When peats are buried, the weight of the overlying sediments squeezes out much of the water from the peat and reduces its volume (called compaction). Continued burial deeper into the earth also exposes the material to higher temperatures. Heating, and to a lesser extent, time and pressure act on the buried peat to change it into coal. The stages of coalification proceed through different ranks of coal (lignite, sub-bituminous coal, bituminous coal, anthracite coal).
The process of coalification involves chemical and physical changes to organic and inorganic compounds. Coalification continues to the rank achieved by the highest heating during burial history. Once a coal achieves a certain rank, the process cannot be reversed.
Learn more about heat, time, pressure, and coalification