Coal, oil and natural gas are three of our most important fuels. They provide three-quarters of the energy we use. All are produced during the formation of sedimentary rocks.
Coal, oil and natural gas are called fossil fuels because, like fossils, they form in the ground over millions of years from the remains of dead plants and animals. When one of these fossil fuels is burned it produces heat, which is used to heat buildings or to power motor vehicles. In power stations, the heat from fossil fuels is used to produce electricity.
HOW COAL IS FORMED
Coal is a sedimentary rock made of carbon. This carbon is the remains of plants that formed low, swampy forests hundreds of millions of years ago. Many of these plants are related to the ferns and club mosses that still grow today.
As the coal-forest plants died, they fell into the swamps in which they grew. Over time, the swamps grew deep in dead and decaying plant materials. Then the sea invaded the swamps and covered the plant materials with mud, sand, and clay. Later, the sea retreated and a new forest grew upon the mud, sand, and clay. This happened many times until there were several layers of dead plants. The weight of the mud, sand, and clay above them gradually squashed these layers, and so helped to compact them into thick underground layers, or seams, of coal.
EXTRACTING AND USING COAL
If coal deposits are at or near the surface, they are dug out of vast pits called opencast mines. If the coal is deep down, underground mine shafts are dug down to the seams.
Coal is no longer used in huge quantities to heat homes. Some power stations still burn coal to produce the steam that drives electricity generators. When coal is heated without air it produces coke, which is used in making steel. Bitumen for surfacing roads comes from coal, and coal also contains chemicals used to make dyes, drugs and some plastics.
OIL AND NATURAL GAS
Oil was formed when millions of tiny sea animals died and sank to the bottom of the oceans. Their bodies turned to slime and this became buried by layers of mud and sand. After millions of years, the slime, mud, and sand turned to shale and sandstone rocks, while the compressed slime turned to oil and natural gas.
Oil and natural gas filled the tiny pores, or gaps, in porous rocks such as sandstone. Inside these rocks, the oil floats on top of the water, and any natural gas floats on the oil, as gas is lighter than oil or water.
Today, deposits of oil and natural gas are found by drilling deep under the seabed, or underground, because some areas that were once sea are now dry land.
USING NATURAL GAS AND OIL
Natural gas is used as a fuel in homes for heating and cooking and as a source of heat for industry. Some power stations burn gas to produce electricity.
The crude oil taken from the ground or from under the sea contains many useful substances, including petrol, diesel, kerosene and a vast collection of chemicals. Thousands of useful products are made from these, including detergents, dyes, explosives, pesticides and a vast array of plastics and artificial fibres.
PROBLEMS WITH FOSSIL FUELS
The world depends on fossil fuels, but as we are using them up much quicker than they can form, so eventually they will run out. Burning fossil fuels also produces smoke, fumes and waste gases that pollute the air, causing illnesses and damaging buildings and other structures. One of these waste gases is carbon dioxide, which enters the atmosphere and contributes to global warming.