Inside The Earth

Plate tectonics - the structure of a ‘plate’

The earth is forever moving like a giant engine. Without a solid and strong crust to our planet, plate tectonics could never occur. This is because the solid surface or crust of the earth, known as the lithosphere, is cracked into ‘plates’. The Lithosphere (0-70km deep) is floating on the weak and partially melted asthenosphere (70-250km deep), allowing the plates to move about. The asthenosphere is in turn lying on top of the Mesosphere or deep mantle.

Journey to the Centre of the Earth?

Unfortunately the possibility of journeying to the centre of the earth remains in the realms of science fiction. We are still, however, able to build up a good picture of the inside of our planet and these various layers have been identified by looking at waves produced by earthquakes. When an earthquake occurs, it transmits seismic waves in all directions through the earth. These seismic waves can be detected using seismographs. There are various types of waves travelling at different speeds. The slowest waves travel through the crust, but other waves travel through the earth and can be recorded by seismographs on the other side of the earth.

Shadow zones

By measuring the distance and the time taken for the waves to reach different parts of the earth from the epicentre, it is possible to plot the speed of the waves. They increase with depth. The paths of the waves through the body of the earth are curved due to changes in density, and it is this change in density that causes the waves to bend due to a type of reflection (refraction). But there are some areas, called shadow zones, where the waves from the earthquake are not received. This indicates that there is an area or core within the earth of a different density. At the boundary, wave are reflected back into the earth away from the surface. A picture of the internal structure of the earth has been built up from this data.

Next: What is plate tectonics?