Organic sedimentary rocks are one of three types of sedimentary rocks, along with clastic sedimentary rocks and chemical sedimentary rocks. Coal is an organic sedimentary rock. Like other organic sedimentary rocks, coal is made from particles of formerly living things, called organic matter. Peat, lignite, and bituminous coal are all made from accumulated plant matter exposed to varying degrees of burial pressure, and all are considered different ranks of coal. Peat experiences the least pressure, making it more porous with visible plant pieces, while lignite is solid, but crumbly. Bituminous coal is more dense and sooty. The latter is what we mainly think of as coal, and it is the coal used as an energy source. It is often referred to as “soft coal,” but the name has nothing to do with the actual hardness of the rock.
An ideal environment and series of events is required for coal to be created. In general, a swampy environment is best. Swamp waters are typically oxygen deficient. When the plant material dies and falls into the water, the lack of oxygen slows the decaying process drastically. The water also protects the matter from organisms likely to consume it on land. In order for coal to form, the accumulation of dead plants must be greater than the rate of decay. Plant matter accumulates extremely slowly over thousands of years, and the swamp waters must remain stable in that time – too deep and the plants drown, too shallow and the dead plants are not protected. To produce a foot of coal, approximately ten feet of matter is required.
Coal formation is actually an odd occurrence in nature because bodies of water do not remain stable for long periods of time. Two situations make it possible to maintain the perfect water level for coal: First, a rising water level proportionate to the plant matter accumulation. Second, sinking land keeping pace with the plant matter accumulation. It is believed that most coal seams were created under the latter situation.
Finally, flooding rivers then cause sand or mud to wash into the swampy region, covering the organic sediments. This new material is the source of pressure, which slowly transforms the matter in rock. Coal requires more pressure than peat and lignite, but too much pressure will result in the metamorphic rock called anthracite coal.
Despite the improbable series of events required to form bituminous coal, it is the most abundant. It is used for electricity and heat production, as well as in the manufacturing of plastics, synthetic rubber, linoleum, and some medicines. It has a higher heating value than other types of coal and a carbon content between 77 and 87%. It can be either low volatile, medium volatile, or high volatile. Coal, as a sedimentary rock, is formed from sediments, but in this case they are mainly dead plant matter, which makes it an organic sedimentary rock.