Geology - Interior of the Earth

Lithosphere :- The solid Earth is known as Lithosphere. An idea about the constituents of this solid body of about 6370 km radius , can be had on the basis of several available indirect observations. Say for example , the average density of the whole earth is 5.5 , whereas that for the rocks making most of the upper crust of the Earth is 3 , which infers that the material beneath the surface is of a much higher density. The temperature indications show that it increases with the depth. The hat lava erupted by the volcanoes also indicates that the deep interior parts of the Earth are very very hot. The pressure as also been observed to be increasing with depth. An analysis of the meteorites falling on earth indicates that they essentially consist of iron and nickel in their core. Since the earth is also a celestial body like That of the meteorites , it can be easily inferred that out of Earth may also contains iron and nickel in it's core.
From all such indirect evidence , it has been generally established that the matter inside the earth is of a higher density , and the deep interior part of the Earth may be in liquid state ; or because of high pressure , it may be in semi-liquid state.
In the modern times , however , our knowledge regarding the interior of the Earth has been vastly increases by the experimental results obtained with the help of seismic waves. During an earthquake ( natural or artificial produced by an explosion ) , three types of waves are generated , known as primary (P) , Secondary (S) , and Long (L) waves. These waves radiated in all directions passing through the earth body , moving faster in the higher density medium. The L - waves which confines themselves to the crystal part , travel faster along the ocean floors. This indicates that the ocean floor composed of a material denser than that of the upper crust of the continent basins. The P and S waves which propagate through the deeper parts of the Earth , are subjected to reflection and refrection with the changes in density. The S-Waves are further characterised by their capablity of passing only through the solids. 
Based upon such experimental work, carried out with the help of such seismic waves , the interior of the Earth can be predicted today , with an enough degree of confidence.
In all , the entire 6370 km thickness of the Earth can be divided into the following three major zones :-
(A) The Crust
(B) The Mantle
(C) The Core

(A) The Crust :- The topmost thin layer of the earth body , having solid thickness of about 30 to 40 km In continents and 5 to 6 km in the oceans , is known as the crust. The base of the crust is marked by a sharp boundary , called Mohorovicic discontinuity or M - discontinuity. The depth of the M - discontinuity is greater beneath the continents than beneath the ocean basins.
Intact , it has been concluded that in the continents , the total depth is about 35 km , out of which the bottom of 5 km depth consists of denser Basalt rock ( density = 3.0 ) , and the top 30 km consists of lighter Granite rock ( Density = 2.7 ) . There is no such granite beneath the oceans , and only 5-6 km thick basalt occurs. These granitic rocks of the continents and the basaltic rocks of the oceans , are generally covered by a top layer of unconsolidated sediments deposited since the formation of these continents or oceans.
The earth crust thus provides us with very useful hard and soft rocks , and are classified as , Igneous , Sedimentary , and Metamorphic rocks , depending upon the certain factors.

(B) The Mantle :- Beneath the M - discontinuity , lie the still denser rocks ( average density 3.3 ) known as the ultra basic rocks , and this portion of the earth is known as the Mantle. It's thickness is about 2860 km and it's density increases with depth.

(C) The Core :- Below the depth of about 2900 km from the surface of the earth , starts the cores. It has been inferred by the study of seismic waves that the earths core is probably 12 or more , but it's exact compositions is not known. Most of the scientists is favour in iron - nickel composition , while some other support the idea of iron - silicate core.
An abrupt increase in the velocity of P - waves at about 2170 km below the core boundary , suggests the presence of an inner core.
This may mean a change from the liquid to the solid state , but this is not well established fact.
The presence of earth's magnetic field may also be explained under this theory of the presence of a liquid core. This may be due to the electric current set up by motion in the liquid core. This is possible because the core is thought to be metallic and , therefore , a good conductor.