Coal is a fragile sedimentary rock formed from the accumulation of plant material in a low oxygen environment, such as a swamp, and buried in sediment. The plant material is subjected to heat and pressure from burial and transforms into coal. A variety of factors contribute to the quality of coal produced, such as the type and amount of plant material, temperature and pressure, and length of time buried. There are four main types (ranks) of coal, from lowest carbon content to highest: lignite, sub-bituminous, bituminous, and anthracite. Bituminous coal is further divided into thermal coal and metallurgical (coking) coal. Thermal coal is used for energy production and metallurgical coal is used for steel manufacturing. Geopolitical concerns have driven recent increases in demand for thermal coal for power generation and demand for metallurgical coal is expected to remain strong.