Damberger, Heinz H. Illinois State Geological Survey, Champaign, Illinois.
Last reviewed:March 2019
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- Coal seams
- Coal rank
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A brown to black combustible carbonaceous sedimentary rock that is mined and used primarily as a fuel. Coal originated by accumulation and subsequent physical and chemical alteration of plant material over about 300 million years, and contains at least 50 percent by weight carbon. The plant debris accumulated in various wet environments, commonly called peat swamps, where dead plants were largely protected from decay by a high water table and oxygen-deficient water. The accumulating spongy, water-saturated, plant-derived organic material, known as peat, is the precursor of coal. Over time, many changes of the original vegetable matter were brought about by bacteria, fungi, and chemical agents. The process progressively transformed peat into lignite or brown coal (lowest quality and carbon content), subbituminous coal, bituminous coal, and anthracite (highest quality and carbon content, Fig. 1). This progression is known as the coalification series. Increasingly deeper burial under hundreds to thousands of meters of younger sediments is required to advance coalification to the bituminous coal and anthracite stages. The pressure exerted by the weight of the overlying sediment and the heat that increases with depth, as well as the length of exposure to them, determine the degree of coalification reached. See also: Coal paleobotany; Coal mining; Fossil fuel; Kerogen; Lignite; Peat; Sedimentary rocks
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