Internal Structure of the Earth

On the basis of seismic investigations, the earth can be divided into three major layers: crust, mantle, core.
Differentiation of layers of Earth

Crust: The outer superficial layer of the earth is called the "crust". In continental regions, the crust can be divided into two layers.
The upper layer which is less dense and granitic in character, is known as "sial", while the lower layer which is basaltic in character is known as "sima".
It extends down to 30 or 40 kilometer beneath continents and to about 10 km beneath ocean basins

Mantle: The mantle is located beneath the earth's crust and has a thickness of about 2900 km. It has been divided into two layers: 
(i) upper mantle, and
(ii) lower mantle.
The boundary between these is at about 700 km depth.
The upper mantle contains a most important zone called the "asthenosphere". It is located at depths between 50 to 100 km.
This zone provides lava for volcanic eruptions.

Core: The core (inner core and the outer core) accounts for just about 16 per cent of the earth's volume but 33% of earth’s mass. Like Mantle, core can also be distinguished into two layers namely outer core and inner core.   
The outer core is composed of iron mixed with nickel and trace amounts of lighter elements.      
The outer core is not under enough pressure to be solid, so it is liquid even though it has a composition similar to the inner core.