How does coal form from dead plant material?

Coal forms from dead plant material through a multi-stage process known as coalification, which begins in wet, low-lying environments like swamps.12

The Formation Process

1. Peat Accumulation: In warm, humid regions, plant matter accumulates in stagnant, water-logged areas. Because these environments are often anaerobic (oxygen-poor), the plant material is protected from complete decomposition by fire or microorganisms.12
2. Biochemical Stage: Microorganisms break down the plant material, converting it into peat. This stage occurs in the upper few meters of a deposit where some oxygen may still be present.1
3. Geochemical Stage: As the peat is buried by increasing layers of sediment, it is removed from the reach of microbial life. The transformation into coal is then driven by physical changes associated with depth, primarily increasing temperature.13

As the material is subjected to higher temperatures over time, it progresses through various ranks—from lignite to subbituminous, bituminous, and finally anthracite. Evidence of this plant origin remains visible in coal through fossils and microscopic structures like cell walls and spores.32