Describe briefly about the formation of coal.

The process of formation of coal is described below:

1. Coal was formed by the bacterial decomposition of ancient vegetable matter buried under successive layers of the earth.
2. Under action of high pressure and temperature, in absence of air, the decayed vegetable matter converted into coal through a series of steps as shown in the figure below.
3. With each successive layer of coal formed, the amount of carbon present in the deposit increased and level of impurities decreased.
4. This process of formation of coal is known as carbonization which took millions of years.