AQA A Revision Guide – The Restless Earth

1. Why do we get volcanoes and earthquakes? It is all to do with PLATE TECTONICS. This is a relatively new theory (only

really been around for 100 years and been universally accepted for the last

40-50 years) and it looks at the structure of the Earth and examines how

vast areas of rock or PLATES can move. To understand this theory you need

to have an idea of the structure of the Earth. The diagram on the right shows

this. The crust floats on the semi-molten rocks of the Mantle. The high

temperatures in the Mantle result in convection currents that enable the

crust to move.

2. Is the Crust just one piece? NO! It is split up into a series of

PLATES which fit together like a giant

jigsaw. The map on the left shows this-

note the underlying land masses in

blue. You can see that we are on the

Eurasian plate!

3. Are all these plates made of the

same material?

No! Some are made of continental

crust and some are made of oceanic crust. It is important to understand this as this will help you understand what

happens at the different plate boundaries. This table summarises the differences:

4. So what happens when the plates meet?

It depends! As I have already said it is determined by the type of crust involved, but also in terms of the direction in

which the plates are moving. The different types of plate boundary are discussed on the next page.

TIP: AT THIS POINT GO TO THE ‘s-cool’ SITE http://s-cool.co.uk/gcse/geography/tectonics/plate-boundaries.html

Data /

Type Oceanic crust Continental crust

Crust

depth shallow, 5 - 12 km

deeper, average 40km, 75km beneath young

mountain range

Age Young rocks, less than 200 Million years old Older, generally over 1500 Million years

Density 3.0 x 103 Kg m-3 DENSE! 2.6 x 103 Kg m-3 NOT AS DENSE AS OCEANIC

CRUST!

Rock

type,

geology

Mainly basalt Range of granites with thin covering of

sedimentary rocks

Destructive (Compressional) Plate Boundary (volcanoes &

earthquakes) - These cause violent volcanoes and

earthquakes, as well as deep-ocean trenches and fold

mountains. An oceanic plate and continental plate move

towards each other. The denser oceanic plate dives under

the lighter continental one, creating a deep ocean trench.

As the oceanic plate goes deeper into mantle it melts in the

subduction zone, due to friction and the increased

temperature. The newly molten rock is lighter that that

which surrounds it, so it will rise towards the surface and

cause volcanoes on the Earth's surface. The continental

crust is crumpled by the collision of the two plates creating

Fold Mountains.

Constructive (Tensional) Plate Boundary (volcanoes- some

earthquakes) - Although often not as violent as those on

destructive plate boundaries, volcanoes and earthquakes do occur

on constructive plate boundaries. They also cause mid-ocean

ridges to form. Two plates move away from each other. Molten

rock (magma) rises from the mantle to fill the gap between the two

plates. This forms a mid-ocean ridge. Volcanoes can also form here,

along the edges of the plate boundary, due to the rising magma.

These volcanoes are called shield volcanoes.

Conservative Plate Boundary – (no volcanoes, but severe earthquakes)

The main effects of a conservative plate boundary are earthquakes, which can

be fairly violent and frequent. Two plates slide past each other, without

creating or destroying any land. As they move past each other they often get

stuck, building up great pressure until finally they jolt past each other. This

sudden movement is what causes earthquakes.

The best-known example of a conservative plate boundary

is the San Andreas Fault, where the North American and Pacific plates are actually moving in the same direction, but at a different speed.

Collision (Compressional) Plate Boundary - (no volcanoes, but frequent earthquakes)

Where two continental crusts collide neither can sink. Instead they push into each other forcing material to be folded up into huge mountain ranges. Often this movement and pressure can cause earthquakes, but no volcanoes will occur on these boundaries.

The best example is found where the Indian plate collided with the Eurasian plate to form the Himalayas.

5. How do the plates move?

The crust sits on top of the mantle. Within the mantle, heat

rises from the core of the earth and this causes convection

currents (the red arrows on the diagram). These currents move

about as quickly as our nails grow, but it enough to get the

plates to move. This movement sets off volcanoes and

earthquakes.

