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Theory:

Fossil fuels and their formation
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Fossil - dead remains of plants and animals buried under the soil, sea over billions of years ago.
Fuels- Substances that burn in the presence of oxygen to produce heat and energy.
 
Fossil fuels are non-renewable resources as they are available in limited quantity in nature. Fossil fuels are formed due to the degradation of dead plants and animals buried under the soil, sea billions of years ago due to natural calamities.
 
The degradation of plants and animals occurs under extreme conditions such as the absence of air, high pressure and temperature. Coal, natural gas, crude oil are called fossil fuels.
 
Formation of Coal:
 
It takes \(300\) million years for the formation of coal. Due to Natural calamities like floods, earthquakes, and volcanic eruptions lead to the burial of dense forests under the earth's surface.
 
Thus, the dead vegetation get compressed slowly in extreme conditions such as a rise in temperature and pressure along with the absence of air.
 
The formation of coal takes in a slow step process where there are four stages before the formation of coal. They are
PeatLigniteBituminousAnthraciteCoal
 
The slow process of conversion of dead vegetation into coal is called carbonisation.
 
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Formation of Petroleum:
  
Over millions of years ago, the dead sea organisms such as algae and plants that lived in shallow seas settled down the bottom of the sea. They got covered with sand clay, and they got compressed with high pressure and temperature with the absence of air leading to the formation of petroleum.

Petroleum is also called crude oil as it is a mixture of hydrocarbons, and when distilled, they give many byproducts.
  
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Formation of Natural Gas:
  
This gas is a naturally occurring hydrocarbon. Natural gas is formed by the anaerobic decomposition of dead plants and animals under extreme conditions like high pressure and temperature.
  
The formation is similar to petroleum; the difference is in the chemical composition of both. Natural gas is mainly made of methane group. Natural gas is found above petroleum as it is less dense compared to crude oil.
 
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Note: The amount of pressure and temperature along with the type of substance buried determines the material formed, i.e. coal, petroleum or natural gas.