Coal
Coal is an organic sedimentary rock made from sedimentary soil with plant remains which is accumulated into coal by pressure.
Origin
Coal forms from the accumulation of plant debris, usually in a swamp environment. When a plant dies and falls into the swamp, the standing water of the swamp protects it from decay. Swamp waters are usually deficient in oxygen, which would react with the plant debris and cause it to decay. This lack of oxygen allows the plant debris to persist. In addition, insects and other organisms that might consume the plant debris on land do not survive well under water in an oxygen-deficient environment.
To form a coal seam, the ideal conditions of perfect water depth must be maintained for a very long time.The ideal conditions are: 1) a rising water level that perfectly keeps pace with the rate of plant debris accumulation; or, 2) a subsiding landscape that perfectly keeps pace with the rate of plant debris accumulation. Most coal seams are thought to have formed under condition #2 in a delta environment. On a delta, large amounts of river sediments are being deposited on a small area of Earth's crust, and the weight of those sediments causes the subsidence.
For a coal seam to form, perfect conditions of plant debris accumulation and perfect conditions of subsidence must occur on a landscape that maintains this perfect balance for a very long time. It is easy to understand why the conditions for forming coal have occurred only a small number of times throughout Earth's history. The formation of a coal requires the coincidence of highly improbable events.
'Coalification'
Coal knows different stages of its organic metamorphism. In each stage it loses an amount of its water and it becomes a harder soil or rock type. In the table below every stage and description can be found.
Coalification table
| Stage | Description |
|---|---|
| Peat | A mass of recently accumulated to partially carbonized plant debris. Peat is an organic sediment. Burial, compaction, and coalification will transform it into coal, a rock. It has a carbon content of less than 60% on a dry ash-free basis |
| Lignite | Lignite is the lowest rank of coal. It is a peat that has been transformed into a rock, and that rock is a brown-black coal. Lignite sometimes contains recognizable plant structures. It has a carbon content of between 60 and 70% on a dry ash-free basis. |
| Sub Bituminous | Sub bituminous coal is a lignite that has been subjected to an increased level of organic metamorphism. This metamorphism has driven off some of the oxygen and hydrogen in the coal. That loss produces coal with a higher carbon content. |
| Bituminous | Bituminous is the most abundant rank of coal. Bituminous coal is formed when a sub bituminous coal is subjected to increased levels of organic metamorphism. It has a carbon content of between 77 and 87% on a dry ash-free basis. |
| Anthracite | Anthracite is the highest rank of coal. Unlike other types of coal, it is usually considered to be a metamorphic rock. It has a carbon content of over 87% on a dry ash-free basis. |
