Coalification

Coalification is the geological process by which plant-derived material is transformed into coal through diagenetic and low-grade metamorphic changes as organic matter is buried and subjected to increasing heat and pressure over long timescales. The initial stage involves accumulation of plant debris in wetlands, forming peat. With burial, peat is compacted and loses moisture and volatiles, while chemical rearrangements produce more ordered, carbon-rich substances.

As burial depth and temperature increase, the material passes through a series of ranks: lignite, sub-bituminous coal, bituminous coal, and, at still higher grades, anthracite. Each rank is characterized by higher carbon content, lower hydrogen and oxygen contents, reduced moisture, and greater energy density. The transformation also involves changes in the maceral composition and the development of a more coal-like structure.

Coalification is controlled by time, temperature, pressure, and chemical environment. It commonly occurs in sedimentary basins where thick sequences of peat-bearing sediments are buried by subsequent deposition and tectonic compression. The process can take millions of years and is often accompanied by compaction, dehydration, and the expulsion of coalbed fluids.

Geochemically, coalification is assessed by proxies such as coal rank indicators, including vitrinite reflectance, fixed carbon content, and calorific value. It is distinct from the unrelated maturation of kerogen into liquid and gaseous hydrocarbons, which occurs at higher temperatures (catagenesis). Coalification explains why coal deposits vary in quality and are distributed unevenly in the geological record.