Plate Tectonics
Structure of the Earth
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inner core (solid due to heat and pressure from outer layers)
outer core (liquid)
mantle
crust
Oceanic-oceanic divergence
When two oceanic plates move apart from each other, magma rises to fill the gap that is created at the plate
boundaries. The magma cools and solidifies to form new sea floors which extend the existing floors in a process
called sea floor spreading. The resulting feature is an oceanic ridge.
Oceanic-oceanic convergence
When two oceanic plates collide with each other, the denser plate is forced to subduct under the less dense plate.
The subducted plate melts to form magma under intense heat and pressure. The magma forces its way up
through fractures in the Earth’s surface to form underwater volcanoes. Over time, these volcanoes may build up
and form volcanic islands.
Oceanic-continental convergence
When an oceanic plate collides with a continental plate, the denser oceanic plate is forced to subduct under the
less dense continental plate. The subducted plate melts under intense heat and pressure to form magma. The
magma rises through fractures in the Earth’s surface and cools and solidifies to form volcanoes. The sediments
in the continental crust also compresses and folds under pressure to form a range of fold mountains.
Continental-continental convergence
When two continental plates collide into each other, the sediments and minerals in the edges of both plates
compress and fold to form a range of fold mountains.
Volcanoes
Volcanoes
Volcanoes are conical or dome-shaped landforms formed from the accumulation of lava that has been emitted
onto the Earth’s surface.
Formation of volcanoes
Volcanoes are formed when plate movements at convergent or divergent boundaries cause magma to rise up
through fractures in the Earth’s crust. The magma gathers in a chamber called the magma chamber until the
pressure in it becomes too great. The magma then erupts through a volcanic vent to the Earth’s surface. During
an eruption, lava, volcanic ash and dust are ejected onto the Earth’s surface, where the lava cools and solidifies
near the vent. Over the years, they accumulate to form a taller volcano.
Types of lava
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Basic lava
Basic lava has very high temperatures ranging from 1100°C to 1200°C. It has a low viscosity so it flows
quickly. It takes a longer time to solidify because it is hotter. The slopes of the volcano are gentle because
the lava flows over a long distance. Basic lava is found at fixed hot spots and divergent plate boundaries
because the magma comes directly from beneath the Earth’s surface and not the melting of plates, which
would add to its mineral content and hence its viscosity.
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Acidic lava
Acidic lava has lower temperatures ranging from 800°C to 1000°C. It has a high viscosity so it flows
slowly. It takes a shorter time to solidify because it is cooler. Therefore, the slopes of the volcano are
steeper. Acidic lava is formed at convergent plate boundaries which is the reason why it is rich in
minerals. The molten plate which is the magma is high in mineral content. This is why acidic lava has a
high viscosity.
Types of volcanoes
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Shield volcanoes
A shield volcano has gentle slopes and a relatively flat top. It is formed from basic lava which has a low
viscosity. This is why the lava is able to flow over a long distance before solidifying, giving the volcano
its characteristic gentle slopes/broad base. Also, as gases are able to escape easily through the less
viscous lava, the eruption is usually non-violent.
Acid volcanoes
Acid volcanoes are formed from acid lava which is viscous and solidifies near the vent of the volcano.
The eruption of an acid volcano is usually violent because the acid lava solidifies quickly near the vent
of the volcano, blocking rising magma and gas from escaping from the vent. This causes a build-up of
pressure inside the magma chamber which in turn causes a violent eruption. Also, as the lava cools and
solidifies quickly, the slopes of the acid volcano are steep.
Composite volcanoes
This is the most common type of volcanoes which consists of alternating layers of ash and cinder and
acid lava. As the volcano erupts, lava forces its way up through the main vents and solidifies quickly.
However, sometimes the magma forces its way through other channels and new vents are created on
the side of the volcano. These new vents allow the viscous lava to cool and solidify quickly on the
surface, blocking the gases and magma from escaping. The increasing pressure causes a violent
eruption of ash and cinder, followed by lava. This type of volcano is steeper at the top and gentler near
the base due to the size and weight of volcanic materials ejected. Smaller and lighter objects like ash
and cinder are carried farther by the wind and tend to settle at the base. Heavier and larger objects such
as solidified lava are deposited near the crater.
