Geography tectonics topic Edexcel GCSE

What is plate tectonics?

Plate tectonics is the theory that explains the way plates on the Earth’s crust move and how and why volcanoes and earthquake occur.

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What are tectonic plates?

The Earths crust is divided into 7 major and a number of minor plates, these move in different directions and are powered by convection currents in the semi-molten mantle.

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What is the general distribution of earthquakes and volcanoes?

Earthquakes and volcanoes generally have a linear distribution as they are found at plate boundaries.

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What are the differences between the distribution of earthquakes and volcanoes?

Earthquakes occur at ALL plate boundaries but volcanoes are only found at divergent (constructive) and convergent (subduction zones). The biggest concentration is around the Pacific. Hot spots are an exception and are found in the middle of plates w

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Name and describe the different types of plate boundary

Divergent or constructive – moving apart. Convergent (includes both subduction zones and collision zones) – moving towards each other. Conservative or passive – sliding past each other.

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Name and describe the two different types of crust

Continental – older and lighter (less dense) so is not subducted. Oceanic – newer (created at constructive margins) and heavier (more dense) so sinks beneath continental crust.

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Describe and give an example for divergent (constructive) boundaries

These are where 2 plates move apart from each other (convection currents moving away from each other). At the Mid-Atlantic Ridge the North American and Eurasian plates are moving apart at about 2.5 cm per year. Molten rock (called magma) rises up t

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How is a ridge formed at divergent (constructive) boundaries (give example)?

The rising magma pushes the crust upwards to form a ridge e.g. the Mid- Atlantic Ridge. As the plates pull apart the centre drops down to form a central rift valley.

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How are submarine volcanoes formed at divergent (constructive) boundaries?

In some places submarine volcanoes form. This happens where the rising magma is able to break through the crust and erupts at the surface forming shield volcanoes.

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Why do earthquakes occur along divergent (constructive) boundaries?

There are frequently earthquakes along tensional plate boundaries. This happens because the plates do not move apart smoothly, tension gradually build up until they suddenly move. It is the sudden movement that releases great amounts of energy and

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Name the three types of convergent (destructive) margins and the types of plate involved in each

Three types of convergent boundary: (a) subduction zone – oceanic & continental crust (b) subduction zone oceanic and oceanic crust (c) collision zone – continental & continental crust

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Describe give and an example for a subduction zone (oceanic to continental) at a convergent (destructive) margin

The oceanic crust is forced down (subducted) beneath the continental crust e.g. NAZCA plate beneath South American Plate

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Explain the presence of trenches at a subduction zone (oceanic to continental) at a convergent (destructive) margin

Trenches are formed where the edges of the plate are warped downwards. Earthquakes - occur in the Benioff zone where plates lock together and then suddenly move releasing huge amounts of energy. Fold mountains - formed on inland side from pressure o

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Explain the presence of earthquakes at a subduction zone (oceanic to continental) at a convergent (destructive) margin

Earthquakes occur in the Benioff zone where plates lock together and then suddenly move releasing huge amounts of energy.

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Explain the presence of fold mountains at a subduction zone (oceanic to continental) at a convergent (destructive) margin

Fold mountains are formed on inland side from the pressure of collision e.g. the Andes in South America.

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Explain the presence of volcanoes at a subduction zone (oceanic to continental) at a convergent (destructive) margin

Volcanoes (composite cone, very explosive – acidic magma) are created from magma created when subducted plate melts.

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Describe give and an example for a subduction zone (oceanic to oceanic) at a convergent (destructive) margin

When 2 oceanic plates move together e.g. the Pacific and Philippines plates, the one that is moving fastest is subducted.

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What are the similarities/differences between an oceanic to oceanic and an oceanic to continental subduction zone

The same processes except that there are no fold mountains but a volcanic island arc is formed e.g. the Philippines. Volcanic eruptions and earthquakes can be particularly violent here as magma is very acidic.

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Describe give and an example for a collision zone (continental to continental) at a convergent (destructive) margin

They are the same density so neither subducted. Plates crumpled and forced up, create huge fold mountains e.g. Himalayas and cause earthquakes when pressures is suddenly released e.g. Kashmir earthquake 2005.

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Describe give and an example for a conservative boundary

Plates move parallel to each other e.g. the Pacific and North American Plates along the San Andreas fault.

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Explain the occurrence of earthquakes at a conservative boundary with an example

Earthquakes occur when plates lock together, pressure builds up and then snap past each other releasing the energy e.g. San Francisco 1989. No volcanoes as no subduction or rising magma.

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What is an earthquake?

An earthquake is the violent shaking of the ground caused by movements in the Earth’s crust.

