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Unit 2-Section A Topic 3 Tectonic Landscapes
World distribution of earthquakes and volcanoes. Where and why.
Why do
Plate tectonics= theory that explains why earthquakes and volcanoes are found where they are.
earthquakes and
1-The Earth’s crust is divided into seven large and 13 smaller plates.
volcanoes both
There are two different types of crust: Oceanic crust is younger, heavier, can sink and is constantly being
occur where they
destroyed and replaced. Continental crust is older, lighter, cannot sink and is permanent. It is the
do?
differences between the two types of crust that account for the variation of activity at plate boundaries.
2-Plates move a few centimetres a year. They float on the molten mantle. Convection currents in the
mantle cause the plates to move. The convection currents are a result of heat from the interior of the
Earth.
3-Three different types of plate movement due to convection currents:
- Some plates move towards each other. This is called a convergent plate boundary.
- Some plates move away from each other. This is called a divergent plate boundary.
- Some plates move alongside each other. This is called a conservative/transform plate boundary.
4-The plate boundaries are where most of the earthquakes and volcanoes occur. Exception: Volcanoes at
hotspots.
Hotspots
Some volcanoes do not occur on plate boundaries. These volcanoes are formed 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. At areas where the pressure is greater in the mantle, magma erupts through the crust as
when the plume reaches the crust it causes the crust to dome (and crack).
Volcanoes are created if the magma rises above the ocean surface.
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.
The characteristic features of convergent, divergent and conservative plate boundaries.
2 types of constructive or divergent boundaries
Constructive or
Most of the constructive boundaries are located in oceans but they can also be found on land.
divergent
1- Convection currents pull plates apart. 2- As the plates move apart, magma from the mantle rises to fill
boundaries in
the gaps and forms new oceanic crust. 3- A mid-ocean ridge is formed by the new crust. 4- Magma rises
oceans
due to convection currents, leading to pressure and doming of the crust (oceanic). Magma rises through
the weaknesses in the crust. Eventually (low viscosity) magma erupts onto the surface. The continued
movement of plates pulls the plates apart leading to more effusive eruptions. Submarine volcanoes
appear along the ridge. Some may grow to form islands, e.g. Iceland
Constructive or
The East African Rift Valley formed around 25 to 30 million years ago where the Somalian and Nubian
divergent
plates are pulling away from the Arabian Plate. The East African Rift valley is opening up and new land is
boundaries in land being formed in the bottom of the valley. The rift is a narrow zone in which the African Plate is in the
process of splitting into two new tectonic plates. It includes a number of active as well as dormant
volcanoes among them: Mount Kilimanjaro and Mount Kenya. At The Red Sea, the plates have fully
separated and the central rift valley has dropped below sea level and forms a new ocean.
3 types of convergent or destructive boundaries
Oceanic1- Movement of plates towards each other leads to subduction of the denser oceanic plate. 2- Friction and
continental
pressure cause earthquakes to occur along the subduction zone. 3- High temperatures and friction lead to
boundary
the melting of the edge of oceanic plate. Less dense melt rises through the mantle, and pressurises the
continental crust. Cracking of the continental crust due to collision provides a path for rising magma.
Rising magma contains gas and is highly pressurised leading to explosive eruptions. 4- A trench forms
where the oceanic plate is being subducted. 5- Fold mountains form where the two plates collide. The
force of the collision leads to the upthrust of the plates forming mountains with folded geology. E.g. Nazca
and South American Plates
Oceanic-oceanic
Change to 3 above: As the subducted oceanic plate begins to melt, magma escapes to the surface to
boundaries
form volcanoes, a few kilometres from the trench. After several eruptions, these volcanoes break the
ocean surface to form islands. When several volcanic islands form together they are called an island arc.
No formation of fold mountains. E.g. Eurasian and Philippine plates
Continental-to1- When two continental plates meet at a destructive boundary, a slow collision takes place as both plates
continental
have a low density. 2- This results in intense folding and uplift and leads to the formation of fold
boundary
mountains. 3- There is no subduction. Therefore, there are no volcanoes. 4- However, earthquakes occur
due to the collision of the plates. E.g. Eurasian and Indian Plates
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Transform/conservative boundaries are where plates just slide past each other with crust neither created nor destroyed.
E.g. The San Andreas Fault in California marks the junction of the North American and Pacific plates. Both plates are moving
north-west but at different speeds. Instead of slipping smoothly past each other, they tend to ‘stick’. The pressure builds up until
suddenly the plates move forward and an earthquake occurs.
