The formation of earth took place as a cold aggregate of planetesimals. These were mainly made of compounds of silicon, Iron and magnesium with traces of other elements. As more and more planets collided with the earth and struck to it, their kinetic energy was converted into heat. Also due to radioactive disintegration of uranium, thorium and potassium and compression of the earth the planet got heated.
At last after 800 million years of its formation, it melted due to force of gravity and the earth began to reorganize itself. The increased temperature of the earth melted iron. The drops of molten iron began to fall to the centre of the earth and consequently, the lighter substance came on the surface of the earth and formed earth’s crust. The molten iron, which fell at the centre, made core of the earth. This organized earth into different layers. This state is called differentiation. During this process, gases and water vapors trapped in the molecules of primitive materials were liberated and formed the atmosphere and the oceans.
Due to differentiation, the earth was organized into three major layers:
1. The crust of the earth
2. The mantle
3. The core
Three forth part of the crust of the earth is covered with water and above that there is an envelope of atmosphere.