface
Fumaroles : superheated water turns to steam as its pressure drops when it emerges from the
ground

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Types of Volcanic Eruptions
Volcanic eruptions, especially explosive ones, are very dynamic phenomena. That is the
behavior of the eruption is continually changing throughout the course of the eruption. This
makes it very difficult to classify volcanic eruptions. Nevertheless they can be classified
according to the principal types of behavior that they exhibit. An important point to
remember, however, is that during a given eruption the type of eruption may change between
several different types.
 Hawaiian - These are eruptions of low viscosity basaltic magma. Gas discharge
produces a fire fountain that shoots incandescent lava up to 1 km above the vent. The
lava, still molten when it returns to the surface flows away down slope as a lava
flow. Hawaiian Eruptions are considered non-explosive eruptions. Very little
pyroclastic material is produced.
 Icelandic These are eruptions of low viscosity basaltic magma. The magma erupts
through fissures rather than a central vent.
Icelandic eruption

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 Strombolian - These eruptions are characterized by distinct blasts of basaltic to
andesitic magma from the vent. These blasts produce incandescent bombs that fall
near the vent, eventually building a small cone of tephra (cinder cone). Sometimes
lava flows erupt from vents low on the flanks of the small cones. Strombolian
eruptions are considered mildly explosive, and produce low elevation eruption
columns and pyroclastic fall deposits.
Strombolian Eruption
 Vulcanian - These eruptions are characterized by sustained explosions of solidified or
highly viscous andesite or rhyolite magma from a the vent. Eruption columns can
reach several km above the vent, and often collapse to produce pyroclastic
flows. Widespread pyroclastic falls are common that contain mostly angular
blocks. Vulcanian eruptions are considered very explosive.
 Pelean - These eruptions result from the collapse of an andesitic or rhyolitic lava
dome, with or without a directed blast, to produce glowing avalanches or
nuée ardentes, as a type of pyroclastic flow known as a block-and-ash flow. They may

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also produce surges with resulting surge deposits. Pelean eruptions are considered
violently explosive.
 Plinian - These eruptions result from a sustained ejection of andesitic to rhyolitic
magma into eruption columns that may extend up to 45 km above the vent. These are
the most explosive eruptions. Eruption columns produce wide-spread fall deposits
with thickness decreasing away from the vent, and may exhibit eruption column
collapse to produce pyroclastic flows and surges. Plinian ash clouds can circle the
Earth in a matter of days. Plinian eruptions are considered violently explosive.
Types of eruptions

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VOLCANIC ISLAND ARCS

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As with oceanic-continental convergence, when two oceanic plates converge, one usually
subducts under the other, just the same as when an oceanic plate collides with a continental
one. The denser of the two usually subduct. The older oceanic plate is the denser or heavier
one. As one plate subduct a deep trench is formed on the sea bed. This is like a deep valley in
the sea. Trenches are the deepest part of the ocean floor. The deepest trench in the world is
the Marianas Trench in the Pacific Ocean at approximately 11,000 m. Here the older pacific
plate plunges or subducts beneath the Philippine plate. The trench mark the area where the
oceanic plate starts to subduct into the mantle. They generally run parallel to volcanic island
arcs. As the oceanic plate goes deeper into mantle it melts in the subduction zone, due to
friction and the increased temperature.
The newly formed magma is lighter or less dense than the one in the mantle. Being lighter it will
rise towards the surface where it will protrude through the oceanic crust to form a chain of
volcanic islands or island arc. Examples of Volcanic island arc are the eastern Caribbean island
arc in the Caribbean, the Aleutian Islands in the pacific and Japan. As the plate subducts, the
overriding plate scrapes sediments as well as projecting portions of the ocean floor off the
upper crust of the lower plate. This creates a zone of deformed rocks that attaches itself to the