The measurement of earthquake magnitude (the Mercalli and Richter scales). Focus and epicentre.
Differences between the Mercalli and the Richter scales.
Mercalli Scale
Measured on a scale of 1-12
Measures the intensity of an earthquake
It is an arbitrary scale based on observations of damage
to building and the environment by people.
Richter Scale
Measured on a scale of 1-9
Measures the magnitude of an earthquake
It is a scientific scale based on accurate measurements by
a seismograph.
Explain how earthquakes occur with reference to the epicentre and focus of earthquakes.
An earthquake is the shaking and vibration of the Earth's crust due to movement of the Earth's plates. Earthquakes can happen
along any type of plate boundary. Earthquakes occur when tension is released from inside the crust. Plates do not always move
smoothly alongside each other and sometimes get stuck. When this happens pressure builds up. When this pressure is
eventually released, an earthquake tends to occur. The point inside the crust where the pressure is released is called the focus.
The point on the Earth's surface above the focus is called the epicentre. Earthquake energy is released in seismic waves. These
waves spread out from the focus. The waves are felt most strongly at the epicentre, becoming less strong as they travel further
away. The most severe damage caused by an earthquake will happen close to the epicentre.
The reasons why people continue to live in areas of volcanic and earthquake activity.
People chose to live in areas of volcanic and earthquake activity because they felt that the advantages outweighed the
disadvantages.
3 main types of reasons (SEE) to live in an area:
- Social reasons (people)
- Economic reasons (money)
- Environmental reasons (physical reasons)
Social
Volcanic activity:1- Poverty. Poor people, particularly in LICs, can’t afford to leave. E.g. Mount Merapi in
Central Java, Indonesia, is one of the most active volcanoes in the world. In spite of the risk, about one
million people live within 20 miles of Merapi. 2- People don’t want to leave family and friends. 3- People
believe that the chance of the volcano erupting is very low as volcanoes can remain dormant for hundreds
of years, e.g. Mount Sinabung in North Sumatra province had been mostly dormant for hundreds of years
until its 2014 eruption.
Earthquake activity: HICs: People feel safe due to earthquake proof buildings. E.g. All new buildings in
Japan have to be earthquke proof. Schools with two storeys or more have evacuation chutes which
children can slide down to safety and earthquake drills take place every month .
Economic
Volcanic activity:1-Fertile soils: Although people know that living near the Merapi volcano is dangerous,
they perceive that the risk is worth taking. This is because material ejected from the volcano rapidly
breaks down (weathers) into a fertile soil that is rich in minerals needed to plant growth. The productive
soil, which gives high crop yields, has encouraged intensive farming. 2- Geothermal energy: In Iceland:
Geothermal power facilities currently generate 25% of the country's total electricity production and about
9/10 households are heated with geothermal energy.
Earthquake activity:1- Istanbul, a city of at least 14 million people, is located at the western end of the
North Anatolian Fault. On Aug. 17, 1999, a 7.4 magnitude earthquake hit the cities just east of Istanbul and
over 17,000 people were killed, including at least 300 people in Istanbul's Avcılar district. However, areas
such as Istanbul in Turkey provide many jobs for the locals. 2- In November 2014, 24 workers were killed
and 52 injured in a coal mine fire that followed a shallow 1.6 magnitude earthquake in China's northern
Liaoning province. People accept the risk as they can make a good living from mining.
Environmental
Volcanic and earthquake areas are often beautiful areas 1-which attract tourists. E.g. Mount Etna on the
Italian island of Sicily is one of the most frequently visited active volcanoes on Earth, the total number of
visitors exceeding one million per year. Since 1987 the volcano and its slopes have been part of a national
park. Tourism is one of the prime sources of income for the local population. 2- where people buy houses
despite earthquake risks e.g. Malibu, California.
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The causes and effects of a volcanic eruption: Montserrat, 1997
Explain the causes 1- Volcanic island of Montserrat situated on a destructive plate boundary. North American plate
subducted by the Caribbean plate. 2- This happens because the oceanic North American plate is denser
than the Caribbean plate. 3- Convection currents pull the denser North American plate into the mantle
where intense heat and friction cause the rock to melt. 4- This molten rock is lighter than the surrounding
rock, forcing it to rise through cracks in the rock towards the Earth’s surface forming the island.
