How does temperature affect the coalification process?

Temperature is the primary factor driving the coalification process, which transforms vegetable matter into coal of increasingly higher rank, such as lignite, bituminous, and finally anthracite.12

As organic material is buried, it is exposed to the Earth's geothermal gradient, which typically increases temperature with depth. While this gradient averages about 30 °C per kilometre, it can vary significantly based on regional geological activity. Research indicates that temperatures below 200 °C are sufficient to reach the highest coal rank of anthracite.1

In contrast to temperature, increasing pressure from burial is not considered a driver of coalification and may even hinder the process by trapping volatile compounds like methane that need to escape. While time is a factor in laboratory settings, its role in nature remains less clear, as some low-rank brown coal deposits have remained unchanged despite being buried since the Carboniferous Period.1