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overriding plate. This zone is known as an Accretionary prism or wedge. Barbados to the east of
the Lesser Antilles volcanic chain is an emergent part of the accretionary prism
The Eastern Caribbean Islands
The South American Plate is moving westwards due to sea floor spreading at the Mid Atlantic
Ridge. Where it meets the Caribbean Plate, it descends (subducts) beneath it. This is because
the oceanic lithosphere of the South American Plate is denser than that of the Caribbean Plate.
The subduction causes low density ocean floor sediment to be scraped off the surface of the
South American Plate and thrust onto the Caribbean Islands as accretionary wedges, in a
process called obduction. Barbados to the east of the Lesser Antilles is an emergent part of the
accretianory prism. The line of subduction is marked by the deep sea Puerto Rico Trench
the South American plate continues to descend into the mantle, it starts to melt. The newly
formed magma is less dense and rises through the oceanic crust to form the Eastern Caribbean
Volcanic Island Arc. The Caribbean volcanic islands form a curved linear chain or ‘volcanic island
arc’ parallel and to the west of the Puerto Rico Trench.

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Saba is the northernmost island while Grenada is the southernmost island. The Caribbean island
is an area known for volcanic and earthquake activity.
Marianas Island
The Marianas Trench, which runs parallel to the Mariana Islands, has formed where Pacific
Plate converges with Philippine Plate. The Marianas deepest point of the Marianas trench is
approximately 11 000 m deep.

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Marianas island in the pacific
The Pacific plate converges with the Philippine plate to form the Marians Islands. Where pacific
plate meets the Philippines Plate, it descends (subducts) beneath it. This is because the oceanic
lithosphere of the Pacific plate is denser than that of the Phillippine Plate. The line of
subduction is marked by the Marianas Trench which is the deepest
the Pacific plate continues to descend into the mantle, it starts to melt. The newly formed
magma is less dense and rises through the oceanic crust to form the Marianas Islands.

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The Aleutian Islands (west of Alaska), the Kurile-Kamchatka Arc, Japan, Philippine Islands, and
Marianas Islands, New Zealand, and the Indonesian Islands, along the northern and western
margins of the Pacific Ocean are zones where oceanic lithosphere is being subducted beneath
oceanic lithosphere. These are all island arcs.
Fold Mountains
Fold Mountains are associated with convergent plate margins. Fold Mountains occur at
continental continental convergence and oceanic and continental convergence.
When two plates converge there is no subduction as both plates are of the same density.
Instead the rocks crumple and buckle. The plates push into each other creating crustal
thickening, folding and overriding some of the most complex structures in the world. The
continental crust is around 75 km thick. The collision of the Indian and Eurasian plates have
thrown up the Himalayas and the Tibetan plateau. The Himalayas now rise to 8,854m. The
Indian plate is moving northwards at a rate of about 5cm annually producing earthquakes that

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affect several countries in the region such as India, Pakistan and China. The collision of the
Africa and European plates formed the folded Alps of Southern Europe.
When an oceanic and a continental plate collide the oceanic plate sinks and the continental
plate is folded and pushed upwards to form a fold mountain. They were formed as a result of
the convergence of the Nazca plate and the South American plate. The heavier oceanic crust is
pushed towards the South American plate, and because it is denser is subducted underneath.
The South American plate is less dense so sits on top of this subduction zone but the rocks of
the South American plate have been folded upwards and crumpled into Fold Mountains. The
Juan de Fuca and North American plates collided to form the Rockies of North America.

