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. On average, heat in the earth rises one degree Farenheit per 70 to 100 ft of depth. Heating during burial can also occur through interaction with hydrothermal fluids, or through contact metamorphism (Hower and Gayer, 2002; Ruppert and others, 2010). Increased pressure can influence some processes at higher ranks, and increasing time influences coalification by providing more chances for heating (Stach and others,1982; Bustin and others, 1995).
Heating causes hydrocarbon compounds (compounds composed of hydrogen, carbon, and oxygen) in the peat to break down and alter in a variety of ways resulting in coal. In general, moisture and gases (e.g., methane, carbon dioxide) are systematically expelled from the peat and resulting coal with increasing burial and heat. This results in increasingly carbon-rich coals. The stages of this trend proceed from peat through different ranks of coal (lignite, sub-bituminous coal, bituminous coal, anthracite coal). The process of coalification continues to the rank achieved by the highest heating achieved during burial history. Once a coal achieves a certain rank, the process cannot be reversed.