What made the
Volcano produced thick lava (andesite) which builds up at the top of the volcano in a dome until it
eruption so
becomes too heavy and the dome collapses. As the dome collapsed huge ash cloud and pyroclastic
catastrophic?
flows
Effects on people
1- 19 people, mostly farmers, killed by a pyroclastic flow as they were in the exclusion zone set up in 1995.
They were too far away to hear the warning sirens. 2- The villages of Farm and Trant were completely
buried by ash flow deposits. 3- Common injuries: severe burns in the feet as a result of walking on ash
deposits, burns to various parts of the body, including burns to the nose and mouth due to breathing in
the hot gases. 4- 100-150 houses destroyed, partially buried or burned by the intense heat or destroyed by
direct impacts of rocks, up to 5m in size.
Effects on the
1-Pyroclastic flows: - flattened thousands of trees. - baked soil hard - many rivers such as the Belham
environment
River were blocked causing flooding. 2-Ash and rock deposits: - a total of 4 square kilometres of land was
covered by the deposits. - formed a thick, broad fan at the mouth of the Paradise River just north of
Bethel.
Prediction and prevention of volcanic eruption and earthquakes
Earthquakes
It is not possible to Justification can be positive or negative or a combination.
predict
Negative
earthquakes.
A variety of methods are being used by scientists to predict earthquakes but they are not completely
Justify this
accurate. Despite sophisticated monitoring equipment it still remains very difficult to pinpoint exactly
statement. (3)
when an earthquake will happen. (1)
Positive
A variety of instruments can be used. A seismometer is an instrument that measures ground vibrations
caused by earthquakes. (1) Seismometers record ground movements which are converted to radio signals.
(1) These signals are transmitted to computers that record the earthquakes 24 hours a day. (1)Tiltmetres
measure tiny changes in the slope angle or "tilt" of the ground.
Education
In Japan, 1 September is Disaster Prevention Day. On this day emergency drills organised by local
governments are held throughout the country.
In the USA, information is given by the Federal Emergency Management Agency (FEMA) on how to
prepare for an earthquake. It states ways to plan ahead. Three of them are: Check for hazards in the
home. Identify safe places indoors and outdoors. - Have disaster supplies on hand.
Planning
In Japan: Buildings are made earthquake proof with the aid of deep foundation and massive shock
Earthquake proof
absorbers that dampen seismic energy. Another method allows the base of a building to move semibuildings
independently to its superstructure, reducing the shaking caused by a quake.
The new San Francisco International Airport uses many advanced building technologies to help it
withstand earthquakes. One of these technologies involves giant ball bearings. The 267 columns that
support the weight of the airport each ride on a 1.5-meter -diameter steel ball bearing. The ball rests in a
concave base that is connected to the ground. In the event of an earthquake, the ground can move 51 cm
in any direction. The columns that rest on the balls move somewhat less than this as they roll around in
their bases, which helps isolate the building from the motion of the ground.
Volcanic eruptions
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Prediction
What can be done
to limit the effects
of volcanic
eruptions?
Most volcanic eruptions are preceded by clear warning of activity from the volcanoes. After the first
seismic activity was detected in 1992 in Montserrat measures were taken to predict if and when the
volcano was going to erupt.
Monitoring the volcano:
Method
Explanation
Sulphur dioxide
Sulphur dioxide is produced by magma in large quantities and so its
measurement
emissions are monitored as they are a good indicator that magma is near
the surface
Seismometers
As the magma collects in the magma chamber, its movement creates small
earthquakes and tremors. Seismometers close to the volcano record ground
movements. E.g. In Montserrat, 14 seismometers used to record ground
vibrations.
Tiltmeters
Tiltmeters measure tiny changes in the slope angle of the ground e.g. Five
electronic tiltmeters record ground movement in Montserrat.
1- Hazard map: Three Zones have been defined in Montserrat after the first eruption in 1995 including an
exclusion zone in the south with no admittance except for scientific monitoring and National Security
Matters.
2- Three ways to divert or stop lava flows:
(i) Divert lava flows by detonating explosives, for instance in 1996, the Italian army used explosives to
block a lava flow from Mount Etna threatening villages below.
(ii) Construction of earth walls can deflect lava. Used successfully in July 2001 eruption of Mount Etna to
protect tourist facilities.
(iii) Spraying large volumes of water can cool an advancing flow. Two small lava flows from Mount Etna
were cooled by using this method.