6. Where are the Plate boundaries found?

7. What landforms are found along the plate boundaries?

Fold mountains, ocean trenches, shield volcanoes and composite

volcanoes.

8. What are Fold Mountains?

The diagram on the right shows this. Sedimentary rocks form in a

geosyncline. The sediment turns into rock (sedimentary rock such as

limestone). The plates begin to move and squeeze the rocks. This

causes the rocks to fold producing anticlines (upward folds) and

downward folds (synclines). Examples include the Alps, Rockies and

Himalayas.

9. What are ocean trenches?

These are found along destructive margins. As the oceanic crust sinks below the continental crust, during the process of

subduction, a deep trench is created. You can see this on the diagram of a destructive plate boundary below (right). A

famous example is the Marianas Trench in the Pacific Ocean.

10. What are

volcanoes and

where are they

found?

Volcanoes are created

where magma escapes

through a vent, which is a

crack in the Earth’s crust.

Volcanoes are found at

Destructive

(Compressional) and

Constructive (Tensional)

plate boundaries – see

diagrams on the left.

11. What are the two types of volcano that I need to know?

SHIELD and COMPOSITE volcanoes. The differences in their characteristics are due to their location and the type of

magma feeding them. Shield volcanoes have BASIC LAVA, whereas Composite volcanoes have ACID LAVA. Basic lava

is runny and therefore doesn’t allow pressure to build up producing effusive eruptions; acid lava is the opposite as it

is rich in silica and therefore is very sticky. Pressure can build up and eruptions are explosive. The diagrams below

summarise the differences between the two.

12. Using landforms at plate boundaries: Fold Mountain case study – Alps (see your

A3 sheet)

Where: Mountains along the borders of France/Italy/Switzerland/Austria

Why: The collision between the African plate and the Eurasian plate. Limestone

sediments formed in the Geosyncline were folded up into the peaks of the Alps.

When: The last 60 million years. However, Glaciation in the last 100,000 years has

sculpted the landscape further with frost-shattered peaks and deep U-shaped valleys.

How has the landscape been used (how have people adapted to the landscape)?:

Farming, Hydroelectric Power, Tourism. The valleys are used for roads and railways. A

more detailed discussion is provided below:

Human use Details and how people have adapted

Tourism Winter Sports – glaciated valleys have produced steep slopes. The height has enabled a long snow

season. Resorts such as La Plagne (see Glaciation revision guide) and Chamonix have profited from

this.

Summer Sports – Hiking, mountain biking, abseiling etc. Warm temperatures ideal for this. The

Montenvers train taking people to the Mer De Glace near Chamonix is an example of this.

Hydroelectric

Power

The steep/deep U shaped valleys created by glaciation provide perfect conditions for the

constructed of reservoirs. Power is created by the release of the water. This has enabled industry to

set up in the valley floor.

Farming Pastoral (animals) on the valley slides. These animals are placed on the High Alp (saeter) during the

summer months and then moved down the slopes in the winter. This is called ‘transhumance’. Dairy

farming is crucial; pipes send milk down the mountainside in summer to the dairies. Soils are deep

and fertile in the valley floor, so arable (crop) farming is done.

13. What are the volcanic threats?

These can be split into primary and secondary effects. In fact, the same can be said for the effects

of earthquakes

1. Primary effects- i.e. things that happen directly as a result of the eruption in the short term.

Things like:

The volcanic blast- power of the blast can blow things over- this happened at Mount Saint Helens, where all the trees near the volcano were flattened.

Building destroyed by fires

Livestock and crops destroyed

Pyroclastic flows - occur where the ash cloud that has been ejected into the air loses its ability to stay suspended and comes falling back to earth. This resultant clouds of searing hot ash and gases (often 600°C) rush down the sides of the mountain (often in valleys) at great speeds.

Lava flows - Lava clearly is a hazard due to its ability to destroy most things in its path. However, lava flows can be diverted (Heimaey, 1973 in Iceland) and people are normally able to get out of the way!

