fossil fuel

The formation of coal. Coal forms where vegetable matter accumulates but is prevented from complete decay and forms peat beds. Over time it becomes buried and compressed, forming lignite. Increased pressure and temperature produces bituminous coal with a higher carbon content. At great depths, high temperatures reduce methane and anthracite is formed with a very high carbon concentration.

The formation of oil and natural gas. Oil forms when marine plants and animals die and accumulate in stagnant water lacking in oxygen. They are quickly buried by clay and so do not completely decay but form hydrocarbon-rich muds, broken down by anaerobic bacteria. Increasing heat and pressure transform the hydrocarbons into fatty acids, which are then changed into an asphaltic material, keragen. Further increases in temperature and pressure cause oil to form and natural gas collects above the oil.

Deposits of oil and gas are found close together because gas is given off during the formation of oil.

Combustible material, such as coal, lignite, oil, peat, and natural gas, formed from the fossilized remains of plants that lived hundreds of millions of years ago. Such fuels are non-renewable resources - once they are burnt, they cannot be replaced.

Fossil fuels are hydrocarbons (they contain atoms of carbon and hydrogen). They generate large quantities of heat when they burn in air, a process known as combustion. In this process carbon and hydrogen combine with oxygen in the air to form carbon dioxide, water vapour, and heat.

Extraction of coal and oil causes considerable environmental pollution, and burning coal contributes to problems of acid rain and the greenhouse effect.