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The Origins of Coal

Coal deposits come from many epochs, but the best and most abundant are from the forests in the warm, swampy river deltas of the Carboniferous period, some 320 million years ago. Long before the dinosaurs reigned, there was a time ruled by forests of giant ferns, reeds, and mosses. The earth was a warmer, steamier place back then and plants thrived, growing taller than our tallest trees today.

As these plants died, they fell into the swampy waters to form rich layers of peat. These layers were typically sandwiched between layers of sediment like shale, deposited when waters temporarily retreated. During the Permian period, about 290 million years ago, the seas receded entirely, and many coastal plains turned to desert. Sedimentary rocks like sandstone were laid down over the shale and peat. Later, limestone was laid down when the waters returned.

In time, the weight of the upper layers pressed down on the lower layers, causing tremendous pressure and heat. This triggered chemical changes in the peat, forcing out oxygen and hydrogen and leaving behind rich deposits of carbon, called coal.

The deeper the coal, the more pressure was exerted on it. This caused different grades of coal—from hard, pure anthracite to softer lignite. Between these layers lay bituminous coal, which is used in today’s electric utility power plants. About 52% of U.S. coal reserves are bituminous coal.