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Positive impacts of volcanic activity
Fertile Soils
Volcanic materials produce fertile soils. Ash and cinder are natural fertilizers as they are rich in
minerals. As lava cools and is eroded by the elements it also adds to soil fertility. For this reason
agriculture often thrives in volcanic regions, e.g. near Mt Vesuvius, which is an important
vegetable production are in southern Italy. The Brazilian plateau is another region where
ancient volcanic activity has enriched the soils and allowed coffee production to thrive. In New
Zealand volcanic soils are used in the dairy industry and fruit production.
Land surface
Erupting volcanoes are thought to be the source of the first land as the hot planet cooled.
Eruptions continue in the sea creating new land. All the volcanic islands are created in this way.
The shape of Montserrat has changed when the last eruptions extended the land in the south.
(but more than a half of the land area is now uninhabitable. There are so many islands that
were created as a result of volcanic activity. The Eastern Caribbean islands, the Aleutian Islands
and the Marianas islands in the pacific are all volcanic island arcs and were formed as a result of
volcanic activity. Volcanic island arcs are formed as a result of oceanic-oceanic convergence.
The older and denser oceanic crust subducts into the mantle at first. As the oceanic plate sinks
into the mantle it starts to melt forming new magma. The newly formed magma is less dense
than the magma in the mantle. The magma now rise through the oceanic crust forming Volcanic
island Arcs. The eastern Caribbean island was formed from the collision of the Caribbean and
South American plates. Hawaii is in the middle of the pacific and was formed as a result of intra
plate volcanism. Sometimes mid oceanic ridges at divergent plate boundaries break the surface
of the sea to form islands such as Iceland and Surtsey in the Atlantic and Easter Island in the
pacific.

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Minerals
Many useful materials are formed directly or indirectly from volcanic activity. Building materials
such as granite and marble, precious minerals such as diamonds as well as silver and copper are
formed from magma and in and on the earth’s crust. Other mineral resources such as gold,
silver, nickel, copper, and lead are sometimes found around volcanic activity.
Geothermal energy
Geothermal energy production in Iceland is another positive of volcanic activity. Iceland is on
the Mid-Atlantic ridge (plates separating) and for this reason has a lot of volcanic activity.
Magma rises close to the surface of the crust and this heats the groundwater. This water is
heated to well beyond boiling point (up to 200 degrees Celsius) and becomes “super-heated”.
Wells are drilled into the rock and the hot water is pumped out. As this hot water reaches the
surface it does so as steam due to the intense heat. This steam is then used to drive turbines
and create electricity. The steam then cools slightly and becomes hot water, which is then
piped to homes and offices in Icelandic towns to heat them. Today, over 90% of homes in
Iceland are heated through geothermal energy. Once the water is used to heat buildings, it is
then used in green-houses as the still warm water is piped under the soil to allow the

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production of flowers and vegetables. If there is a lack of groundwater, cold water is pumped
into the rock through specially drilled wells, which is then heated by the hot rock and pumped
back up.
Tourism
Volcanic peaks. Whether active or formant are natural tourist attractions in many parts of the
world. From the majestic Mt Fuji in Japan to the ever flowing Hawaiian cones, tourist visit and
wonder at this evidence of the earth’s interior. Volcanoes are also important tourist attractions.
Mount Vesuvius in Naples and Mount Etna in Sicily are major tourist attractions in southern
Italy. In Iceland, the geysers and hot springs caused by volcanic activity bring many tourists to
the island. This tourism generates jobs and money in areas that may not have many other
sources of employment. Jobs are generated in areas such as accommodation, transportation,
sight-seeing and retail (shops).Geysers are also used as tourist attractions such as the
Yellowstone National Park.

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Tour on Mount Vesuvius in Italy
A group of people watching a geyser (old faithful) in the Yellow Stone national park erupting

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Building Construction
When lava/magma is cooled it form igneous rocks. Some igneous rocks such as granite are
being used in the construction industry for thousands of years. Granite is very durable and
strong igneous rocks that is used for all kinds of structures. Because pumice is so light it is used
quite often as a decorative landscape stone. Basalt is also quarried in some part of the world
and is also widely used in the construction industry.
Science
Volcanic activity allows scientist to study the materials of the interior earth’s crust. This is very
important since it is too hot and far from the surface for direct investigation.
Plant development
Volcanic slopes are often steep but sometimes they are inaccessible. Thus they can provide
refuges for rare plants and animals from the ravages of humans and livestock.
Positive impact of Earthquake activity
Positive – Better Engineering and Building Structures
Because earthquakes are unavoidable and unpredictable, scientists and engineers create ways
to make structures quake-resistant and more stable. Places like California, where earthquakes
constantly occur, have buildings and structures designed to survive earthquakes. Engineers
build quake-resistant buildings by using lighter materials and creating structures that can
handle sideway loads, as high-rise structures tend to “sway” during major earthquakes.
Understanding the Earth’s Interior
Measuring small earthquakes allows geologists to study areas underground. Geologists can
measure the way that the vibrations of earthquakes travel and make inferences about the type
of material the vibrations pass through. There are two main categories of seismic waves: body

