coal, fuel substance of plant origin, largely or almost entirely composed of carbon with varying amounts of mineral matter.
There is a complete series of carbonaceous fuels, which differ from each other in the relative amounts of moisture, volatile matter, and fixed carbon they contain. Of the carbonaceous fuels, those containing the largest amounts of fixed carbon and the smallest amounts of moisture and volatile matter are the most useful to humans. The lowest in carbon content, peat, is followed in ascending order by lignite and the various forms of coal—subbituminous coal or black lignite (a slightly higher grade than lignite), bituminous coal, semibituminous (a high-grade bituminous coal), semianthracite (a low-grade anthracite), and anthracite.
Lignite and subbituminous coal, because of the high percentage of moisture they contain, tend to crumble on exposure to the air. Bituminous coal, being more consolidated, does not crumble easily; it is a deep black in color, burns readily, and is used extensively as fuel in industries and on railroads and in making coke. Anthracite, which is nearly pure carbon, is very hard, black, and lustrous and is extensively used as a domestic fuel. Cannel coal, a dull, homogeneous variety of bituminous coal, is composed of pollen grains, spores, and other particles of plant origin. It ignites and burns easily, with a candlelike flame, but its fuel value is low.
The vegetable origin of coal is supported by the presence in coal of carbonized fibers, stems, leaves, and seeds of plants, which can be detected with the naked eye in the softer varieties and with the microscope in harder coal. Sometimes carbonized tree stumps have been found standing in layers of coal. The general interpretation of these facts is that coal originated in swamps similar to present-day peat bogs and in lagoons, probably partly from plants growing in the area and partly from plant material carried in by water and wind. From the thickness of coal seams, it is assumed that the coal swamps were located near sea level and were subject to repeated submergence, so that a great quantity of vegetable matter accumulated over a long period of time.
The initial processes of disintegration and decomposition of the organic matter were brought about by the action of bacteria and other microorganisms. Peat, the first product formed, is altered to form lignite and coal through metamorphism. The pressure of the accumulated layers of overlying sediments and rock upon the submerged plant matter forced out much of the water and caused some of the volatile substances to escape and the nonvolatile carbon material to form a more compact mass. The greater the stress exerted in the process of metamorphism, the higher was the grade of coal produced. Cannel coal was probably formed in ponds, rather than in lagoons or swamps, as it occurs in lenticular masses and is frequently found to contain fossil fish. Coal was formed chiefly in the Carboniferous period of geologic time, but valuable deposits date also from the Permian, Triassic, Jurassic, Cretaceous, and Tertiary periods.
Coal is found in beds or seams interstratified with shales, clays, sandstones, or (rarely) limestones. It is usually underlaid by an underclay (a layer of clay containing roots of plants). The coal is removed by strip (surface) mining or underground mining methods (see coal mining).
The chief coal fields of the United States are the Appalachian (from N Pennsylvania into Alabama), the Eastern Interior (Illinois, Kentucky, and Indiana), the Northern Interior (Michigan), the Western Interior (Iowa, Kansas, Missouri, Oklahoma, and Arkansas), the Rocky Mountain (Colorado, Wyoming, Utah, New Mexico, Montana, and North Dakota), the Pacific (Washington), and the Gulf Coast (Texas, Arkansas, and Louisiana). In Europe the chief coal-producing countries are Germany, Russia, Ukraine, and Poland. There are valuable coal fields in China, India, Indonesia, Australia, South Africa, and Korea but only a few in South America, mainly in Colombia.