When the earth melted about 800 million years after its formation, its material was reorganized under influence of gravity into three distinct layer of different density.
The three layers thus formed are:
1. The outer-most crust
2. The mantle
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3. The central core.
Image Source: loe.org/content/2013-03-15/Layers.jpg
i) The outer-most crust of earth :
The outermost layer of the earth called, outer crust, is just like the skin around an apple or an orange. Its thickness varies from place to place. Under continents, its thickness ranges from 35 to 60 km. Under oceans, its thickness is about 10 km. It is generally composed of light elements.
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ii) The mantle :
The region between the crust and the ore of the earth is called mantle. This layer extends from the base of the crust downwards to depth of about 2900 km. It is believed to be made up largely of iron and magnesium silicates.
iii) The central core :
The central core of the earth consists of inner solid sphere surrounded by an outer fluid shell. In this part of the earth the temperature is about 4000 degree Celsius and the pressure is about 3.7 million times the atmospheric pressure. This layer mainly contains molten iron with some nickel. Due to high pressure solid iron is also present.