Coal the heat and pressure of deep burial causes the loss of water, methane and carbon dioxide and an increase in the proportion of carbon. Thus first lignite (also called "brown coal"), then sub-bituminous coal, bituminous coal, and lastly anthracite (also called "hard coal" or "black coal") may be formed. The wide, shallow seas of the Carboniferous Period provided ideal conditions for coal formation, although coal is known from most geological periods. The exception is the coal gap in the Permian–Triassic extinction event, where coal is rare. Coal is known from Precambrian strata, which predate land plants—this coal is presumed to
Environmental impact of the coal industry the environment. While these substances are trace impurities, enough coal is burned that significant amounts of these substances are released. During combustion, the reaction between coal and the air produces oxides of carbon, including carbon dioxide (CO, an important greenhouse gas), oxides of sulfur (mainly sulfur dioxide, SO), and various oxides of nitrogen (NO). Because of the hydrogenous and nitrogenous components of coal, hydrides and nitrides of carbon and sulfur are also produced during the combustion of coal in air. These include hydrogen cyanide (HCN), sulfur nitrate (SNO) and other toxic substances. SO and nitrogen oxide react in the atmosphere