The crust is the outer layer of the earth and is relatively thin. The crust is split into plates of varying size and at plate margins it is most liable to move. The slabs of crust float on the semi-molten upper mantle. The heat from the core drives convection currents of the magma in the mantle which then creates convection currents which sometimes moves the plates together or moves them apart.
Continental (Sial) plate:
* A much older plate in comparison with the Oceanic (Sima) plate
* Less dense
* Cannot sink
* Granitic/acidic rocks
* Deeper and denser, around 30km.
Oceanic (Sima) plate:
* A much newer plate in comparison with the Continental (Sial) plate
* Much denser – will subduct
* Alkaline rock, low silica content
* Can be renewed and destroyed.
Types of plate margin:
Destructive (compressional) plate margins:
Convection currents in the mantle cause the plates to move together, if one plate is oceanic whilst the other is continental crust, then the denser Oceanic plate will subduct under the lighter continental plate. The Oceanic plate is then destroyed form magma (can create grey volcanoes due to the impure magma and the friction can also cause earthquakes).
If two continental plates meet each other, this will form a collision boundary which they will collide and will ‘buckle’.
Constructive plate margins:
This is when plates move apart. Magma is able to force its way through the cracks and surface to form volcanoes, and new land is formed this way, a mid ocean ridge is formed through this.
Conservative plate margins:
The plates are sliding past each other. They are moving in similar directions, but at slightly different angles and speeds. As one plate moves quicker than the other plate, they will get stuck and the build-up of pressure will cause them to release which will cause an earthquake.
There was an earthquake on the San Andreas Fault in 1906 where the earthquake was 7.9 on the Richter scale and 3,000 people were killed and 225,000 people were homeless. The San Andreas Fault is between the Nazca and the North American plate.
Fold mountains and ocean trenches:
Young fold mountains are the highest mountains in the world. There are fold mountains in central Asia including Everest. Ocean trenches form some of the deepest parts of the ocean, and both these landforms occur primarily at plate margins.
Sediment can be collected on the ocean floor, and as two continental plates collide, the sediment is forced up to create a fold mountain.
Ocean trenches form at destructive plate margins when the oceanic plate subducts under the continental plate, where there is the region of the ‘deepest’ waters in the ocean.
* They form at destructive, mainly subduction zones.
* ‘Grey’ clouds as it is impure lava from the molten Oceanic plate.
* Explosive as the vents can block, and it has high viscosity.
* Pyroclastic flows.
* Forms clouds of ‘tephra’ – ash.
* Has steep slopes and is formed of layers of ash and lava.
* They form at constructive plate margins.
* Less dangerous, ‘red’ volcanoes.
* Lava extrudes through fissures and cracks.
* Frequent eruptions – low gaseous content so is not explosive.
* Pure molten magma with a low viscosity.
Fold Mountains case study:
The Andes are a range of young fold mountains which formed 65 million years ago and are 7000km long.
Farming – In Bolivia, many subsistence farmers grow a variety of crops on its steep slopes, including potatoes. Terracing gives many advantages to farm in this harsh environment, as the flat areas retain water and also limits the downwards movement of the soil. Cashcrops like soybeans, rice and cotton are too grown.
Llamas have carried materials for irrigation and people who settle in Machu Picchu rely on llamas to transport materials. They can carry over 25% of their body weight, and the mining businesses have often relied on them. Males are used for transport while females are used for meat, milk and their wool for clothing and rugs.
Mining – More than half of Peru’s exports are from mining. The Yanacocha gold mine is the largest gold mine in the world, it is an open pit and daily dynamite blasts loosen the rocks. The nearby town of Cajamarca has grown from 30,000 to 240,000 in 2005 as it brings along a lot of jobs, but crime rates too have increased.
Hydroelectric power – The steep slopes and narrow valleys which limit farming, it is an advantage for hydroelectric power generation. Narrow valleys can more easily be dammed and the melting snow in spring increases the supply of water.
Tourism – There are many natural attractions in the Andes like the mountain peaks, volcanoes, glaciers and lakes. Some attractions show how people have settled in these inhospitable areas like the remains of the Machu Picchu. The Inca trail shows the Inca ruins, various different species of orchid and birds.
Shield volcano – LEDC
When- 17th January 2002
Where- Nyiragongo volcano in the Democratic republic of Congo.
Why- A movement of plates along the African rift valley.
Speed of lava reached 60 kph, lava flowed across the runway at Goma airport and through the town. The lava destroyed many homes as well as roads and water pipes, set of many explosions in fuel stores and killed 45 people. (Primary effects)
Half a million people fled from Goma into Rwanda.
They spent the night sleeping on the streets of Gisenyi. There was no shelter, electricity or clean water.
Many residents returned within a week in the hope of receiving aid.
