Fossil fuels come from the ancient remains of plants, algae, and tiny sea animals that were buried and changed inside Earth over millions of years.

Basic idea

Over hundreds of millions of years, layers of dead plants and microscopic organisms built up in swamps, lakes, and oceans, then were buried by mud and sand. Heat and pressure deep underground slowly turned this buried organic material into coal, oil, and natural gas.

Coal

Coal mainly comes from ancient plants and ferns that lived in swampy forests long before humans existed. When these plants died and were buried without much oxygen, they were gradually compressed and heated until they turned into coal.

Oil and natural gas

Oil and most natural gas come from tiny marine organisms like plankton and algae that lived in ancient seas. After they died, they sank to the seafloor, were buried by sediments, and over millions of years were transformed by heat and pressure into liquid oil and gaseous hydrocarbons.

Why they’re called “fossil” fuels

They are called fossil fuels because they come from ancient, once-living organisms whose remains were preserved and altered in Earth’s crust, not because they mainly come from dinosaurs. Because this process takes millions of years, fossil fuels are considered non-renewable resources on human time scales.