The study of the structure of the earth focuses on its layered structure, and the variations in the density and temperature, at various depths. The shape of the earth is that of a spheroid with mean equatorial radius of 6378-388 km. and polar radius of 6356912 km. concisely speaking, the earth is a globe having a radius of 6371 kilometers.

Direct observation of the interior of the earth is not possible as the interior becomes hotter with depth which is convincingly indicated by the volcanic eruptions. Apart from the seismological studies, other important sources of data, even though indirect, logically prove that the earth’s body comprises several layers, which are like shells resting one above the other. These layers are distinguished by their physical and chemical properties, particularly, their thickness, depth, density, temperature, metallic content and rocks.

The layered structure of the earth-developed during the process of its transformation from a hot-gaseous state to the present state. During these processes, the heavier material sank down and the lighter material floated up and consequently because of the differen­tial densities of the materials constituting the earth, they got separated and formed layers of different densities.

Broadly, the earth’s interior has been divided into three major parts:


1. The Crust.

2. The Mantle.

3. The Core.

Inferences obtained through seismological studies:


The seismic waves which result during the occurrence of earthquakes are chiefly of three types as.

(a) Primary waves (P-waves).

(b) Secondary or Sheer Waves (S-waves).

(c) Rayleigh (R) waves which are also known as ‘L’-waves.


These seismic waves differ from each other in respect of their propogation velocity, wave-length and path of travel. Their nature of vibration is also different as some of them have longitudinal- vibration, others have vibrations of transverse-nature.

From several seismological studies, it has been inferred that

(I) The three segments of the earth i.e., Crust, Mantle and Core, are separated by two sharp breaks, which are usually known as major discontinuities.

(II) The crust is having a thickness of about 33 kms.


(III) The crust is composed of heterogeneous rocks.

(Iv) The second major segment of the earth i e., the mantle extends from below the crust to a depth of 2900 kms.

(v) The third major segment of the earth extends from below the mantle up to the centre of the earth and is known as ‘the core’.

Apart from the aforesaid facts, it also gives evidences regarding the existence of a number of minor discontinuities within the earth, which may be because of


(a) Changes in the chemical composition of the materials.

(b) Changes in the density of the materials.

(c) Changes in the state of a given material i.e., whether it is in solid, liquid or viscous state.

(d) Changes in the physical properties of minerals, etc.


1. The crust:

It is the top-most layer of the earth. Its thickness over the oceanic areas is generally 5 to 10 kms, whereas on the continental area, it is 35 kms and the thickness ranges from 55 to 70 kms in orogenic belts.

Sub-divisions of the crust:

The crust of the earth is sub-divided into sub layers as follows :

(i) Sial (ii) Sima

(i) Sial:

It is also known as the Upper-Continental-Crust. It consists of all types of rocks like igneous, sedimentary and metamorphic, which are exposed at the land-surface. It is rich in -silica and aluminium.

Its composition is usually granitic to grano-dioritic. In the ocean-basins, they are floored by a basaltic-horizon which are poorer in potassium and richer in aluminium than the basalts of the land- surface and are called ‘Oceanic-tholeiites’.

Conrad Discontinuity separates the sial-layer from the under­lying sima-Iayer. This discontinuity is a second-order discontinuity and is located at a depth of 11 kms.

(ii) Sima:

It is also known as Lower-Continental crust. Its thickness is about 22 kms. It includes two parts :

(а) Outer Sima

(б) Inner Sima.

Together they are basaltic in composition and this layer is rich in silica and magnesium.

(a) Outer sima. extends up to a depth of 19 kms and comprises rocks of intermediate composition.

(b) Inner sima is located at a depth of 19 kms and extends up to 33 kms. It is of basic to ultra-basic in composition.

2. Mantle:

It is separated from the over-lying crust by the Mohorovicic-Discontinuity which is a first-order discontinuity. Its thickness is about 2865 kms. If forms 83 % of the earth by volume and 68% by mass.

It is the source-region of most of the earth’s internal energy and of forces responsible for ocean-floor spreading, continental drift, orogeny and major earthquakes.

The material is olivine-pyroxene complex, which exists in a solid state. It is believed that the upper mantle has a mix of 3-parts of ultramafic rocks and 1-part of basalt and the mix is known as Pyrolite. The lower mantle extends from 1000 km to the core- boundary.

Within the mantle, a number of second-order discontinuities have been located, which are as follows :

(i’) Density break at 80 km depth ; density changes from 3’36 to 3’87 above and below the level, respectively.

(ii) Gravity break at 150 km depth ; gravity changes from 984 cm/sec2 to 974 cm/sec2 till it reaches at a depth of 1200 kms.

(Iii) At 700 km-depth, there changes the capability of the materials in storing amount of elastic-strain energy. Up to 700 kms the capability is more.

(iv) Repetiti discontinuity. At 950 km depth. It marks the lower limit of the very rapid rise in the velocity of seismic vibrations.

(v) Gravity-break. At 1200 km depth, gravity attains its minimum value i.e., 974 cm/sec2, thereafter it rises up to 1068 cm/sec2 at the core-boundary.

3. The core:

It is separated from the mantle by the Guttenberg Weichert Discontinuity and extends up to the centre of the earth. It consists of three parts :

(i) Outer-core, (ii) Middle-core, and (iii) Inner-core.

(i) Outer core. It extends from 2900 to 4982 kms. It is considered to be in a state of homogeneous fluid and it does not transmit S-Waves.

(ii) Middle core. It is a transition layer, extends from 4982. kms to 5121 kms. The material is in a fluid to semi-fluid state.

(iii) Inner core. It is believed to contain metallic nickel and iron and is called ‘nife’. It is probably solid with a density of about 18. Its thickness is 1250 kms.

Other Important Facts:

1. The central-temperature is estimated to be 6000°C.

2. The central-pressure is 392 X108 bars in C.G.S unit.

3. Density at the centre is 18 gm/cm8.

4. Lithosphere constitutes the upper horizon of the crust only up to a depth of about 16 km.

5. Asthenosphere is the layer beneath the lithosphere which virtually has no strength to resist deformation.