Water was supplied in tankers, aid agencies including Christian Aid and Oxfam were involved in distributing medicine, food and blankets.
Mt. St Helens, USA:
When- 18th May 1980
Where- Cascade Range in the Washington state.
Why- The Juan de fuca plate subducted under the North American plate which caused friction causing an earthquake of 5.1 Richter to follow.
57 people were killed.
* Many Lahars (mud slides) – roads, railways and bridges were destroyed.
* It had a grey core, hence an explosive lateral blast.
* A lot of ash and pumice was expelled from the volcano.
* Pulverised rock, glacial ice and ash wiped all living things 27km from the crater.
* 270 homes, 47 bridges, 24km of railway were destroyed.
* Volcanic ash from the eruption has improved soil fertility.
* Tourism has now increased, and it is becoming better known due to the volcano.
* The landscape after 10 years has got traces of green and wildlife.
* Mobilising helicopters to search and rescue those in the vicinity of the crater.
* The ash clogged air conditioning systems and blocked roads which were even in 1 metre deep of ash, and the ash was cleared.
* President Carter sent out an extra two million of masks.
* Shelters were built for stranded tourists and many had emergency treatment in nearby towns.
* The ash increased the fertility of the soil.
* Buildings and bridges needed rebuilding.
* Drainage had to be fixed due to all the debris that covered it.
* The forest in the north needed to be replanted by the forest service after removal of fallen timber.
* Roads had to be rebuilt for future tourist’s consideration.
* $1.4 million was spent to transform the area so it can allow access for 3 million visitors a year.
* The road to the North of the volcano was opened in 1990 and to the Johnston Ridge Observatory in 1997.
Monitoring and predicting volcanoes:
Earthquakes are a frequent sign of an impending eruption, though you cannot know exactly when the eruption will be.
The bulge that appeared on the northern flank of the Mt St Helens was evidence of magma movement. This could have been monitored easily by ’tiltmeters’ which can identify small, subtle changes to the environment.
GPS can use satellites to detect movement of as little as 1mm and the change in temperature of the surface can be seen on satellite images.
Digital cameras placed on the rim of the volcano can monitor changes.
Robots can monitor changes by collecting samples to check the amounts of sulphur dioxide (increasing levels suggest that there will be an eruption).
From previous patterns of lava flows, ash movement and lahars, it allows for people to prepare for an eruption by organising the evacuation and arranging supplies.
Supervolcanoes are on a much bigger scale and do not have characteristic cones, but they have large depressions called calderas. Fissures are lines of weakness which allow magma to escape. During the explosion there are vents which allow for the magma to reach the surface.
Can affect North America and the whole world. There is evidence that the magma beneath Yellowstone is shifting and the caldera is bulging up beneath the lake. The ground has risen 70cm in places.
Considering the side of its 80km long magma chamber, it is potentially five times the minimum size for a supervolcanic eruption. An eruption is likely to destroy 10,000 km^2 of land, kill 87,000 people and 15cm of Ash would cover buildings within 1000km and 1 in 3 people will die. The ash would affect transport, electricity, water and farming. Global climates would change and many crops will fail.
Where an earthquake takes place beneath the crust it’s called the ‘focus’. Deep-focus earthquakes cause less damage than shallow-focus earthquakes. The point above the focus where the earthquake is most strongly felt is the epicentre (above the focus).
Then many waves are sent out.
* Firstly the primary waves which are relatively weak and warn of worse to come.
* Then the secondary waves which moves at right angles to the main waves.
* Then the transverse waves move side to side which cause a lot of damage.
Seismographs can record the extent of the shaking by producing a line graph called a seismogram.
The Richter scale is logarithmic, so there is a tenfold increase every time the scale increases by 1. There is no upper limit to the scale. The Richter scale measures how much seismic energy is released called it’s magnitude through seismogram readings.
The Mercalli scale measures the effect of earthquakes on a scale from 1-12, and uses descriptions of how bad the damage is.
Where do earthquakes occur?
Earthquakes tend to occur at plate boundaries, as friction and pressure builds up which causes earthquakes.
As destructive plate margins, the pressure from the subducting plate and its melting can trigger strong earthquakes as the pressure is released.
At constructive plate boundaries, the earthquakes tend to be less severe as the friction and pressure of plates moving apart is less intense than at destructive plate boundaries.
Earthquakes at conservative plate margins tends to be of a greater strength as when the plates slide past each other, they could get caught up which would build up stress and pressure. Then the pressure would be released in a sudden release of the plate which causes an earthquake.
Earthquake – MEDC
When – 17th January 1995
Where – Southern Japan
Why – The Philippines Plate shifted beneath the Eurasian plate, and the collisions led to an earthquake on the Nojima fault.
What – 7.2 on the Richter scale, with tremors lasting 20 seconds. The damage caused in excess of $220 billion.
