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12.5: Fossil Fuel Formation

Difficulty Level: At Grade Created by: CK-12
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What exactly is powering this car?

There was an old ad suggesting that you put a tiger in your tank was referring to the strength and speed of these wild cats. But it might also have been referring to the use of organic material to power an engine. When your tank is full of gas, it doesn't have a tiger in it, but it does have ancient plants, plankton, and other formerly living creatures. Not a pegasus though!

Formation of Fossil Fuels

Can you name some fossils? How about dinosaur bones or dinosaur footprints? Animal skeletons, teeth, shells, coprolites (otherwise known as feces), or any other remains or traces from a living creature that becomes rock is a fossil.

The same processes that formed these fossils also created some of our most important energy resources, fossil fuels. Coal, oil, and natural gas are fossil fuels. Fossil fuels come from living matter starting about 500 million years ago. Millions of years ago, plants used energy from the Sun to form sugars, carbohydrates, and other energy-rich carbon compounds. As plants and animals died, their remains settled on the ground on land and in swamps, lakes, and seas (Figure below).

This wetland may look something like an ancient coal-forming swamp.

Over time, layer upon layer of these remains accumulated. Eventually, the layers were buried so deeply that they were crushed by an enormous mass of earth. The weight of this earth pressing down on these plant and animal remains created intense heat and pressure. After millions of years of heat and pressure, the material in these layers turned into chemicals called hydrocarbons (Figure below). An animated view of a hydrocarbon is seen here: http://www.nature.nps.gov/GEOLOGY/usgsnps/oilgas/CH4_3.MPG.

Ball and stick model of methane

Hydrocarbons are made of carbon and hydrogen atoms. This molecule with one carbon and four hydrogen atoms is methane.

Hydrocarbons can be solid, liquid, or gaseous. The solid form is what we know as coal. The liquid form is petroleum, or crude oil. Natural gas is the gaseous form.

The solar energy stored in fossil fuels is a rich source of energy. Although fossil fuels provide very high quality energy, they are non-renewable.


  • Hydrocarbons are molecules made of one carbon and four hydrogen atoms.
  • Ancient living organisms are buried quickly and altered by intense heat and pressure to form fossil fuels.
  • Fossil fuels include solid coal, liquid petroleum, and liquid natural gas.


Use this resource to answer the questions that follow.


1. What is natural gas and crude oil made from?

2. Why are these products called fossil fuels?

3. What type of environment did the microorganisms live in?

4. What covered the organisms?

5. What is bio-genesis?

6. How were the biotic materials cooked?


1. Why are coal, petroleum, and natural gas called fossil fuels?

2. How do fossil fuels form?

3. What is the actual source of energy in a fossil fuel?

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Preserved remains or traces of an organism that lived in the past.

fossil fuel

A hydrocarbon created from the remains of formerly living organisms that can be used for energy.


A chemical compound containing hydrogen and carbon that is used for energy.

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Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
Aug 21, 2016
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