How does peat change into lignite?

The transformation of peat into lignite is a key stage in the process of coalification, which converts organic plant matter into coal of increasingly higher rank.12

This transition occurs as peat deposits are buried under layers of sediment. The process is driven by two primary physical factors:32

• Increasing Depth of Burial: As peat is buried deeper, it is subjected to higher temperatures.42
• Increasing Temperature: The geothermal gradient, which generally increases with depth, is the primary driver of coalification. Lignite typically forms at relatively shallow depths and at temperatures lower than 100 °C (212 °F).45

While microbial activity is responsible for the initial formation of peat near the surface, it ceases at depths below approximately 10 metres (about 30 feet). Consequently, the subsequent conversion of peat into lignite is a geochemical process controlled by these changing physical conditions rather than biological activity. This transition is gradual and typically occurs at depths ranging from 100 to 400 metres (approximately 330 to 1,300 feet).342