On the basis of the physical and chemical properties of the earth’* surface, behavioral patterns of seismic waves of earthquakes and the lava erupted form volcanoes, it is generally held that the earth is composed of three layers or parts.
1. The crust: The outer or upper most layer of the earth is the crust. It is solid and is the thinnest and lightest part. It is 6 to 60 kms thick. It has two layers.
a. Continental crust or sial: The upper part consist of sedimentary and granite rocks’ and forms the continents. Its major constituent elements are silica and aluminum. So, it is termed as sial. Its average thickness ranges between 10-12 km.
b. Oceanic crust or Sima: The lower part of other Crust is known as oceanic crust. Its thickness is around 5 km. This layer is rich in silica and Magnesium. Therefore, it is called Sima. The lower continuous layer is mostly composing of silica and magnesium and so it is termed as sima. The mean density of this layer is 2.95 gm. Conrad discontinuity between Sial and Sima and Mohorovicic discontinuity between Crust and Mantle are the major discontinues in the crust.
2. The Mantle: The portion of the earth’* interior, lying beneath the crust and above the core is known as mantle. It is largely made up of basic silicates, rich in iron and magnesium. It extends from 60 to 2,900 km. The density of this layer from 3.3 to 5.7gm. The mantle has two parts.
a. Upper Mantle: This is the outer layer of the Mantle known as Asthenosphre. Most of the earthquakes and volcanoes take birth due to disorder in this layer.
b. Lower Mantle: This is the lower layer of the Mantle known as Mesosphere and it is in solid form. Repetti discontinuity between Asthenosphere and Mesosphere and Gutenberg discontinuity between Mantle and Core are the major discontinues in the mantle.
3. The core: The core is the central part of the earth’* interior. It extends from the lower boundary of the mantle to the centre of the earth for about 6,400 kms. The most important materials of core are nickel and ferrous (Nife). The core consists of two layers, namely.
a. Outer core: It is the outer layer of the Core consisting hard molten rocks. Most of the materials are in molten form. It extends from 2900km to 4980km.
b. Inner core: It is the lower layer of the Core with very high temperature and pressure. Most of the materials are in solid form therefore, it is called solid core. The average temperature of this layer is around 2900° C. Transition discontinuity is found between Outer. Core and Inner Core.
