MOVEMENT AND CHANGE

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MOVEMENT AND CHANGE

• Movements in the Earth

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Structure of the Earth
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Structure of the Earth

• We live on the thin, solid crust.
• The mantle is made of hot rock. It extends half
  way to the centre of the Earth.
• The core is very dense because it is made of iron
  and nickle.

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Structure of the Earth

• The outer core is molten (liquid). The inner core
  is solid.

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Structure of the Earth

• How do we know what the Earth is like inside?
• The answer is that we make use of Earthquakes.

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Structure of the Earth

• An earthquake sends out strong vibrations, called
  seismic waves.
• These travel from one side of the Earth to the
  other and can be detected by seismometers.

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Structure of the Earth

• During an earthquake, seismic waves are produced.
• These can be recorded on seismometers.

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Structure of the Earth

• This picture shows the basic idea behind a
  seismometer.
• They are very sensitive and can even detect
  atomic explosions from the other side of the
  world.

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Structure of the Earth

• This is a seismograph and shows the trace
  produced by a major earthquake.

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Structure of the Earth

• There are two types of seismic waves and it is
  the difference between these two waves that tells
  us about the internal structure of the Earth.

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Seismic Waves

• There are two types of waves caused by
  earthquakes.
• P waves (or primary waves) These are detected
  first by seismometers because they travel faster
  than S waves.
• They travel as longitudinal waves.

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Longitudinal waves (P waves)
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Seismic Waves

• S waves (or secondary waves) These travel more
  slowly.
• They travel as transverse waves.

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Transverse waves (S waves)
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Seismic Waves

• BOTH P and S waves can travel through solid
  materials, but transverse waves, S waves, can not
  travel through a liquid.
• Transverse waves can travel along a liquid
  surface.

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Seismic Waves

• When an earthquake occurs, P and S waves travel
  through the Earth.
• They travel on curved paths as they speed up as
  they go.

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• P waves are partly reflected and partly
  refracted.
• S waves are totally reflected they cannot travel
  through the liquid core.

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• Seismometers around the world detect the waves.
• The fact that S waves are not detected on the far
  side of the Earth from earthquakes give evidence
  that part of the Earths core is liquid.

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MOVING PLATES
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• The outer layer of the Earth made of the crust
  and upper mantle is called the lithosphere.
• The lithosphere is arranged in plates which cover
  the Earth.
• Earthquakes and volcanoes occur at the edges of
  the plates.

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Why do the plates move?

• The hot mantle is heated by radioactive decay in
  the centre of the Earth.
• This heat cause the liquid magma within the Earth
  to move (by convection currents)
• It is these huge convection currents which cause
  the plates to move.

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When plates move apart

• When plates move apart a crack appears between
  them.
• Molten magma flows from the mantle into the
  crack.
• This is seen as volcanic activity.

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When plates move apart

• Most of this volcanic activity occurs hidden from
  view in the depths of the oceans.

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When plates come together

• When two plates collide, one plate will be forced
  beneath another plate.
• Usually the denser oceanic crust is forced under
  the less dense continental crust

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• The ocean crust is forced under the continental
  crust.
• As the rock from the ocean crust goes into the
  mantle, it melts and becomes magma.

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explosive volcanoes
powerful earthquakes
crust melts
mantle
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Slip-sliding plates

• In some places in the world the plates are
  sliding past each other.
• When plates slide past each other they often jam
  for several years.
• This results in very powerful earthquakes.

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Slip-sliding plates

• The San Andreas fault in California is such a
  fault.
• This fault frequently causes large quakes.
• In 1906 it destroyed the city of San Francisco.