Fossil fuels – coal, oil and gas were formed from the remains of living things that died millions of years ago and are preserved as fossils.
Coal began to form about 350 million years ago. At that time parts of the Earth’s surface were covered with swamps and lakes. Forests of huge trees and giant ferns grew in the swamps. When these plants died, they rotted down and gradually changed into a type of dark soil called peat. As the centuries passed the peat was buried under layers of sand and mud. Successive layers pressed down more and more tightly until the peat was compressed into layers of hard, black, shiny rock—coal. Folding and faulting of rock layers, the result of Earth movements over millions of years, together with erosion, have brought some coal layers close to the surface and within reach of underground mines.
Coal was first mined as a fuel on an industrial scale in the 18th century when it was used in furnaces to power steam engines and smelt iron. Today it is used in power stations to produce electricity. Coke, a baked form of coal, is a smokeless fuel used in making iron and steel.
OIL AND GAS
Oil is a very important substance. It is used as a fuel in power stations, cars, ships and aircraft, and is an essential raw material for plastics and chemical industries.
Oil and gas were formed in the seas millions of years ago. When the tiny plants and animals that lived in them died they sank to the bottom and were buried under layers of sand and silt. These were gradually compressed into layers of sedimentary rock. The heat action of bacteria changed the remains into crude oil and natural gases.
Pressures in the Earth force the oil up through the sedimentary rock, which has tiny holes in it like a sponge. The oil rises until it comes to a layer of hard rock. If the hard rock has formed a dome over the soft rock, the oil is trapped under it. Geologists looking for oil study the local rock formations and make test drillings. If oil is found, wells are drilled into the ground. When the drill reaches oil, the pressure may be enough to send it gushing up to the surface. If not, it is pumped. The crude oil from the well is sent by pipeline or tanker to a refinery where it is separated into different substances by distillation.
The oil is boiled at the bottom of a huge tank called a fractionating tower. The vapours that are formed float upwards, cool and condense into liquids at different temperatures. Trays at different heights in the tower collect the liquids as they form. These separate parts, or fractions, are formed into different oil products, such as petrol, kerosene and diesel oil. At the top of the tower, gas comes off. The thickest, heaviest products, such as bitumen, used for making roads, sink to the bottom. They can be refined again to make lubricating oils.
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