Category: Earth Science
When plants die, they most of the time decay rapidly, and the n they are converted by bacteria into water, carbon dioxide and salts. The process of decay is halted when plants are buried quickly in swamps or bogs, and so the partly rotted plants pile up in layers, these layers are compacted into carbonaceous rocks, and these rocks are mostly composed of carbon.
The first stage in coal formation can be seen in soggy swamps, marshes, and shallow lakes. The plants that are decaying form a substance called, â€œpeatâ€. This peat is then cut for fuel but first it must be dried, because 90% of its weight is water. Dry peat contains 60% carbon. The second stage in formation is â€œigniteâ€, or brown coal, which is similar to peat but much more compact and contains 60% to 75% when dry. It is used as a fuel and in the chemical industry. Bituminous or household coal is hard and contains up to 90% carbon. Anthracite, the last stage in coal formation contains 95% carbon, and the shiny black rock is clean to handle.
Most bituminous coal and anthracite mined today was formed around 300 million years ago. At that time, lush forests with high trees, enormous ferns, and mosses grew in swamp. Every once and awhile the swamps were submerged and eroded materials from the land such as sand and mud was spread over layers of the plants. Later the land rose and the forests grew again. As a result coal seams or beds alternate with layers of different rocks. A sequence of coal beds and other rock layers are called a "coal measure". Some of these coal measures may be 5000 feet thick and may contain 20 to 30 coal beds, and these can be between 60 feet thick, to just a few inches.
Some coal beds are sometime on, or just beneath surface, and are removed by open cast mining. Some other are much deeper and tunnels have to cut in order to reach them. In modern coalmines a machines cut the coal out and it is brought to the surface on conveyor belts.