Repeating the process many times and over many years created layers of rotting vegetation, gradually compressed under tons of sediment and debris. With the working of heat, time, and the movements of the Earth's surface, these layers eventually turned into the coal beds we know today. Gradual weathering and reshaping of the Earth's surface brought many of these coal beds closer to the surface and thus within the reach of humans.
Most coal was formed during the Carboniferous period, a geological era that occurred between 360 and 286 million years before today. This period predates the appearance of dinosaurs and most of the natural world we know today, although the physical conditions that lead to the formation of coal can still be observed in the swamps and bogs of the southeastern United States. Evidence suggests that humans have used coal for several thousand years, but it was only around 800 years ago that it was first extensively mined and used. Coal is directly responsible for many of the human achievements associated with the Industrial Revolution, and is currently indispensable for the energy demands of human civilization at the beginning of the twenty-first century.
Coal is defined based on the relative amount of carbon it contains, as well as the heat value it produces. There are four types, or ranks, of coal that are generally recognized, although the variation within a rank can be high, based on local geological conditions.
Lignite coal contains the least amount of carbon. Soft and brown, usually with visible fossilized plants, it is commonly found under the plains and prairies of North America. Lignite is mostly used in coal-fired power plants. If buried further and subjected to time and pressure, lignite would turn into a harder, more recognizable form of coal.
Bituminous coal can be either shiny or a dull black, and has a higher carbon content than lignite. The most common type of coal, it is easy to light and burns with a smoky yellow flame, and is suited for a range of uses, from domestic to industrial production. It is used to produce coke, which is important for iron and steel production. Bituminous coal is found throughout the western United States, including the Cascade Range of Washington.
Sub-bituminous coal has less carbon than bituminous coal, and also a lower sulfur content. This means that sub-bituminous coal is a generally clean burning coal; it s also found throughout the coal regions of the western United States.
Anthracite coal is shiny, hard, and brittle. Formed under intense pressure, it has high carbon content and is much denser than bituminous coal. Difficult to ignite, once lit it burns with a blue flame, generates intense heat and produces almost no smoke. The classic coal fields of the eastern Unites States such as those in Pennsylvania and West Virginia produce anthracite coal.