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waves and surface waves. P waves and s waves are body waves. Rayleigh waves and love waves
are surface waves. The p waves and s waves differ in several aspects. S waves can only travel
through solid rock while p waves can travel through solid and liquid. Both seismic waves travel
through the body of the earth and it helps us to understand the characteristics of the different
layers of the earth.
Insight for geologists:
The subterranean parts of the earth are really hard to explore because mining through various
geographic variations and through magma is really hard and in some cases it is out right
impossible, especially with the current state of technology. Earthquakes however provide a
glimpse into how these regions might look like. Geologists can predict the type of material in
these regions by seeing how the vibrations pass though those materials. Because of
earthquakes geologists are better able to understand how the deeper reaches of the earth
might be. These help geologists predict water aquifers, natural gas deposits, oil deposits and
other important resources. They can also in some cases find the density of the deposits. This
makes natural resource extraction more efficient.
Brings valuable minerals above the ground
The shift of the tectonic plates can either force portions of earth to sink down or move upward
due to this process of shift in the tectonic plates there is a shift in the arrangement of rocks,
which results in a shift of the mineral and ore deposits. This movement of earth sometimes
pushes mineral or metal rich deposits close to the surface of the earth that makes mining them
a lot easier. Such a shift in tectonic plates can also shift other resources like fossil fuels and
makes them easier to extract. Fossil fuels like natural gas, petroleum and so on also might get
pushed up or made more accessible because of earthquakes.

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Value of Folded and Faulted Landscapes
Value of fold mountains
Source of minerals
Source of minerals, for example, coal beds in the Appalachians, as well as minerals intruded
below the mountain as batholiths, for example, tin in the Andes. The Andes Mountains contains
a rich mix of minable materials that are both very valuable and very useful to man. When the
Spanish conquered South America their prime objective was to prospect for gold. The
Yanacocha gold mine in Peru is the largest gold mine in the world. It is an open cast mine and
the rocks containing the gold are blasted with dynamite. The rock is then sprayed with toxic
cyanide and the gold extracted from the resulting solution. The nearby town of Cajamarca has
grown from 30 000 when the mine started tom240,000 people in 2005. There exists large
deposits of Coal, oil and natural gas, iron ore, silver, tin, copper, phosphates and nitrates and
bauxite (for Aluminium) within the Andes Mountain. The Andes are famously rich in mineral
resources and have given birth to a long tradition of metallurgy and mining that has led to one
of the most important metal industries of the world. The principal metals are: Chile
(molybdenum, copper, iron), Peru (copper, silver, lead, zinc, gold, vanadium, iron, arsenic),
Bolivia (tin, silver, lead, tin, bismuth), Ecuador (gold), Colombia (gold, platinum, iron),
Venezuela (gold, iron, aluminum).
Coal is mined in Appalachia by both surface and underground mining techniques. Surface coal
mining methods in the steep terrain of the central Appalachian coalfields include mountaintop
removal, contour, area and high wall mining. Coal mining operations are found in Kentucky,
West Virginia, Virginia, Maryland, Pennsylvania, Ohio, Alabama and Tennessee.
Source of energy
Many of the Fold Mountain regions of the world are prime spots for the generation of
hydroelectric power (HEP). They have a plentiful supply of water; deep, narrow valleys with
quick flowing rivers, and they are sparsely populated, meaning that few people are displaced

