MOVEMENT AND CHANGE Movements in the Earth

Structure of the Earth

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.

Structure of the Earth The outer core is molten (liquid). The inner core is solid.

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

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.

Structure of the Earth During an earthquake, seismic waves are produced. These can be recorded on seismometers.

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.

Structure of the Earth This is a seismograph and shows the trace produced by a major earthquake.

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.

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.

Longitudinal waves (P waves)

Seismic Waves S waves (or secondary waves): These travel more slowly. They travel as transverse waves.

Transverse waves (S waves)

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.

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.

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

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 Earth’s core is liquid.

MOVING PLATES

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.

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.

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.

When plates move apart Most of this volcanic activity occurs hidden from view in the depths of the oceans.

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

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.

explosive volcanoes powerful earthquakes crust melts mantle

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.

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.