Hiiltyminen

Hiiltyminen, known in English as coalification, is the geological process by which plant-derived material is transformed into coal through burial, compression, and chemical alteration under increasing temperature and pressure over long timescales. It occurs in sedimentary basins where organic matter is deposited in wet, anoxic conditions and then buried by additional sediments.

The process begins with the formation of peat from accumulated plant debris. With continued burial and limiting decay, peat is compacted and altered, producing lignite—the lowest rank of coal. Further heating and pressure drive chemical changes that yield progressively higher ranks: sub-bituminous coal, bituminous coal, and, under higher temperatures and pressures, anthracite. The ranking reflects increasing carbon content and energy density and decreasing moisture and volatile matter. A common way to assess coal rank is through vitrinite reflectance, which increases as coal matures.

Several factors influence coalification, including burial depth, geothermal gradient, duration of burial, moisture content, original vegetation, and the chemical environment within the surrounding rocks. The mineral and fluid content during burial can also affect the rate and pathway of transformation. Coalification is a slow, long-term process that records the thermal and burial history of a basin.

Geological significance and applications: coalification helps determine the quality and type of coal available in a basin, guiding resource assessment and extraction strategies. It also provides insights into past environmental conditions and sedimentary basin evolution. Coal deposits formed through this process are distributed worldwide, with major occurrences in many historic coal basins across North America, Europe, and Asia.