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when a reservoir is created. Water falling over high relief has been used for hydroelectricity,
for example in Norway and Canada. There are several examples across the world.
Hydroelectricity is common in the Alps in Europe. Energy companies use the Alps because of
the steep terrain. They are able to build dams across valleys and make manmade reservoirs to
provide Hydroelectric Power (HEP) to the area (60% of Switzerland’s energy comes from HEP
from the Alps). The high rainfall and snowmelt in the spring provides a lot of water to power
the HEP plants.
The deep valley and rivers of the Andes give it huge potential as a region to produce
hydroelectric power. The narrow valleys are ideal to dam as it cuts costs, and the steep relief
increases water velocities allowing electricity generation. Snow melt fuels most of the water
provision, but this means that HEP production can be reduced to small amounts in winter. The
Yuncan dam project dams the Puacartambo and Huachon rivers in northeast Peru
The Yucan Dam Project

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Tourism
Tourism is an important economic activity in regions of Fold Mountains. The tourism industry
generate jobs and income for local people. Several activities are interested to tourists. Some of
these mountain ranges have developed their own flora. They are often untouched and persons
particularly Eco tourist are interested in these scenery. Other activities might include skiing,
hiking and mountain climbing.
Tourism is a massive industry for Peru and the country has a lot to offer. In the East you can
take part in Eco-tourism activities in the Amazon Basin, as found along the Madre De Dios River
near to Puerto Maldonado. Peru has some fantastic coastline as well, but the highlight of Peru
is undoubtedly the Inca Trail. The trail is covered in 4 days and basically covers 43km of old
pathways linking together old Inca settlements in the inhospitable mountains of the Andes. It is
South America's best known trek and is one of only 23 World Heritage Sites (as deemed by
UNESCO) to be classified as important both naturally and culturally. The trail is strictly
controlled and 500 trekkers are allowed to start out on the trail every day.
Tourism is a prime use of the Alps with winter sports, such as skiing, skating a huge attraction to
the area. Local residents run hotels, ski schools and entertainments to support the tourism
industry, which has over 20 million visitors per year. In the summer months, the area offers
walking, climbing and mountain biking opportunities, along with other outdoor activities, and in
doing so, keeps the local workers’ incomes fairly consistent throughout the year
The Himalayan Mountains represent the world’s highest mountain. From trekking and white
water rafting to rock climbing and mountain biking, the Himalayas offer adventurers many
options. As Himalaya snows melt in the spring, slow-flowing rivers become raging torrents of
water crashing over rocks, creating another Himalayan adventure — white water rafting.
Several rivers flow from the Himalayas, including the Teesta, Ganges and Zanskar. Nepal also
has more than 10 rivers suitable for white water rafting. Trekking is another activity as toursits
try to reach the peak at Mount Everest. Mountain biking is also on the list of activities as tourist

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try to experience the Himalayas. There are several different types of flora and fauna in the
mountain and tourist get to enjoy the mountain scenery.
Biodiversity
The Fold Mountains of the world have their own distinct flora and fauna. Smaller micro climates
can be found in these areas and if one should trek some of these mountains there is going to be
a variation in terms of the flora and fauna present. Some of these areas are inaccessible and
they are reserved for scientific enquiry.
Country/ Regional Boundaries
High mountains often serve to separate one area/ country from another.
Defensive sites
Extinct volcanoes may provide defensive settlement sites (Edinburgh)
Farming
Farming is also practice in some areas of Fold Mountains. Some crops are grown for personal
consumption while others are grown for commercial purposes. The mountain slopes of the
Andes are used for a variety of farming practises. The best land can be found on the valley
floors, but an ingenious system of terraces dug into the valley sides and held up by retaining
walls has been used to bring the lands on the valley sides into food production. The flat
terraces help to hold up water in a region where there are marked shortages. Most crops are
grown in the lower areas and include soya, maize, rice and cotton. However, the main staple
crop of the Andes is the potato, and there are hundreds of different varieties found in the
mountains.

