Short Essay on the Internal Structure of the Earth

Direct observation of the interior of the earth is not possible due to the fact that the interior becomes hotter with depth, which is convincingly indicated by the volcanic eruptions. Besides, the deepest hole in the earth, a drill hole, is only about 8 km deep; this is quite negligible in comparison with the radius of the earth.

Since direct observation of the interior of the earth cannot be made conveniently, all the important sources of data on the structure of the earth are indirect and are logically derived and inferred from other evidences.

Apart from the seismological studies, other important sources of data like meteorite analysis etc., even though indirect, logically prove that the earth’s body comprises of several layers which are like shells resting one above the other.

These layers are distinguished by their physical and chemical properties, particularly in their thickness, depth, density, temperature, metallic content etc.

Now-a-days much information about the interior of the earth has been obtained from the study of the propagation of the earth-quake waves through the earth. As it has has been identified, there are three types of earthquake waves:

(i) Longitudinal, Primary or P-waves.

They are similar to sound waves in which the particles move to and fro in the direction in which the wave is travelling. These waves travel in solid, liquid and gaseous media. They have short wave length and high frequency.

(ii) Transverse, Secondary, S-waves or Sheer waves.

These waves are like the waves which run along a string which is fastened at one end, stretched fairly tight and shaken at the other end. In such waves the particles move to and fro at right angles to the path of the wave.

These waves travel only in solid medium. In comparison to the Primary waves, they are slow in motion. They also have short wavelength and high frequency.

(iii) Surface, L-waves, Rayleigh or R-waves.

These are transverse waves and are confined to the outer skin of the crust and are responsible for most of the destructive force of earthquakes. They have low frequency, long wave length and low velocity.

The P and S-wave velocities change with depth and each change can be related to a change in materials. Each region of changing shock-wave velocity demarcates a zone of discontinuity. This also gives the following information about the interior of the earth:

(i) the shells of increasing density are found towards the centre of the earth and it is estimated to be 18 at the centre.

(ii) each shell is formed of different materials i.e. the materials vary in their chemical composition, physical properties as well as in their state (i.e. solid, liquid or gas etc.). These changes attribute to the existence of minor discontinuities.