Coal formation began to develop about 300 million years ago, during the time of Carboniferous period. This period witnessed wide, shallow seas and dense forests. The seas occasionally flooded the forested areas, thereby trapping plants and algae at the bottom of a swampy wetland. Some new plants grew, while some died and fell into the swamp waters. With time, thick layers of dead plants rotting in the swamp compounded and got buried and compressed under the weight of overlying mud and vegetation.
The surface of the earth changed and water and dirt washed in which hindered decaying process. More plants grew up, but they eventually died and fell, forming separate numerous layers one on top of the other. The weight of the top layers and the water and dirt packed down the lower layers of plant matter.
As the plant debris shifted deeper under Earth’s surface, it encountered increased temperatures and pressure. Mud and acidic water prevented the plant matter from coming into contact with oxygen and slowed down the decomposition rate of plant matter extensively These areas of buried plant matter are called peat.
Peat
Peat is an accumulation of partly decayed vegetation that has gone through a small amount of carbonization. Under the right conditions, peat undergoes changes and gets transformed into coal through a process called carbonization. Carbonization takes place under incredible heat and pressure.
**Peat formation is even taking place today in swampy areas like Indonesia.
Coalification
The process of coalification began when dead plant matter was protected from biodegradation and oxidation, by mud or acidic water, and was converted into peat. Heat and pressure of deep burial caused the loss of water, methane and carbon dioxide and increased in the proportion of carbon.
The transformation from a plant substance to a metamorphic rock starts around 3 — 4 kilometres of buried peat sediment. At this depth, with an average rate of temperature increase of 30°C per kilometre, the temperature rises to over 100°C and initiates chemical reactions that can transform the material into coal.
The grade of coal produced depends on the maximum pressure and temperature reached. Lignite is produced under relatively mild conditions, whereas sub-bituminous coal, bituminous coal, or anthracite coal forms under relatively much higher temperature and pressure. Coal is ranked according to how much it has changed over time. As per Hilt’s Law, the deeper the coal seam, the higher is its rank which means that with increase in depth, the material encounters greater temperatures and pressure, resulting in more carbon transformation.