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Value of Faulted Landscapes
Tourism
The Great Rift Valley is a remarkable geological feature and a very popular tourism region in
Kenya. The main attractions of the Great Rift Valley are the wonderful national parks and safari
opportunities enabled by the wealth of wildlife; it is a particularly good area for bird watchers
due to the string of alkaline lakes which attract flamingos, pelicans and many other species to
the scenic valley.
The freshwater ecosystems at Lake Naivasha and Baringo, the nature reserve at Lake Bogoria,
and the Nakuru National Park are a source of endless fascination, teeming with an incredible
diversity of birds and large concentrations of animals. At these destinations visitors can go on
water safaris and get up close to the wildlife of the lakes.
The rift valley especially provides opportunity for man to learn of geological processes that
have shaped the earth and continue to do so. It is therefore a laboratory of geological
processes. For example the Eastern African Rift system. Study of this entire landscape serves to
extend man’s knowledge of geological processes.
VOLCANOES, EARTHQUAKES AND FLOOD AS HAZARDS
Human beings live all over the world even in hazardous areas, since the positive aspects may
outweigh the potential hazards. Extreme events which occur in uninhabited areas of the world
are not hazards. Each natural event can have several impacts on human life and property. Some
events take lives and destroy property directly (primary effect); while others have indirect
effects (secondary effects). Tertiary effects are long term effects which may be permanent.
Flooding has negative impacts on human life and property. Flood waters can sweep people to
their death and cause damage to property. This is the most common hazard experienced
globally and throughout the Caribbean.

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Volcanoes are perhaps the most dramatic extreme natural event. Hot lava spewing out of the
vent; ash and gases exploding into the air. Hot clouds of gases speeding down slopes, are very
spectacular occurrences. The impact can be devastating, obliterating large areas with people
and buildings buried under tons of hot ash/lava. Some volcanic eruptions are not explosive but
can still have a negative impact on the human environment.
Earthquakes are the most sudden in onset, striking with little warning. The violent seismic
waves immediately destroy buildings and kill people persons by burying them under the rubble.
They may trigger landslides and damage dams. Tsunamis may be created by undersea
earthquakes ad flood coastal areas.
Hazard impacts
Hazard risk arises from the intersection of natural events and vulnerable population. The
magnitude, speed, extent and duration of the event, all influence its impact, but the
characteristics of the human environment will also play a big part in the realization of the
hazard, the disaster or the catastrophe.
Generally the impact of any hazard is dependent on the number of people and type of human
activity in the affected area, Populations in LDCs are often unprepared and ill equipped to
respond to hazard impacts. Many lives are lost and it is often difficult for communities to return
to previous living standards. In MDCs, there are greater property losses than loss of life as
higher levels of technology and resources allow more people to prepare, evacuate or recover
from the effects of hazards. For example, flooding of the Mississippi river in the USA has a
different level of impact from flooding in the Ganges delta where the population is larger and
more vulnerable.
The magnitude of an event will directly influence its impact: the greater the size of the event,
the greater the hazardous effects. Its frequency will also affect how it affects people. Generally
people are better prepared if they have experienced an event. Planned land use zoning and
evacuation may lessen the impact
Hazard event characteristics

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Flood Earthquake Volcano
Speed of onset Moderate Fast Moderate
Duration Days/weeks Seconds Days
Area Widespread Concentrated Limited
Each natural hazard can have primary, secondary and tertiary effects depending on the
characteristics of the population and the level of development.
 Primary effects are those caused directly bu the event- for example flood water may
sweep away houses, or earthquakes may cause buildings to collapse
 Secondary effects are those in the aftermath of the event, for example, famine,
diseases or fires after the event is over.
 Tertiary effects are long term/ permanent changes for example, relocation of
settlements
Earthquakes- Primary and secondary effects of earthquakes
The seismic waves generated by earthquakes are most powerful at the epicentre. Depending on
the type of material they pass through and the type of relied, they may have many effects.
Although earthquakes last for a few seconds they present great hazards particularly in the built
environment of tall densely populated buildings.
Primary effects of earthquakes happen straight away and occur as a direct result of the ground
shaking. For example, shaking of the ground causing

