How Coal Forms: Stages of Coal Formation (simple, unlabeled) — free printable diagram

How coal forms, stage by stage

Coal is a fossil fuel formed from the remains of plants that grew in warm, wet swamps and were buried before they could fully decay. The diagram shows the sequence: dead plant material builds up under water, is buried under sediment, and is then changed by heat and pressure over a very long time into progressively harder, more carbon-rich forms.

Starting material

Dead plant matter in waterlogged swamps

Stage 1

Peat — partly decayed plant material

Stage 2

Lignite — soft brown coal

Stage 3

Bituminous coal — harder, darker, widely burned

Stage 4

Anthracite — hardest, highest carbon content

What drives it

Burial, heat and pressure over millions of years

• The plants must be buried in waterlogged ground. Ordinary decay needs oxygen, so swamp water is what stops the plant material rotting away completely and allows it to accumulate.
• As layers of sediment build up above, the buried material is pushed deeper, where it is both squeezed and heated. Water and other compounds are gradually driven off.
• Each stage has a higher proportion of carbon than the one before, which is why the coal becomes harder, darker and releases more energy when burned as the sequence progresses.
• The process takes millions of years, which is why coal is described as non-renewable: it is being used far faster than any new coal could form.
• Peat is the first stage and is still forming today in bogs and wetlands. It is a useful reminder that the diagram shows a process still under way at its start, not one confined to the distant past.
• Oil and natural gas form differently — chiefly from marine organisms rather than land plants — so the coal sequence should not be presented as the general story of all fossil fuels.