Volcanic Bombs- large boulders and stones (pumice) that is hurled out of the volcano during an eruption

2. Secondary effects- i.e. things that happen as a result of the eruption. These may not be instantaneous and the

effects may actually be felt in the much longer term

• Lahars/Mudlfows - Mudflows and lahars are closely associated with volcanic activity. Rain brings soot and ash back to the ground, which becomes a heavily saturated mudflow when combined with melted snow and ice (perhaps from the peak of the volcano). The sediment that is picked up turns into the hazardous mudflow.

• Changes in climate • Food/ water supply disrupted • Homelessness • Businesses forced to close and realted unemployment • Insurance claims

14. Case Studies of Volcanic Eruption: Montserrat

Montserrat is an island is situated in the Caribbean and 12 miles long x 7 miles wide. Much of the population is poor and relies on farming. Before the eruption of 1995, over 12,000 people lived on the island but less than 5,000 do today.

The volcano (Soufriere Hills) was formed as a result of the subduction of the Atlantic plate under the Caribbean plate – a destructive plate boundary.

The Soufriere Hills volcano started erupting in 1995 for the first time in 350 years. However, the big eruption came in June 1997. It caused:

Huge pyroclastic flows over 500⁰C – 3 main ones came down ‘Mosquito Gut’. The final one killed farmers who thought they were safe

50% of Montserrat uninhabitable

Infrastructure destroyed (airport etc)

19 died and many fled the island

Farms and houses abandoned. The capital, Plymouth, was destroyed and abandoned.

Montserrat’s economy was devastated (lack of tourists did not help)

What was the response?

What is the future for Montserrat? Tourists began to come back to the island by 2005. Little Bay has developed as the new capital, with the harbour helping trade. However, the volcano still rumbles on........

15. What is a Supervolcano?

A volcano that can erupt on a much bigger scale than a normal volcano. They can be quite difficult to identify as they are so large, but crucially do not have a recognisable peak or cone. For example, they had to use images from space to check to see if Yellowstone was in fact a ‘Caldera’ (Greek for ‘cauldron’).

16. How do supervolcanoes form?

They are found at ‘hot spots’ where magma his risen towards the surface of the crust. They don’t necessarily need to be on a plate boundary. A huge magma chamber develops inside the Earth and more rocks melt under the intense pressure. This provides the volcano with a huge amount of magma that can be erupted.

17. How big are they? – HUGE!!

See the diagram on the right. Look how small the circle representing the size of the Mount Saint Helens eruption is compare to the VEI 8 eruptions. VEI stands for Volcanic Explositivity Index. Like the Richter scale it is lograithmic – each point on the scale is 10x greater than the previous. To be a VEI 8 – super eruption – 1,000km³ needs to be ejected.

18. Where is the next super-eruption suggested to be?

Many believe that Yellowstone in the USA could erupt again. ‘Utter devastation’ is how professor Bill McGuire describes the effects of a super-eruption. He suggests the following could occur:

Virtually all life within 1,000km would be killed by ash and lava flows

Magma thrown 50km up into the atmosphere

The amount of ash in the atmosphere would block out solar radiation and result in freezing temperatures. Crops wouldn’t grow, economies would collapse and it is believed that society would not survive.

19. What are Earthquakes and where are they found?

Earthquakes occur along faults, which are large cracks in the earth's crust. Most of these are associated with the larger plate boundaries, along which the largest earthquakes usually occur. They are caused by the sudden jerking movements of the fault, either laterally or vertically, and are almost impossible to predict.

20. What key terms will I need to know?

Focus-The actual place where the fracture within the earth’s crust takes

place

Epicentre-The point on the earth’s surface directly above the focus- the

worst place to be in an earthquake!

Seismic Waves-The waves/energy that is produced in an earthquake

Liquefaction- the loss of strength of the ground during earthquake

shaking. The ground becomes ike quicksand!

21. How are earthquakes measured? MERCALLI & RICHTER

They are measured on the Richer Scale. What is special about it? - A scale of magnitude (strength and size of it)- it is special as it is a logarithmic scale. This means that each point on the scale is x10 more powerful than the previous point (see diagram on the right) . Earthquakes can also be measured on the Mercalli scale. This is a visual

obervation of the damage caused by the earthquake and is not reliant on

measuring the magnitude (strength) of the earthquake with scientific

instruments.

