 Coal is formed when dead plant matter decays into peat and is converted into coal by the heat and pressure of deep burial over millions of years.Vast deposits of coal originate in former wetlands—called coal forests—that covered much of the Earth's tropical land areas during the late Carboniferous (Pennsylvanian) and Permian times.As a fossil fuel burned for heat, coal supplies about a quarter of the world's primary energy and two-fifths of its electricity.