A combustible rock that contains more than 50 percent
(by weight) carbonaceous material formed by the compaction
and induration of plant remains, coal is the most
abundant fossil fuel. It is a black sedimentary rock that consists
chiefly of decomposed plant matter, with less than 40
percent inorganic material. Coal is classified according to its
rank and impurities. With increasing temperature and pressure,
coal increases in rank and carbon content. Most coal was
formed in ancient swamps, where stagnant oxygen-deficient
water prevented rapid decay and allowed burial and trapping
of organic matter. In addition, anaerobic bacteria in these environments
attack the organic matter, releasing more oxygen and
forming peat. Peat is a porous mass of organic matter that still
preserves recognizable twigs and other plant parts. Peat contains
about 50 percent carbon and burns readily when dried.
With increasing temperature and pressure, peat is transformed
into lignite, bituminous coal, and eventually anthracite.
Anthracite contains more than 90 percent carbon, and it is
much shinier, brighter, and harder than bituminous coal and
lignite.
See also HYDROCARBON.
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