any of a class of materials of biological origin occurring within the Earth's crust that can be used as a source of energy.

Fossil fuels include coal, petroleum, natural gas, oil shales, bitumens, tar sands, and heavy oils. All contain carbon and were formed as a result of geologic processes acting on the remains of organic matter produced by photosynthesis, a process that began in the Archean Eon more than 3 billion years ago. Most carbonaceous material occurring before the Devonian Period (approximately 415 million years ago) was derived from algae and bacteria.

All fossil fuels can be burned in air or with oxygen derived from air to provide heat. This heat may be employed directly, as in the case of home furnaces, or utilized to produce steam to drive generators that can supply electricity. In still other cases—for example, gas turbines used in jet aircraft—the heat yielded by burning a fossil fuel serves to increase both the pressure and the temperature of the combustion products to furnish motive power.

Since the late 18th century, fossil fuels have been consumed at an ever-increasing rate. Today they supply (coal mining) nearly 90 percent of all the energy consumed by the industrially developed countries of the world. Although new deposits continue to be discovered, the reserves of the principal fossil fuels remaining in the Earth are limited. The amounts of fossil fuels that can be recovered economically are difficult to estimate, largely because of changing rates of consumption and future value as well as technological developments. For example, using current technology, a typical coal bed must be no less than about 60 cm (2 feet) thick and buried no more than about 2,000 m (6,600 feet) to be mined economically. However, advances in technology may make it possible in the future to mine thinner beds at greater depths at reasonable cost, increasing the amount of recoverable coal. Estimates of remaining petroleum resources are equally difficult. As recoverable supplies of conventional (light-to-medium) oil become depleted, it is expected that heavy oil, and oil extracted from tar sands and oil shales, will be exploited as sources of liquid petroleum on a wide scale. See also coal mining; petroleum production.