Coal
Bituminous (top) and anthracite (bottom) coal. Images courtesy U.S. House of Representatives. (fair use policy)
Coal is fossilized peat, dead plant material that once piled deep on the bottom of ancient swamps.
Coal comes in three main types, or grades. First the swampy peat is squeezed and heated to form a brown, soft coal called lignite. In the process, the material releases hydrocarbons, which migrate away and eventually become petroleum. With more heat and pressure lignite becomes the higher-grade bituminous coal. Bituminous coal is black, hard and usually dull to glossy in appearance. Still greater heat and pressure yields anthracite, the highest grade of coal. In the process, the coal releases methane or natural gas. Anthracite, a shiny, hard black stone, is nearly pure carbon and burns with great heat and little smoke.
With still more heat and pressure, coal becomes a true mineral, graphite, made of pure carbon.
Learn more about coal in Coal in a Nutshell and the Coal Geology category. And for a glimpse at a hazard of coal mining, look at this old postcard of a culm fire.
See more sedimentary rocks in the Sedimentary Rocks Gallery.
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