TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Geography > Plate Tectonics
Restless Earth
The earth is made up of around 12 interlocking tectonic plates that fit together like a jigsaw. They sit on top of a semi molten mantle and are moved by large convection currents in the mantle. The places where these plates meet are known as plate boundaries and sometimes plate margins. This is where the majority of earthquakes occur and volcanoes are found.
Plate Boundaries
Destructive:- Here, a denser oceanic plate subducts underneath the lighter continental plate. This cause heat and pressure as the plates rub against each other. This heat and pressure causes the plates to stick and so the pressure builds. This pressure is then released during an earthquake. As well as earthquakes, volcanoes can erupt releasing magma trapped in the destruction of the oceanic plate. Where the two plates first touch, a deep trench is formed
Constructive:- Also known as sea floor spreading. This is where two plates move apart and magma rises to fill the gap creating new crust. An example would be the mid Atlantic ridge. Sometimes the actual land may rise up out of the ocean as in the case of Iceland
Collision:- At a collision plate margin, plates of the same type press against each other. This then forces both plates to push against each other and the pressure is released as the plates move upwards forming a mountain range. An example would be the Himalayas where the Indian plate is pushed north into the Eurasian plate. This causes the crust to fracture into mountains.
Conservative:- Similar to a destructive plate boundary, two plates slide past each other laterally. This causes heat and pressure and so the plates stick together. Pressure builds up and is released in a violent earthquake. Take the example of the San Andreas faultline. The North American plate slides southwards and the Pacific plate slides northward.
Earthquakes
Earthquakes happen when tension builds up between two plates. This usually happens on destructive or conservative plate boundaries. The tension is released in a sudden violent earthquake and so the ground shakes as it moves to where it should be. Earthquakes are measured on the richter scale and the richter scale is a logarithmic scale so magintude 2 is 10 times as powerful as a magnitude 1 quake and a magnitude 4 quake is 100 times a powerful as a magnitude 2 quake and 1000 times as powerful as a manitude 1 quake.
Many people are killed either during the quake and the causes can be split into two groups. Primary Causes happen during the quake and secondary causes happen after the quake.
Primary Causes
- Falling Debris
- Exploding cars
- Building collapse
Secondary Causes
- Diseases from polluted water supply
- Fires from broken gas pipes
- Lack of food, shelter and clean water
- Rescue efforts are made difficult due to e.g. broken bridges, collapsed roads.
Volcanoes
Types of Volcanoes
Case Studies
San Francisco 1989
- Happened at 17:05 Local time
- 17th Oct 1989
- 6.9 Magnitude on the Richter scale
- Conservative plate margin
- Worst affected area was the Marina District where liquefaction of Debris from 1906 quake occurred
- 1.5 Mile long section of the Cypress freeway collapsed.
- No one hurt or killed at candlestick park where people were gathering for baseball game
Future:-
- Greater than magnitude 8 by 2018
- Diaphragms added under Oakland Bay Bridge
- Trusses resist rotational movement around TransAmerica Building
- Shear Bracing added to give lateral support to Alcoa Building
- Pin Jointed trusses added to bay bridge
- Cross braces added to many masonry buildings
- Million gallon reservoir built
- 100 miles of earthquake proof pipe laid
- Array of seismic instruments installed to warn of earthquake
Mount St Helens 1981
Comments