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Describe what and were the focus and the epicentre are

The focus is the location within the Earth (usually on a fault line) where the earthquake happens. The epicentre is the point on the surface vertically above the focus –most damage is usually experienced here.

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Why do earthquakes happen?

Earthquakes occur where there is a sudden release of energy as one plate suddenly moves past another along a fault line, energy has built up as the plates have been locked together.

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What are the two main measurements used to measure earthquakes?

The Richter scale and the Mercalli scale.

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Explain and describe the Richter scale

Measured using a seismometer - tells you the amount of energy released (magnitude). Recorded using Arabic numerals. Each point on the scale, I – X represents a quake 10 times more powerful than one at the previous level.

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Explain and describe the Mercalli scale

Measured by observation – looking at the effects. It uses Roman numbers. Based on the observed damage that has been done, measured on a scale 1 – 12.

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What did the San Francisco Earthquake in 1989 measure on the Richter scale?

Measured 6.9 on Richter scale

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What were the primary impacts of the San Francisco Earthquake in 1989?

Section of Bay Bridge collapsed, Section of Nimitz Highway collapsed

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What were the secondary impacts of the San Francisco Earthquake in 1989?

Gridlock on roads, Fires in Marina District

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What were both primary and secondary impacts of the San Francisco Earthquake in 1989?

63 dead, Several thousand lost homes, Several thousand injured

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What were the short term responses in the San Francisco Earthquake in 1989?

Rescue teams rescued people, Fire service put fires out, Emergency centres set up for homeless, National Guard deployed

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What were the medium term responses in the San Francisco Earthquake in 1989?

Dangerous buildings demolished, People re-housed, roads etc repaired

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What were the long term responses in the San Francisco Earthquake in 1989?

New fire fighting system introduced, More research and monitoring, More buildings retro-fitted, New building standards – better levels of earthquake proofing, Insurance premiums went up, Some people (not many) moved away

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In any natural disaster what factors are important in deciding how much damage and how many casualties are created?

1) Wealth of the country affected 2) The quality of the buildings 3) The amount and quality of rescue services 4) The amount of warning given 5) Communications 6) Population density 7) Time when it happened 8) How big the hazard was

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What are the three classifications for volcanoes?

Volcanoes can be classified as: Active – erupting. Dormant – erupted in the past and likely to do so again in the future. Extinct – will never erupt again

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Where are volcanoes found?

1) Divergent boundaries. 2) Subduction zones (both types). 3) Hot Spots

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Explain and give an example for the occurrence of volcanoes at divergent boundaries

e.g. mid oceanic ridge where North American and Eurasian plates are pulling apart – convection currents rising and so brings magma to surface. Shield volcanoes (‘gentle’ eruptions).

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Explain the occurrence of volcanoes at subduction zones

subducted plate is melted in mantle so produces more hotter magma which rises to surface. Composite cone volcanoes (very explosive).

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Explain the occurrence of volcanoes at hot spots

plumes of magma rise through the crust (shield volcanoes – ‘gentle eruptions).

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What are the two types of volcano that can be formed and what are the features of each?

Composite cones = violent eruptions - pyroclastic flows, ash, volcanic bombs, dust, poisonous gasses etc. Shield volcanoes = gentle eruptions – thin runny lava.

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What is the location of Montserrat?

Island in the Caribbean Sea.

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When did the volcanic eruption occur in Montserrat?

1995 – 1997

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What were the causes of the eruption at Montserrat?

On a subduction zone where the North American Plate is being subducted under the Caribbean plate (oceanic/oceanic) Subducted plate melts, produces new, very hot magma that rises. Magma here is acidic so thick and sticky = explosive eruption

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What were the features of the major eruptions between 1995 and 1997?

Pyroclastic flows thick sticky lava volcanic bombs Huge clouds of ash and dust – layers of 2-3m thick on the ground

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What were the impacts of the eruptions on the people?

1) 23 people were killed and many injured in the 1997 eruptions. 2) Montserrat had a population of 11,000 of these 4,000 moved to the north of the island, the rest were evacuated from it. Many evacuees in the north had to live in makeshift shelters.

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What were the impacts of the eruptions on the economy?

1) The tourist industry stopped. 2) most of the villages and farms in south of the island were destroyed by pyroclastic flows or smothered in ash. 3) Plymouth was evacuated, most of the Islands industry, its port, schools and hospitals were there.

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What were the impacts of the eruptions on transport?

1) The Islands airport was closed in 1995 and so the island was cut off from air travel 2) Most of the roads in the south of the island were buried by ash and closed.

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What were the impacts of the eruptions on buildings?

1) Many buildings were destroyed by Pyroclastic flows or volcanic bombs. Whole villages and much of the capital, Plymouth buried by ash

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What were the impacts of the eruptions on the natural environment?