22. Where are earthquakes found?

They occur in long belts, as shown above

They follow plate margins

However, some are found away from plate boundaries – minor fault lines are responsible for these

23. What are the effects of Earthquakes?

Primary effects – (All occur due to the shaking of the ground) include: 1. buildings collapsing 2. roads and bridges being destroyed 3. railway lines being buckled

Secondary effects - are the subsequent effects of the earthquake, and can be even more devastating then the

primary ones.

o Fires: usually from ruptured gas lines. This was the main cause of death and damage after the San Francisco earthquake in 1906.

o Tsunami: They can travel very quickly across entire oceans, before engulfing land 1000's of miles away. I’m sure you all witnessed the footage of the Indian Ocean Tsunami on 26/12/2004.

o Landslides can often be triggered by earthquakes, causing huge amounts of material to be moved very quickly. This is actually what occurred just before the volcanic eruption on Mt. St. Helens. They are most likely to occur where the land is steep, saturated or weak.

o 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 (LEDCs) 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.

24. Earthquake 1: LEDC – Haiti

When: 12/1/2010

Where: Haiti, with the capital Port-au-Prince hit very badly. Epicentre

13km underground, 25km from port-Au-Prince.

Why: Destructive plate boundary where the Atlantic plate subducts the

Caribbean plate (same as Montserrat), but the earthquake was actually

along a fault line cause by this pressure. In fact, the earthquake was a strike-slip event, which is like what happens at a

Conservative margin.

What were the effects?:

Haiti Primary Haiti Secondary

220,000 killed (approx)

Port damaged, roads blocked

8 hospital collapsed or damaged

200,000 houses damaged and about 1.3 million people displaced

2 million without food and water

Looting – police force collapsed

Damage to port and road prevented medical supplies being distributed effectively

Disease in tented camps. Mass graves as people were left on the streets

Power cuts

What was the response?:

Short term: The port was destroyed and the airport couldn’t cope. Supplies

weren’t distributed effectively. American engineers tried to clear the worst

of the debris in the port so that ships could unload the aid. The USA also sent

10,000 troops and £100 million in aid. Bottled water and purification tablets

were provided.

Longer-term response: Rebuild homes to a better standard. Rebuild port.

Still reliant on overseas aid.

25. Earthquake 2: A richer country – Chile (some have

studied Kobe – read on!!)

Chile is not the richest, but it is more affluent than Haiti. When

hit by an 8.8 earthquake its impact was far less than the 7.0 at

Haiti.

When: 27/2/2010

Where: 115km from the city of Concepcionon the coast of

Chile (see maps)

Why: Nazca plate subducts the South American plate at a

destructive plate boundary.

What were the effects?:

Chile Primary Chile Secondary

500 killed; 12,000 injured

500,000 homes damaged

Bridges and roads destroyed

Santiago’s airport slightly damaged

Power and water supplies lost

Tsunami

Fire at a chemical plant near Santiago

What was the response?:

Short term: They responded very quickly as they are prepared for such events. The President insisted on rapid analysis

and asked for help from countries within a few hours. The route 5 highway was temporarily repaired the day after the

earthquake. Aid was therefore able to get where it was needed. 90% of homes had their power restored within 10 days.

Longer-term: Housing reconstruction plan for the 196,000 households affected. The economy was quite healthy, so they

weren’t reliant on foreign aid.

Some students have studies Kobe (MB’s classes)

Notes provided below:

• Kobe, Japan. January 1995 - The earthquake occurred at

5.46am on the 17th January 1995. It measured 7.2 on the

Richter Scale and lasted 20 seconds.

• - Kobe lies on the Nojima fault, a destructive boundary,

where the Philippine plate dives below the Eurasian plate.

This plate boundary is the reason for Japan’s existence but

also means that there is a constant earthquake threat.

• - Kobe was unlucky in the sense that the focus of the

earthquake was very close to the surface and the epicentre

was right beside the city.

