The Crust of Earth
* It is the outermost and the thinnest layer of the earth’s surface, about 8 to 40 km thick. The crust varies greatly in thickness and composition – as small as 5 km thick in some places beneath the oceans, while under some mountain ranges it extends up to 70 km in depth.
* The crust is made up of two layers- an upper lighter layer called the Sial (Silicate + Aluminium) and a lower density layer called Sima (Silicate + Magnesium).
* The average density of this layer is 3 gm/cc.
The Mantle of Earth
* This layer extends up to a depth of 2900 km.
* Mantle is made up of 2 parts: Upper Mantle or Asthenosphere (up to about 500 km) and Lower Mantle. Asthenosphere is in a semi-molten plastic state, and it is thought that this enables the lithosphere to move about it. Within the asthenosphere, the velocity of seismic waves is considerably reduced (Called ‘Low Velocity Zone’).
* The line of separation between the mantle and the crust is known as Mohoviricic Discontinuity.
The Core of Earth
* Beyond a depth of 2900 km lies the core of the earth.
* The outer core is 2100 km thick and is in molten form due to excessive heat out there. Inner core is 1370 km thick and is in plastic form due to the combined factors of excessive heat and pressure. It is made up of iron and nickel (Nife) and is responsible for earth’s magnetism. This layer has the maximum specific gravity.
* The temperatures in the earth’s core lie between 2200°c and 2750°c.
* The line of separation between the mantle and the core is called Gutenberg-Wiechert Discontinuity.