Rocks and plate tectonics

A brief summary with the aid of a geography revision guide covering:

Plate tectonics

Rocks

Earthquakes

Case study: Kobe, Japan

Volcanoes

Case study: Mount St Helens, USA

Weathering

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  • Created by: Elizabeth
  • Created on: 31-05-09 13:16

Plate tectonics

The Earth's crust is made up of pieces like a jigsaw known as tectonic plates.

From inside out, the Earth is made up of:

  • Inner core - solid and 1,200 km thick
  • Outer core - nickel and iron, molten liquid. 3,500 km thick
  • Mantle - approx 3,000 degrees C and 2,800 km thick
  • Crust - 6-80 km thick

http://discovermagazine.com/2007/jun/journey-to-the-center-of-the-earth/llnl_earth.jpg

Diagram may help make things a little more clear. Copy and paste link

The plates 'drift' or move because the heat in the rock below the crust sets up convection movements (rather like the movement you see in a pan of soup when it is heated).

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Plate tectonics 2

TYPES OF PLATE MARGINS

A destructive plate margin occurs where one plate slides beneath another as they collide. The bottom plate crumples, creating new mountains and volcanoes.

At a constructive plate margin the plates are moving apart. Molten rocks from the mantle below spread out and harden, forming a ridge of new rock.

At a conservative plate margin the plates slide past each other. Pressure builds up until they move with a 'jerk', causing earthquakes.

A collision plate margin is where two plates collide and are crushed against each other. They are pushed upwards, forming new mountains.

Fold mountains

When tectonicplates collide, huge forces cause the rocksto be folded and buckled, forming mountains like the Andes in South America and the Himalayas in Asia.

  • An upfold is called an anticline.
  • A downfold is called a syncline (remember sink = down)
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Rocks

There are three main types of rock:

  • Igneous rocks are formed when the molten magma cools. If it cools beneath the surface, large crystals have time to grow and granite is formed. If it flows to the surface, quicker cooling results in basalt.
  • Sedimentary rocks are usually made of fine material deposited in lakes and seas. Sandstone is formed from sand compressed by the weight of other deposits on top of it. Chalk and limestone are formed from millions of shells of tiny sea creatures.
  • Metamorphic rocks occur when igneous or sedimentary rocks are changed by great heat or pressure from volcanic eruptions or mountain building. In this way chalk and limestone are changed to marble.
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Earthquakes

Earthquakes are usually the result of plate movement, which is not smooth. The strain builds up along a fault line between two plates until they move, causing earthquakes.

The point where the earthquake starts below the Earth's surface is known as the focus. The point directly above the focus on the Earth's surface is known as the epicentre.

http://www.empr.gov.bc.ca/Mining/Geoscience/SurficialGeologyandHazards/Earthquakes/PublishingImages/eq-2b.gif

A diagram that you may find helpful

Measuring earthquakes

The magnitude of an earthquake is recorded by an instrument called a seismometer. It measures the height of the shock waves on the Richter scale. Each point on the scale is 10 times greater than the one below. The seismometer records the earthquake vibrations with a pen on a sensitive arm, marking zigzag lines on a drum of paper.

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Earthquakes 2

Precautions against earthquakes

Individuals

  • Prepare an emergency pack including water, food, blankets, first aid kit, radio, torch.
  • During and after the earthquake, shelter under a table or bed and avoid stairways.
  • Turn off gas, water and electricity.
  • After the earthquake move to open ground.

Authorities

  • Monitor the hazard so people can be warned.
  • Have emergency supplies ready.
  • Make plans for shelter, food and water supplies, for emergency services, fire brigade, police, ambulance, and hospital services.
  • Plan to broadcast information for people affected.

Long-term planning

  • Ensure road and rail communcations are built to reduce the effect of earthquakes.
  • Ensure all new buildings are earthquake-proof.
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Case study: Kobe, Japan

  • The earthquake at Kobe struck on 17 January 1995.
  • It measured 7.2 on the Richter scale.
  • Japan lies at the margins of three plates, on a destructive plate margin and subduction zone.
  • The epicentre was in Osaka Bay, close to the cities of Osaka and Kobe.

PRIMARY EFFECTS

  • Violent shaking on the earth and buildings for 20 seconds.
  • Aftershocks lasting half an hour.
  • Over 5,000 people killed and 25,000 injured.
  • Gas mains fractured, electricity cut off, many fires started.
  • Water mains broken so no water reaching houses.
  • 200,000 buildings collapsed, leaving 300,000 homeless.
  • Transport links disrupted, the elevated motorway in downtown Kobe collapsed, and railway lines blocked, making the job of emergency services very difficult.

SECONDARY EFFECTS

  • Transport disrupted for weeks afterwards.
  • Older houses worst hit, new earthquake-proof buildings survived better. Heavy cost of rebuilding, which takes years to complete.
  • Water and food shortages, so army brought supplies to people living in schools.
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Volcanoes

  • When a volcano erupts violently, it throws ash and volcanic bombs which arepieces of rock, into the air.
  • Hot gases, ash and steam can form pyroclastic flows which move very fast and can cause tremendous damage.
  • When a volcano has been dormant for some time the magma in the vent acts as a plug. When the volcano erupts the plug is blown out, often blowing off te top of the cone and leaving a very large crater known as a caldera.
  • Mud flows called lahars are formed when hot ash melts snow and ice or falls into rivers, They move very fast and are very destructive.
  • There are over 600 active volcanoes in the world.

Types of volcano

Active volcanoes are those which have eruptes recently and are likely to erupt again.

Dormant volcanoes are those which have not erupted for a long time but may erupt again.

Extinct volcanoes are unlikely to ever erupt again.

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Case study: Mount St Helens, USA

Mount St Helens was a popular tourist area for fishing, climbing and walking.

It lies at the margins of the North American Plate and the Pacific Plate. This is a destructive zone.

This volcano erupted on 18 May 1980, having been dormant since 1857. Scientists has warned of the eruption and most people were evacuated.

The eruption was so violent that the top of the mountain was blown off with a blast greater than a 20 kilotonne atomic bomb. A great caldera was left.

The blast covered 400km squared, throwing trees over ridges 500 metres high.

Hot ash and gas flowed down the slopes at 160km/hour, melting snow and ice and forming destructive lahars.

The ash cloud crossed the USA in 3 days and circled the globe in 17 days.

The eruption left 100 people dead, including a geologist at an observation post 8km away.

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Weathering

The rocks which form the Earth's surface are broken down by:

Erosion by water in streams and rivers, ice in glaciers, wind, animals and people e.g. footpath erosion.

Weathering by the constant attack of wind and rain, in other words, the weather.

THE WHOLE PROCESS OF BREAKING DOWN ROCKS IS CALLED DENUDATION.

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Weathering 2

Physical weathering

Freeze-thaw

  • cracks fill with water.
  • water freezes and expands as ice.
  • cracks widen and pieces of rock split off.

Biological weathering

  • seed growns in a crack
  • growing plant pushes against the rock and pieces loosen and break off.

Onion-skin weathering

  • in hot areas heat of the sun causes outer layer of rock to expand
  • cold nights cause outer layer to contract
  • outer layer flakes off.
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