Structure of the Earth is the starting point for the preparation of physical geography and a lot of questions has been asked in the past in the UPSC IAS Prelims exam. So prepare this topic well. Remember physical geography is a conceptual subject, so try to understand all of its aspect.

Radius of the earth: 6370 kms.

So how do we know about the interior of the earth?

Well the direct sources are surface rocks, volcanic eruptions, earthquake waves, deep mine operations, etc.

Mining areas are reliable places to investigate more about the interior.

Important Note: The deepest drill is in Kola Arctic Ocean with a depth of 12 kms.

Indirect sources are:

Measure of Pressure, Temperature and Density

Meteors that are of same material and structure as that of earth.

Gravitation, Magnetic field and seismic activity.

Important Note:

1. Gravitation Force is greater near the poles and less at the equator. This is because of the distance from the centre at the equator being greater than that at the poles.
2. Temperature and pressure increase with the increasing distance from the surface towards the interior in deeper depths. Density of the material also increases with depth.

Structure of the Earth

The Crust

It is the outermost solid part of the earth. It is brittle in nature.

This crust has an upper part called granitic rock and forms the continents. Silica and alumina (Sial) are its main constituents.

The lower part has basaltic rock forming Oceanic floor. Silica, iron and magnesium (Sima) are its main constituents.

Thus, sial and sima forms the earth’s crust.

Continental crust can be said to be floating on the oceanic crust.

Oceanic crust is thinner as compared to the continental crust.
The mean thickness of oceanic crust is 5 km whereas that of the continental is around 30 km.

The continental crust is thicker in the areas of major mountain systems. It is as much as 70 km thick in the Himalayan region.

Conrad discontinuity separates the outer and the inner crusts.

The Mantle

The portion of the interior beyond the crust is called the mantle or the mesosphere.

The mantle extends from Moho’s discontinuity to a depth of 2,900 km.

It is composed mainly of dense rocks rich in olivine.

The upper portion of the mantle is called asthenosphere extending upto 400 km. It is the main source of magma and has a density higher than the crust’s.

Lithosphere comprises the crust and the uppermost part of the mantle.

Its thickness ranges from 10-200 km.

The lower mantle extends beyond the asthenosphere. It is in solid state.

Repetti discontinuity separates the outer and the inner mantle.

The Core

The core-mantle boundary is located at the depth of 2,900 km.

The outer core is in liquid state while the inner core is in solid state.

The core is made up of very heavy material. Nickel and iron are its main constituents. It is sometimes referred to as the nife layer.

Gutenburg-Wiechert Discontinuity separates the core from the mantle.

I hope you have been able to grasp the content so far. Try answering some questions from this Practice Paper on Geography.