How Coal Is Formed

Coal is called a fossil fuel because it was formed from the remains of vegetation that grew as long as 400 million years ago. It is often referred to as "buried sunshine," because the plants that formed coal captured energy from the sun through photosynthesis to create the compounds that make up plant tissues. The most important element in the plant material is carbon, which gives coal most of its energy.

Most of our coal was formed about 300 million years ago, when steamy swamps covered much of the earth. As plants and trees died, their remains sank to the bottom of the swampy areas, accumulating layer upon layer and eventually forming a soggy, dense material called peat.

Over long periods of time, the makeup of the earth's surface changed, and seas and great rivers caused deposits of sand, clay and other mineral matter to accumulate, burying the peat. Sandstone and other sedimentary rocks were formed, and the pressure caused by their weight squeezed water from the peat. Increasingly deeper burial and the heat associated with it gradually changed the material to coal. Scientists estimate that from 3 to 7 feet of compacted plant matter was required to form 1 foot of bituminous coal.