Useful Notes on Earth’s Interior

An understanding of the earth’s interior is essential to follow the nature of changes going on over the earth’s surface which are related to the deep laid internal forces operating from within the earth.

This understanding of the earth’s interior is based mainly on indirect sources, because so far it has not been possible to have access to the inner levels of the earth’s structure.

Evidences about Earth’s Interior:

1. High Levels of Temperature and Pressure Downwards:

The recurrent volcanic eruptions throwing out extremely hot, molten material from the earth’s interior and the existence of hot springs, geysers etc. point to an interior which is very hot. Although the average rise in temperature from the surface downwards is 32°C per metre, this rise is not uniform throughout.

In the upper 100 km the increase is estimated at 12°C per km, while it is 2°C per km in the next 300 km and 1°C per km after that. As per this calculation, the temperature is 2000°C at the earth’s crore. The high temperatures are attributed to the internal forces, automatic disintegration of the radioactive substances, chemical reactions and other sources.

Although ideally the innermost part of the earth should be in a liquid or a gaseous state due to the high temperatures, yet, because the pressure also increases with depth, the core is a rigid mass. The layer enveloping the core is in a semi-solid or plastic state.

2. Behaviour of Earthquake Waves:

The earthquake waves are measured with the help of a seismograph and are of three types—the ‘P’ waves or primary waves (longitudinal nature), secondary waves or ‘S’ waves (transverse in nature) while the surface waves are long or ‘L’ waves. The velocity and direction of the earthquake waves undergo changes when the medium through which they are travelling changes.

Thus the velocity of P waves decreases towards the interior pointing to a less solid layer (a characteristic of longitudinal waves), but increase for a while when passing through the inner core only to decrease as it comes out of the core. This point to a solid core surrounded by a partially molten layer.

Similarly, the S waves cannot pass through a liquid medium and are only transmitted through a rigid or solid medium. The S waves get deflected while travelling inwards and come out at the earth’s surface. This again points to a molten, semi-solid layer below the crust and mantle. The L waves do not pass and do not go deeper inside the earth.

3. Evidence from the Meteorites:

The meteorites are solid bodies freely travelling in space which accidentally come under the sphere of influence of the earth’s gravity and as a result fall on earth (or collide with it). Their outer layer is burnt during their fall due to extreme friction and the inner core is exposed. The heavy material composition of their cores confirms the similar composition of the inner core of the earth, as both evolved from the same star system in the remote past.

From the above scientific evidence, a fairly convincing picture of the earth’s interior can be drawn. (See Fig. 1.9)