1) New land created in the sea by pryoclastic flows 2) Some valleys filled with Pyroclastic flow deposits and rivers blocked by so flooded. Pyroclastic flows destroyed thousands of trees.

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What were the responses to the eruptions?

1) Plymouth was completely evacuated 2) 4,000 moved to the north of the island, the 7,000 were evacuated from it 3) £50 million of Aid to redevelop the north of the Island and to help refugees offered 4) Montserrat Volcano Observatory set up.

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What reasons do people have to remain in areas of volcanic activity (economic)?

Farming: Volcanic soils very fertile. Plains of Campania (including the slopes of Mt Vesuvius (Italy) In Kenya around Lake Naivasha. Mineral extraction: Silver & copper etc - Andes. Geothermal power: Iceland and Kenya

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What reasons do people have to remain in areas of volcanic activity (environmental)?

Tourist attractions (also partly an economic reason): Mt Etna, Mt Vesuvius - Italy. Natural beauty: Some places e.g close to Mt St. Helens in Washington State

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What reasons do people have to remain in areas of volcanic activity (social)?

Poverty: No choice where to live - LICs. Religious beliefs: Gods live in the volcano. Don’t see the problem: Some people think that their is little chance of eruption and so decide to live where they are. Particularly true if it's been dormant.

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What reasons do people have to remain in areas of earthquake activity (economic)?

Tourism: Live in places such as beach areas of southern California despite the risk. Mineral extraction: opportunity to profit from

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What reasons do people have to remain in areas of earthquake activity (environmental)?

Natural beauty: E.g. Malibu, California

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What reasons do people have to remain in areas of earthquake activity (social)?

Technology: Many people in HICs feel safe in earthquake zones because of modern earthquake proofing of buildings. E.g. In Japan, Don’t see the problem: think it wont happen to them.

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What methods can be used to predict volcanic eruptions?

Gas measurements, surface deformation, tremors

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How are gas measurements used to predict volcanic eruptions?

Sulphur dioxide is a major volcanic gas that is often monitored using COSPEC (correlation spectrometry) sampling.

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How is surface deformation used to predict volcanic eruptions?

when magma rises the increased pressure changes the shape of the volcanoes’ surface, this can be measured using tilt meters.

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How are tremors used to predict volcanic eruptions?

Rising magma will often cause small earthquakes (called harmonic tremors) – monitored using seismometers.

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What three techniques can be used to stop or divert lava flows?

Using Explosives. Constructing Earth Walls and digging diversion channels (ditches). Spraying flows with large volumes of water

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What methods can be used to predict earthquakes?

Examining past seismic records. Measuring radon gas emissions. Ground water. Levelling and laser reflectors. Electrical discharges. Monitoring animal behaviour.

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How is examining past seismic records used to predict earthquake activity?

There may be patterns in where earthquakes occur – so by studying where they have been jt might be possible to predict where the next one will be

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How is measuring radon gas emissions used to predict earthquake activity?

Recent studies have shown a correlation between emissions of radon gas and seismic events. It seems that the rate of release increases when the rocks are fractured by deformation.

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How is ground water used to predict earthquake activity?

It has been shown that the level of water in wells rises and falls in response to stresses in the earth prior to earthquakes.

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How is levelling and laser reflectors used to predict earthquake activity?

They can be used to measure movements across fault lines. They may reveal sudden changes in movement – or the lack of it which might suggest a build up of pressure prior to a quake.

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How are electrical discharges used to predict earthquake activity?

There is some evidence that electromagnetic disturbances in the atmosphere directly above areas about to experience earthquakes might be detected

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How is monitoring animal behaviour used to predict earthquake activity?

Animals are able to detect low frequency sounds that occur before an earthquake. It has been noted that there have been significant changes in behaviour prior to quakes.

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How can be done to help protect against earthquakes?

Train and educate, updated building design and earthquake proofing, planning laws, well trained and equipped emergency services.

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How can education and training help to protect against earthquakes?

Drills and other training can be used to teach inhabitants where to go that is safest, open spaces etc. e.g in Japan and USA

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How can building design help to protect against earthquakes?

Earthquake proofing of new buildings and retro-fitting of old ones make it less likely for the buildings to collapse. E.g. TransAmerica Tower, San Francisco, Taipei 101 in Taiwan.

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How can planning laws help to protect against earthquakes?

Preventing new buildings being built on or close to fault lines so fewer people live near the areas of high risk.

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How can well trained and equipped emergency services help to protect against earthquakes?

Supplies of emergency materials (food, clean water etc) can be brought in quickly, citizens can be taken to safety and the injured can be treated quicker to reduce the chance of death

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Geography tectonics topic Edexcel GCSE

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