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 People may be killed by collapsed buildings- Majority of deaths from an earthquake are
a result of collapsed buildings.
 Destruction of roads and bridges
 windows shattering;
 roads cracking; bridges toppling over
 Ruptured underground gas mains / electricity / water pipes – in Developed countries gas
lines are sometimes constructed underground. Ground shaking can rupture these gas
lines. Electric poles/cables/wires can also be destroyed.
 The natural environment is also affected by ground shaking. Ground shaking can cause
to trees to topple over.
 Fissures/cracks open in ground
 Sewage lines can be destroyed
 Tsunamis – Tsunamis are large tidal waves. Tsunamis can travel across large expanse of
ocean encroaching or engulfing lands that are 1000s of miles away. When tsunami
approaches shallow water it increases in height sometimes reaching heights of 27ft or
90m or more. The Boxing Day Tsunami in 2004 had devastating consequences. The 1964
Alaskan earthquake caused considerable damage in several Californian coastal areas.
The boxing day Tsunami in 2004 caused significant damage in that area. The death toll was
close to 250, 000
 Landslides- The ground shaking can destabilize slopes causing mass landslide. They are
most likely to occur where the land is steep, saturated or weak.
Liquefaction
This is where a saturated soil loses strength and rigidity because of applied stress, normally an
earthquake. The changes in its state causes the ground to behave like water allowing things to
sink into it. Buildings often topple over or sink into the ground as a result of this.

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Secondary effects occur as a result of the primary effects, and they happen later. For example,
 Fires caused by ruptured gas mains- This was the main cause of death and damage after
the San Francisco earthquake in 1906. This can also emanates from destroyed electrical
poles, fallen wires and cables.
 Disease - Diseases can spread very quickly in the unsanitary conditions often left behind
by massive earthquakes. Water becomes contaminated very quickly, and in Less
Economically Developed Countries (LEDC's) especially; access for the medical services
can be badly hampered by the damage caused by the quake. The most common
diseases to be associated with earthquakes are therefore water-borne ones like cholera
and typhoid. Sometimes bodies are not buried quickly and an accumulation of dead
bodies only makes the situation worse. Ruptured sewage lines can contaminate water
as well.
 Famine- Shortage of food after an earthquake and if enough food is not provided by aid
famine can occur.
 Joblessness- This is a temporary situation after the earthquake where persons are
jobless due to destruction of infrastructure such as banks, schools, factories and other
work places. If infrastructural damage are serious and recovery is slow then it can last
for a longer period of time.
 Homelessness- Homes/houses sometimes are destroyed during an earthquake. So
people are left stranded and are homeless. Tents or temporary housing are usually
erected but often times the place is crowded and sanitation is an issue
 Disruption of waste and sewage disposal systems.
 Lack of potable water as water gets contaminated
 Flooding from tsunami - tsunamis can cause coastal flooding. Coastal communities
across the world are at risk from tsunamis. Crops can be destroyed as a result of this
also.

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These secondary effects can cause greater loss of life in remote or isolated areas. People die of
exposure in cold mountainous winter weather, or die of starvation because aid cannot reach
them properly.
Tertiary effects may include permanent evacuation of the area and changes in relief, for
example, displacement of land along faults.
Flooding
There are different types of flooding: riverine, estuarine, coastal and flash floods. Flooding is
one of the deadliest hazard and cost countries billions of dollars annually.
 People are washed away and drown
 Animals, Cars and buildings are washed away
 Property and crops are destroyed by inundation
 The flood water often leave deep layers of mud on flooded surfaces destroying buildings
and their contents.
 Flood waters can contaminate drinking water supplies and cause diseases
 Destruction of underground utilities
 communication links and infrastructure such as power plants, roads and bridges are
damaged and disrupted,
 Floods can also traumatize victims and their families for long periods of time. The loss of
loved ones has deep impacts, especially on children. Displacement from one's home,
loss of property and disruption to business and social affairs can cause continuing stress.
For some people the psychological impacts can be long lasting
Major floods in China, for example, killed about 2 million people in 1887, nearly 4
million in 1931, and about 1 million in 1938

Module 3 cape unit 1 geography.

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