Where does coal come from?

Fossils of dichrodium ferns found in silt parting in coal beds, infers that such ferns gradually became coal. These fern fossils were found in the Surat Basin in Queensland.
Source: Photo by Firstashore .

Coal is formed from plant remains that rotted in swamps or were buried underground.

Over time, these remains were covered by layers of rock, soil and vegetation and compacted by heat and pressure. This caused a series of chemical changes to take place turning the remains into a thick liquid substance known as peat.

Over further time peat that remained buried underground was subjected to further heat and pressure. This caused it to hardened into brown coal, or lignite.

When left for even more time further chemical processes turned the brown coal into black coal.

The process of turning plant remains into coal is called coalification.

A layer of rotting plant material on the floor of a lake is covered with layers of sediment and mud. Over a very long period of time, this plant material becomes deeply buried. Increasing heat and pressure cause changes and the slowly rotting plant matter turns to peat. As the peat ages heat and pressure continue to cause changes and brown, then eventually black coal is formed.
Source: Image courtesy of Australian Coal Association

Source: Image courtesy of Australian Coal Association

Scientists believe the process of coalification takes millions of years.

It is estimated that Australia's oldest reserves of black coal, found in New South Wales and Queensland, were formed between 225 and 180 million years ago.

Younger black coals mined in Queensland, South Australia and Tasmania are believed to be between 140 and 180 million years old, and Victoria's brown coal reserves were formed less than 45 million years ago.