Effects of a volcanic eruption
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Physical:
Forests are destroyed due to landslides and mudflows caused by the pyroclastic flow.
Aquatic ecology negatively affected due to pollution by mudflows. Biodiversity impacted.
Economic
Lots of funds will be channeled to repair the damage.
Flights will be cancelled due to the state of the country and poor vision caused by volcanic ash.
Social
Trauma caused by the loss of loved ones.
Respiratory problems caused by volcanic eruption.
Why people still want to stay near volcanoes
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Physical
Ash deposits and weathered lava rich in minerals make the land extremely fertile for farming.
Economic
Revenue gained from the generation of geothermal energy by tapping on the steam.
Increased output from fertile soil means increased income for farmers.
Social
Confidence in scientists that they will be able to accurately predict an eruption for early evacuation.
Sense of attachment to the land because it is their ancestral land and they are against the idea of
abandoning it.
Earthquakes
Earthquakes
Earthquakes are vibrations or tremors of the Earth’s surface caused by the sudden release of energy
stored in the rocks in the Earth’s crust. They are the results of plate movements and are thus often
found near plate boundaries. They occur when there is a subduction between two plates and when two
plates move apart of slide past each other. In all these situations, the plate movements are not smooth
and there is friction between the plates.
Effects of earthquakes
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Collapse of infrastructure
This occurs in populated areas when buildings cannot withstand the tremors. Although some buildings
may not fall, their foundations will be weakened which may cause them to fall in the future.
Fires
These are often indirectly caused by earthquakes when gas pipes, wires, stoves etc. are damaged by
the shaking. Fires may also cause the collapse of infrastructure.
Landslides
When the ground shakes, the soil on land at a gradient will be loosened and landslides may occur.
People and infrastructure are buried in landslides.
Tsunamis
These are caused by underwater earthquakes which cause series of huge waves.
Impacts of earthquakes
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Loss of lives
Lives are lost when there are fires, collapse of infrastructure, tsunamis or landslides. Sometimes,
victims of the earthquake who survived with severe injuries die when relief effort is not given in time.
Rescuers’ lives are also endangered when aftershocks occur.
Negative economic impact
The economy will be affected because funds need to be channeled into relief effort and the repairs of
damage. Investors will also think twice before they invest in the country because the country is in a state
of emergency.
Spread of disease
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Survivors are usually mass-housed in tents or makeshift shelters where there is overcrowding and poor
sanitation. This facilitates the rapid spread of diseases and dead bodies and poisonous materials may
also pollute water supplies.
Trauma
Survivors find it very difficult to recover emotionally from the shock of the earthquake and also the loss
of their friends and relatives.
Disruption of jobs
The economy will be impacted when people who rely on the physical environment lose their jobs.
People such as farmers and fishermen are the greatest impacted and the tourism industry is also
affected because tourists do not want to visit a country in such a dire state.
Factors influencing extent of damage
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Level of preparedness
There is less damage when people are able to evacuate in time. If they are more prepared, they will
panic less and be more organised. The authorities that are more prepared will also have sufficient
emergency plans than those who are not so prepared, thus they will suffer less damage.
Magnitude of earthquake
The magnitude of the earthquake affects the extent of damages. According to the Richter scale, an
earthquake can only be detected with a seismometer if it measures 2 or lower and is considered major if
it measures 7 or more.
Depth of focus
The depth of focus is the distance from the Earth’s surface where the earthquake occurred. Usually,
earthquakes with a shallow focus are more devastating that those with a deep focus. An earthquake
which occurred 0-70 km beneath the Earth’s surface is considered shallow, 71-300 km is intermediate
and 301-700 km is deep.
Proximity from epicentre
The further an area is from the epicentre, the less the damage.
Geology of epicentre
If the area is built of soft stilt instead of solid rocks, it will suffer more damage.
Design of infrastructure
Buildings with reinforcement are less likely to suffer damage than older and flimsier ones as they are
less likely to topple.
Tsunamis
Earthquakes generated in the sea cause tsunamis which have devastating effect on coastal areas.
Density of buildings
Places with a high building density will suffer more severe damage than those with less density.
Extensive fires can also break out when buildings collapse. The high density will only facilitate the
spread of the fire.
Adaptation to Earthquakes
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Education
Planning of infrastructure