- Created by: Jawaaad
- Created on: 11-04-17 17:46
What are Plate Tectonics
Plate Tectonics is a theory which gives an explanation for the location of earthquakes and volcanoes:
- The Earth's crust is divided into plates
- These plates move a few centimetres every year as they are floating on the mantle
- This movement is caused by convection currents in the mantle
- This movement causes the plates to collide or move apart
- The pressure created at these boundaries is what causes earthquakes and volcanoes
What are Hotspots
A hotspot is another place where volcanoes occur:
- A mantle plume is a column of hot semi-molten rock below the lithosphere (crust). The plume is stationary.
- The plume directs a string of magma, from the mantle, up towards the lithosphere.
- The magma breaks through the lithosphere, erupts, and forms a volcano
- The plate continues to move making the magma string breaks off. This makes the volcano extinct and it will eventually erode to sea level.
- The process is then repeated and a continuous chain is made.
Examples are the Galapagos Islands and Hawaii
The Two Types of Crust
- Found under the continents. This crust is less dense (lighter) than the oceanic crust, but thicker (up to 60km)
- Found under the oceans. This crust is thinner (around 10km), but is denser (heavier)
Convergent Plate Boundaries
This is between an oceanic crust and a continental crust:
- Plates move together due to convection currents
- Oceanic crust is heavier than continental crust so is forced beneath it
- As it sinks (subducts) into the mantle the plate melts due to high temperature and the friction
- Deep earthquakes occur in the subduction zone. A deep sea trench forms where the plate is forced under.
- The newly formed magma, from the destroyed oceanic crust, is lighter than the mantle as it is mixed with sea water.
- Magma will rise to the surface through the cracks that formed in the collision.
- The magma contains gas which causes explosive eruptions. Sometimes it can create fold mountains
Features: Explosive volcanic eruptions and deep earthquakes
Examples: Nazca and South American Plate
Other Types of Convergent Plate Boundaries 1
If it is between two continental plates:
- Also referred to as a Collision plate boundary
- Plates move together due to convection currents
- The continental crusts are both the same density so they don't sink
- The plates are forced upward. Fold mountains form
- Movements can cause earthquakes
Features: Mountains are formed. Violent earthquakes occur. No volcanoes.
Example: Indian plate and the Eurasian plate forming the Himalayan mountains.
Other Types of Convergent Plate Boundaries 2
If it is between two oceanic plates:
- The denser of the two oceanic plates will be forced down
- No fold mountains are formed as there is no continental crust to be uplifted
- The volcanoes that occur produce volcanic islands because the eruptions happen on the seabed
Examples: The Pacific Plate subducting below the Philipines Plate in the eastern Pacific
Convergent Plate Boundary Diagram
Divergent Plate Boundary
This is usually between two oceanic plates:
- This mostly occurs under oceans
- Plates move apart due to convection currents
- Magma rises up forming new crust. The pressure causes it to dome
- Magma flows out gently. The volcanoes are not explosive
- The lava cools and solidifies to form ridges
- Islands form as the volcanoes rise above the ocean surface
Features: Gentle volcanic eruptions and some small earthquakes.
Example: Iceland & the Mid-Atlantic ridge.
Divergent Plate Boundary Diagram
Conservative Plate Boundary
This happens when plates are moving in different directions or at different speeds to one another:
- Plates stick or jam together and huge amounts of pressure builds up
- Pressure releases and the plates move suddenly causing a violent earthquake
- The plates move on a few millimetres (or more)
- No crust is destroyed or created - it is conserved
Features: Shallow (violent) earthquakes. No volcanoes as no crust is created or destroyed.
Example: San Andreas fault in California
Conservative Plate Boundary Diagram
Two Methods Used To Measure Earthquake Magnitude
The main characteristics of the Mercalli Scale are the following:
- It's a measurement of the intensity of the earthquake completed from eyewitness accounts
- It is measured on a scale of 1-12
- The scale is quite subjective and uses whole numbers
- The person measuring has to be present at the earthquake site
The main characteristics of the Richter Scale are the following:
- It is a measurement of the magnitude of the earthquake completed by a seismograph
- It is measured on a scale of 1-9 where each number of the scale is split into 10 points
- The scale is objective
- The measurement can be done from a different location from the earthquake site
The Characteristics of the Focus and Epicentre of
- This is on the Earth's surface
- It is the place where the worst of the effects of the earthquake are felt
- If the area is inhabited, this will be the place where the worst destruction to buildings and infrastructure will occur
- This is a point below the surface of the Earth
- The depth below the surface varies with different earthquakes
- This is the point where the pressure build-up is released; shock waves move out from this point
Reasons Why People Continue to Live in Areas of Vo
- Volcanic soil is very fertile. Coffee is grown on the slopes of volcanoes in Columbia
- Cheap geothermal energy is produced in areas of volcanic activity. Cheap electricity produced from geothermal energy in Reykjavik, Iceland
- People think there is little risk of a volcano exploding. People in USA near Mt St Helens refused to evacuate as they did not believe it would erupt as it had been dormant for years
- People in LICs can't afford to move. People living on Mt Merapi, Indonesia
- The area around the volcano is very scenic and many tourists want to visit. Mt Etna in Sicily has many tourist guides so provides jobs for the local people
Reasons Why People Continue to Live in Areas of Ea
- People have jobs in the mining industry in earthquake areas. Copper mined in Chile. Two people were killed in a mine during an earthquake in 2007
- People think that the government has taken the precautions against the likelihood of an earthquake (in MICs and HICs). Local people feel safe as many buildings in japan are made to be earthquake proof.
- Some earthquake areas have been developed for the wealthy, so people would want to live there for the fantastic environment. Malibu, an expensive coastal resort, has developed on the Californian coast of USA
Prediction and Prevention
- Earthquake: Seismometers can measure movement. They can predict when an earthquake is likely to take place
- Volcanoes: Satellites record changes in the shape of a volcano. Eruption of Mt St Helens was predicted through the movement of tiltmeters placed on the slopes.
Building design and defence
- Earthquake: Buildings have special features. Automatic shutters, rolling weights on roof to counteract shock waves, open areas where people can assemble if evacuated
- Volcanoes: Diversion channels can be built around villages. Lava flows can be cooled with water to make it move more slowly and eventually stop
Education and planning
- Earthquake: Earthquake drills such as ducking under tables like in schools in Japan. Info packs on earthquake prep are provided by USA government
- Volcanoes: People taught to look for signs. On Sicily, people are taught to look at changes in lava consistency.
Case Study: Turkey Earthquake 1999
- Tukey lies between three continental plates. Eurasian, African and Arabian.
- The movement of these plates caused the earthquake to occur
Effects on people
- Approximately 18,000 people were killed
- Approximately 300,000 people became homeless
- Transport routes between Ankara and Istanbul were destroyed causing disruption to emergency services
Effects on environment
- There was a leak from the chlorine factory at Yalova
- Caused damage to sewage works at Pekim causing sewage to leak into local rivers and groundwater