• - Primary effects included a death toll of approximately 5,500, with another 30,000 injured and 250,000 made

homeless. Over 100,000 buildings collapsed. Infrastructure damage included a 1km stretch of elevated road,

numerous railway bridges, and 120 of the city’s 150 quays were damaged (for ships).

• - Secondary effects included the fact that electricity, gas, water and sewage systems were all hugely disrupted.

Emergency services found it very difficult to get into the city due to the massive destruction of the roads. Many

temporary shelters were required, as well as food and medicines. Cold weather meant that diseases spread

quickly.

• - A week after the earthquake fires still were burning, 2 million homes still were without power and 1 million

were without water. The fires destroyed over 7,000 more homes. Hundreds of aftershocks, 74 strong enough for

people to feel, meant people were too afraid to return to their homes for weeks after the event. It cost about

$100 billion in damage.

• - Tough new laws, building codes and emergency plans were brought in after criticism of the Japanese

Government. Work is continuing to try to predict future earthquakes, but as yet there is very little way of giving

any significant warning time.

26. Why were the impacts/ response different between the two earthquakes? Yellow highlighted stuff relates to

Kobe

PREDICTION, PREPARATION and the ability to ACT.

Prediction is a realistic proposition for volcanoes, but not so for earthquakes. You can work out roughly where they

could happen but no more. You have already seen that Chile (and Japan-Kobe) was far better at dealing with their

earthquake when it happened than Haiti, but why was this?

Chile experienced the largest ever earthquake in 1960 (9.5),

so they didn’t want to take any chances. Buildings built after

1960 had reinforced concrete columns with a strengthened

steel frame. Conversely, Haiti hadn’t had a devastating

earthquake in living memory, so nothing had been done

about building for earthquakes. The buildings in the capital

were poorly built and collapsed (pancaking?). Japan had also

experienced earthquakes – Tokyo 1923 – so had built to

protect against them.

Government is far stronger in Chile (and Japan). They

introduced a plan in 2002 to set out responsibilities for local

authorities in response to an earthquake. It worked well in

2010. However, Haiti has a weak government and had made no such preparations. Japan has a hazard drill day on

the 1st September and people are well educated about the volcanic/earthquake/tsunami risk.

Anti-disaster drills organised by the government take place every few months. This education has prepared people

well; they know what to do in an earthquake – not so in Haiti. As has been mentioned, this is done in Japan.

Money seems to be the issue, but you can still prepare for earthquakes by using ‘appropriate technology’ to build

houses - i.e. bamboo houses in Costs Rica = cheap, flexible and grows quickly. Older building s can be ‘retro-fitted’ too.

27. What are tsunamis?

A series of large waves that from when an earthquake

occurs on the sea bed. The permanent movement up

or down of the sea bed displaces a huge volume of

water and large waves are formed as a result.

28. Why are they so big?

In deep water, the waves move at high speed (no

friction), but in shallower water the wave slows down

and the waves behind still keep coming in. The water

builds up to a great height as a result.

29. Case Study: Boxing Day Tsunami, Indian Ocean.

When: 26/12/2004

Why: Subduction of Indo-Australian plate under Eurasian plate.

9.3 earthquake.

Impacts:

70% of coastal population in Indonesia missing

400,000 people lost jobs in Sri Lanka due to loss of

tourism and fishing boats

Thailand’s tourist industry suffered greatly. 1,700 foreign

holiday makers died

In India, arable land was destroyed by the salt water

About 290,000 people died in total

What was the response?:

Clean water, food, tents and plastic sheeting arrived as aid

$7 billion donated worldwide for the affected countries

£330 million donated by people of the UK

UN World Food Programme provided food aid for more than 1.3 million people

What can/was be done in the long term to protect people from a similar event?

A Tsunami warning system was installed in the Indian Ocean; warnings

are sent to each country and these are relayed on radio, TV and email.

Warning bells/loudspeakers on mosques will also provide warnings.

However, there are natural ways of providing protection through

allowing mangroves to develop. These mangrove swamps absorb wave

power, but many have been cleared to make room for hotels. The

restoration of these will provide some protection.

END OF SECTION