* 6,434 people were killed, 40,000 were injured and 300,000 were homeless.
* Gas mains ruptured, water pipes fractured, elevated roads collapsed, railway lines bucked and only 30% of the Osaka to Kobe tracks were usable.
* 2 million homes were without electricity and 1 million without water for 10 days.
* Fire engulfed parts of the city, like west of the port which destroyed the wooden structures.
* Aftershocks further damaged roads and water supply made it hard to stop the fires.
* People huddled in blankets and in tented shelters in parks in fear of returning to the buildings.
* Hospitals had to operate in corridors.
* Major retailers like 7-Eleven helped to provide essentials.
* Motorola maintained free telephone charges.
* 80% of railways were made operational within a month.
* A year later the Hanshin Expressway and ports were reopen.
* Buildings were built to a ‘1981 code’ which meant buildings were built further apart to prevent a domino effect.
* High rise buildings needed to have flexible steel frames and others built with concrete frames were reinforced with steel not wood.
* Rubber blocks were put under bridges to absorb shocks.
* The Japanese now practise an earthquake drill every year.
Earthquake – LEDC
When – 8th October 2005
Where – Kashmir, Pakistan
Why – The Eurasian and Indian plates colliding into each other.
What – 7.6 on the Richter scale, buildings were not earthquake resistant.
* Around 80,000 deaths from collapsed buildings and hundreds of thousands were injured.
* Entire villages and thousands of buildings were destroyed.
* Around 3 million people were made homeless.
* Water pipelines and electricity lines were broken.
* Landslides buried buildings and people. They blocked access roads and cut off water supplies, electricity supplies and telephone lines.
* Diarrhoea and other diseases spread due to the lack of clean water.
* Freezing winter conditions shortly after caused more casualties and rescue and rebuilding operations were harder.
* Help didn’t reach many areas for days or weeks. People were rescued by hand without the help from emergency services.
* Tents, blankets and medical supplies were distributed within a month, but not to all areas affected.
* International aid and equipment such as helicopter and rescue dogs were brought in, as well as teams of people from other countries.
* Around 40,000 people have been relocated to a new town from their destroyed town of Balakot.
* Government money is been given to people whose houses have been destroyed.
* New health centres have been set up.
* Training is provided to help rebuild more buildings that are earthquake resistant.
Prediction, protection and preparation:
This can reduce the impact of earthquakes.
Prediction involves trying to forecast when an earthquake will happen, Japan tries to monitor earth tremors with belief that a warning can be given – but was not useful in the case of Kobe. Experts know when the earthquakes will happen, but are unsure about when. They also do not know where on the margin it will occur, but animal behaviour has been used in China, due to the strange animal behaviours.
Building to an appropriate standard and designs to withstand movement can ensure protection.
Preparation involves hospitals, emergency services and inhabitants practicing for major disasters, like having drills and codes of practice (like in Japan), so people know what to do to reduce the impact and increase their chances of survival.
Tsunamis are usually triggered by earthquakes. The crust shifting is the primary effect and the secondary effect is the displacement of the water above the crust. When the tsunami waves get near the land, and the shore becomes shallower, the bottom begins to slow down and the end of the wave catches up with the front, which is why it gains height.
Tsunami – LEDC
When – 26th December 2004
Where – Countries that bordered the Indian Ocean, i.e. Indonesia, Thailand, India and Sri Lanka.
Why – The Indo-Australian plate subducted beneath the Eurasian Plate.
What – Measured 9.1 on the Richter scale, the highest wave to come ashore was 25m.
Effects of the tsunami:
* Around 230,000 people were killed, 650,000 were injured and 2 million became homeless.
* Whole towns and villages were destroyed.
* Infrastructure of many countries (roads, water pipers and electricity lines) was damaged.
* Public buildings like schools and hospitals were wiped out.
* About 1,500 settlements were wiped out.
* 5-6 million people needed emergency food, water and medical supplies.
* There was massive economic damage, fishermen lost their jobs and the tourism was affected.
* There was environmental damage as the salt water affects growth and coral reefs were destroyed.
* Hundreds of millions of pounds had been given to foreign governments, NGOs and individuals to give survivors water, food, shelter and medical attention.
* Foreign countries set ships, planes and soldiers to rescue people, distribute food and water and to begin to clear up.
* UK government gave 75 million pounds and the public gave 100 million as well.
* The international community gave fresh water, water purification tablets, food, sheeting and tents as aid.
* Billions of pounds had been given to re-build the infrastructure.
* Programmes had been set up to re-build houses and to help people get back into work.
* A Tsunami Warning System had been put up in the Indian Ocean in June 2006.
* Disaster management plans were set up, and volunteers have been trained who will know how to react to a similar situation.
* Plans to spend 190 million to build 20,000 houses for 100,000 homeless people.