Coal forms from ancient plant material through a process called coalification, driven by heat, pressure, and time.
- Peat formation: In swamps, plant remains accumulate in waterlogged, oxygen-poor conditions. Partial decay produces peat.
- Burial and diagenesis: Over time, sediment covers peat. Compaction and chemical changes begin as heat and pressure increase.
- Formation of lignite (brown coal): Continued burial and mild heat transform peat into lignite, a soft, darkened coal.
- Transition to sub-bituminous coal: More burial and higher temperatures expel water and increase carbon concentration, creating sub-bituminous coal.
- Formation of bituminous coal: Even higher heat and pressure yield dense, energy-rich bituminous coal, often used for industry.
- Formation of anthracite: In regions with significant tectonic compression and high temperatures, bituminous coal can metamorphose into anthracite, the hardest, highest-grade coal.
Notes: Coal types vary by carbon content and energy. The stages are a general progression; not all coal forms reach all stages in every location.