There are 3 main layers that make up the earth: the core, the mantle and the crust. The crust is broken up into chunks called tectonic plates. These plates move around the earths surface because of convection currents. Heat is radiated from the radioactive elements in the core, which causes the mantle to heat up. The hot magma rises to the surface where it cools again, moves parallel to the crust and then sinks. This creates movement in the matle, which moves the plates in the crust.
There are 3 different types of plate boundaries
Compressional Two plates moving towards each other. If it is an oceanic (dense, made of basalt) against continental (less dense, made of granite) then the heavier oceanic plate will subduct (sink under) the continental plate, creating a ridge. If two oceanic or two continental plates meet, then the rock will crumple under the equal pressure from both sides and create fold mountains. Also called a destructive plate boundary. Creates violent volcanoes and earthquakes.
Tensional Two plates moving apart. Usually with two oceanic plates, so oceanic ridges are created. Also called constructive boundary, as new land is created when magma rises through the gap and cools. Creates volcanoes, though they are not usually very violent.
Passive Also called conservative. Two plates moving in parallel. Creates violent earthquakes.
There are 2 types of volcano.
Sheild volcanoes are small and wide based. This is because they are made of basic lava which is runny with a low silica content so it travels far.
Composite cone volcanoes are taller and cone shaped because they are made up of layers of thick, viscous acid lava with a high silica content so the lava doesn't travel far. It builds up higher with each eruption. When the lava cools in the cone of the volcano it requires a very violent eruption to dislodge it.
MEDC Souffreire Hills Volcano-Cause and Effects
Montserrat lies on a compressional plate boundary. The volcano is caused by subduction of the North American/ Atlantic plate under the Carribbean plate. As the two plates merge the oceanic plate is forced down or subducted under the continental plate. As it is forced down pressure increases which triggers earthquakes and at the same time to rise to the surface and when it succeeds will form a volcano such as the one in Montserrat.
- Primary effects:4 sq km of land was covered by pyroclastic flows
- 2/3 of the island covered in ash
- 50% of population covered in ash
- 23 people died in 1997
- The capitla, Plymouth, became a ghost town
- Floods because valleys were blocked with ash
- Airport and port closed, then destroyed by pyroclastic flows
- Farmland destroyed
- Forest fires caused by pyroclastic flows
- Schools and ospitals destroyed
- Secondary effects: Remaining inhabitants of the island living in the harsher north of the island
- Transport is a problem for people travelling to and from the island-the port and airport remain closed
- Over half the population left and didn't return.
Soufriere hills volcano - prediction and response
Scientists at the Montserrat Volcano Observatory (MVO) were taking measurements of ground deformation, siesmic activity and gas emmissions. They measured ground deformation by placing 5 tiltmeters around the slopes of the soufriere hills volcano. these measure tiny changes in the angle of the slope. Gas emissions are measured by looking through a plume of gas at the sun with a spectrometer, which can distinguish between gases by looking at the colour of the sun. Siesmic activity is measured by siesmometers. They measure the strength of small earthquakes caused by movement of magma in the magma chamber of a volcano.
- Response: £41 million given by the british government.
- Money was given to individuals to help them move to other countries.
- Riots occured as locals complained that the British government wasn't doing enough to help the people of the island.
- A risk asesment was done to help islanders understand which areas are at risk and reduce problems for the future.
Earthquakes are caused by a sudden release of pressure between two tectonic plates. The plates snag and pressure is built up, until it gets too much and the plates are released causing shockwaves to radiate from thefocus and the epicentre.
Earthquakes are measured on the Richter scale. It is logarithmic: a score of 9 is 10 times stronger than a score of 8, and 8 is 10x stronger than 7, and so on.
Shock absorbers on bridges often take the form of large steel tubes between sections that move in and out to make the bridge expand and contract. Other measures used to protect bridges include Shear links - beams that connect towers of cable stay or suspension bridges to reduce wobbling, bearings - plastic sheets between steel sheets which are like shock absorbers. Bridges are also made of ductile materials like steel. This makes them less brittle and more likely to withstand large shocks.
Smaller buildings are protected by using cross-bracing with steel beams. Shear walls (solid concrete walls embedded with steel rods) may be built, and the centre of the building is strenthened (a shear core).
Medium sized buildings are built with shock absorbers between the building and its foundations called base isolators. These are usually made of alternating layers of steel and rubber.
Skyscrapers are very vulnerable to earthquakes because of their height. They need extra deep foundations and reinforced framework with strong joints which make the building flexible.
In LEDCs, cheaper methods and materials are used. The roof of a building needs to be cleared of mud so that it is lightler, the walls are reinforced with diagonal bracing using timber or steel if it is available, the wall corners are sometimes reinforced with steel mesh but that is hard to find and pay for. In new buildings, the walls are built with shaped stones and cement. A concrete ring beam can be constructed at roof level.
Case study - Kobe earthquake Japan - effects
Kobe is close to a passive boundary fault line between the philippines and south Japan tectonic plates. In January 1995 about 50km of the fault moved causing the Great Hanshin earthquake.
- Effects: Primary: over 100 000 houses collapsed
- 5500 people killed
- 30 000 people injured
- Roads and railways wrecked
- Electricity, gas and sewerage systems destroyed
- 10% schools destroyed
- 12% industry and 14% services destroyed
- Emergency services disrupted
- Secondary: 300 000 people homeless
- Huge fires
- A flu epidemic
- 2million homes without power
- 1million homes without water for 10 days
- Health hazards in the makeshift shelters
Kobe earthquake - Responses
- Emergency services and army brought in to help
- Heat seeking equipment used to find people under rubble
- Emergency shelters set up
- Moblie telephone systems established
- Water and food supplies brought in
- Hospitals set up in schools
- Fires tackled
- Bulldozing unsafe buildings
- Rebuilding homes, schools, roads, hospitals etc
- Repairing water, electricity, phone and sewarage systems
- Provide help for those in shock
- More monitoring equipment
- Introduce an emergency action plan
effects of tectonic hazards in different areas
LEDC's are more vulnerable to damage from tectonic hazards because they have less money to spare to spend on improving buildings and structures to make them safe, and when an earthquake or volcano does happen, they have less infrastructure and funding to repair the damage and rehabilitate people.
Urban areas are at more risk from tectonic hazards than rural areas, because the population is denser, as are the buildings, so there is more structure to be destroyed. There is more to lose. Also, after the disaster has happened, emergency services have a harder job of finding people under all the fallen buildings in cities, and repairing the massive amounts of damage than they do in rural areas, where there are fewer buildings, and they are cheaper to repair.
Fold mountains are created at compressional boundaries where two continental plates are pushed together and are forced upwards, folding to create mountains.Examples of fold mountains: Himalayas, Alps.
Human Activity in the Alps:- Farming- mostly on the valley floor where it is sheltered, flatter, warmer and deeper soils. Traditionally dairying. Main crops are hay and cereals. some vines and fruit in warmer areas. Use of upland pastures in summer.
- Tourism - Tourists are attracted by winter sports such as skiing, beautiful scenery, alpine climate,Winter and summer holiday resorts.
- HEP and Industry - Industry needs lots of electricity, can be generated from fast flowing streams.Traditional industry includes clock-making, paper and furniture.
- Forestry- Conifers cover the slopes up to about 1800 metres. The wood is used for fuel building chalets and for paper making.
Benifits of living near volcanoes
Vesuvius and the Plain of Campania
Fertile soils - for wheat, tomatoes, peaches, almonds and vines with yeilds 5 times higher than the national average.
Tourism - Trips to Vesuvius and hot springs, museums at Pompeii and Herculaneum.
Minerals - the sulphur wasteland nearby at the Phlegraean Fields.
For some people, it is the best farmland available, and some people may not be able to afford to move to a safer area. Others believe that an eruption will not happen in their lifetime.