Lesson Objectives

  • Describe the natural processes that form the different fossil fuels.
  • Describe different fossil fuels, and understand why they are non-renewable resources.
  • Explain how fossil fuels are turned into useful forms of energy.
  • Understand that when we burn a fossil fuel, its energy is released as heat.
  • Describe how to a nuclear power plant produces energy.


Millions of years ago, plants used energy from the Sun to form sugars, carbohydrates, and other energy-rich carbon compounds that were later transformed into coal, oil, or natural gas. The solar energy stored in these fuels is a rich source of energy. Although fossil fuels provide very high quality energy, they are non-renewable.
In large part, non-renewable energy sources are responsible for the world’s lights seen in this animation: http://www.nature.nps.gov/GEOLOGY/usgsnps/animate/LIGHTS_3.MPG.

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 trace from a living creature that becomes a 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. As plants and animals died, their remains settled on the ground on land and in swamps, lakes, and seas (Figure below).
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This wetland may look something like an ancient coal-forming swamp.
Over time, layer upon layer of these remains collected. 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.
Ball and stick model of methane.

Hydrocarbons are made of one carbon and four hydrogen atoms. This 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.


Coal, a solid fossil fuel formed from the partially decomposed remains of ancient forests, is burned primarily to produce electricity. Coal use is undergoing enormous growth as the availability of oil and natural gas decreases and cost increases. This is happening particularly in developing nations, such as China, where it is cheap and plentiful.

Coal Formation

Coal forms from dead plants that settled at the bottom of ancient swamps. Lush coal swamps were common in the tropics during the Carboniferous period, more than 300 million years ago (Figure below). The climate was warmer then.
Map showing the continents during the Carboniferous period. Notice that quite a lot of land area is in the region of the tropics.
Map showing the continents during the Carboniferous period. Notice that quite a lot of land area is in the region of the tropics.

The location of the continents during the Carboniferous period. Notice that quite a lot of land area is in the region of the tropics.
Mud and other dead plants buried the organic material layer upon layer in the swamp, and burial kept away oxygen. When plants are buried without oxygen, the organic material can be preserved or fossilized. Sand and clay settling on top of the decaying plants squeezed out the water and other substances. Millions of years later, what was left was a carbon-containing rock that we know as coal.
Coal is black or brownish-black. The most common form of coal is bituminous, a sedimentary rock that contains impurities, such as sulfur. Anthracite coal, seen in Figure above, has been metamorphosed, and is nearly all carbon. For this reason, anthracite coal burns more cleanly than bituminous coal (Figure below).
Bituminous coal, an example of a sedimentary rock.
Bituminous coal, an example of a sedimentary rock.

Bituminous coal is a sedimentary rock.

Coal Use

Around the world, coal is the largest source of energy for electricity. The United States is rich in coal (Figure below). California once had a number of small coal mines, but the state no longer produces coal. To turn coal into electricity, the rock is crushed into powder, which is then burned in a furnace that has a boiler. Like other fuels, coal releases its energy as heat when it burns. Heat from the burning coal boils the water in the boiler to make steam. The steam spins turbines, which turn generators to create electricity. In this way, the energy stored in the coal is converted to useful energy as electricity.
US coal producing regions in 1996.
US coal producing regions in 1996.

United States coal-producing regions in 1996.

Consequences of Coal Use

For coal to be used as an energy source, first it must be mined. Coal mining occurs at the surface or underground by methods that are described in the Earth's Minerals chapter (Figure below). Mining, especially underground mining, can be dangerous. In April 2010, 29 miners were killed at a West Virginia coal mine when gas that had accumulated in the mine tunnels exploded and started a fire.
Coal being mined by mountaintop removal.
Coal being mined by mountaintop removal.

Coal being mined by mountaintop removal.
A coal-fired power plant in Utah.
A coal-fired power plant in Utah.

A small coal-fired power plant in Utah.
Some possible types of environmental damage from mining are discussed in the Earth's Minerals chapter. Coal mining exposes minerals and rocks from underground to air and water at the surface. Many of these minerals contain the element sulfur, which mixes with air and water to make sulfuric acid, a highly corrosive chemical (Figure above). If the sulfuric acid gets into streams, it can kill fish, plants, and animals that live in or near the water.


Oil is a liquid fossil fuel that is extremely useful because it can be transported easily and can be used in cars and other vehicles. Oil is currently the single largest source of energy in the world.

Oil Formation

Oil from the ground is called crude oil, which is a mixture of many different hydrocarbons. Crude oil is a thick dark brown or black liquid hydrocarbon. Oil also forms from buried dead organisms, but these are tiny organisms that live on the sea surface and then sink to the seafloor when they die. The dead organisms are buried by layers of dead creatures and sediments and so are kept away from oxygen. As the layers pile up, heat and pressure increase. Over millions of years, the dead organisms turn into liquid oil.

Oil Production

To be collected, the oil must be located in a porous rock layer trapped beneath an impermeable layer where it stays until we can extract it from the rock (Figure below).
Anticlinal structural trap - trapping oil between rock layers.
Anticlinal structural trap - trapping oil between rock layers.

Oil (red) is found in the porous rock layer (yellow) and trapped by the impermeable layer (green). The folded structure has allowed the oil to pool so a well can be drilled into the reservoir.
To separate the different types of hydrocarbons in crude oil for different uses, the crude oil must be refined in refineries like the one shown in the Figure below. Refining is possible because each hydrocarbon in crude oil boils at a different temperature. When the oil is boiled in the refinery, separate equipment collects the different compounds.
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Refineries like this one separate crude oil into many useful fuels and other chemicals.

Oil Use

Most of the compounds that come out of the refining process are fuels, such as gasoline, diesel, and heating oil. Because these fuels are rich sources of energy and can be easily transported, oil provides about 90% of the energy used for transportation around the world. The rest of the compounds from crude oil are used for waxes, plastics, fertilizers, and other products.
Gasoline is in a convenient form for use in cars and other transportation. In a car engine, the burned gasoline mostly turns into carbon dioxide and water vapor. The fuel releases most of its energy as heat, which causes the gases to expand. This creates enough force to move pistons inside the engine to power the car.

Consequences of Oil Use

The United States produces oil, but only about one-quarter as much as the nation uses. The United States has only about 1.5% of the world’s proven oil reserves and so most of the oil used by Americans must be imported from other nations.
The main oil-producing regions in the United States are the Gulf of Mexico, Texas, Alaska, and California (Figure below). An animation of the location of petroleum basins in the contiguous United States can be seen here: http://www.nature.nps.gov/GEOLOGY/usgsnps/oilgas/BASINS_3.MPG.
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Most of California
As in every type of mining, mining for oil has environmental consequences. Oil rigs are unsightly (Figure below) and spills are too common (Figure below).
Drill rigs at Kern River Oil Field, California.
Drill rigs at Kern River Oil Field, California.

Drill rigs at the Kern River Oil Field in California.
Gulf of Mexico oil spill in April 2010.
Gulf of Mexico oil spill in April 2010.

A deadly explosion on an oil rig in the Gulf of Mexico in April 2010 led to a massive oil spill. When this picture was taken in July 2010, oil was still spewing into the Gulf. The long-term consequences of the spill are being studied and are as yet unknown.

Natural Gas

Natural gas, often known simply as gas, is mostly the hydrocarbon methane (refer to Figure above for the structure).

Natural Gas Formation

Natural gas forms under the same conditions that create oil. Organic material buried in the sediments hardened to become a shale formation that is the source of the gas. Although natural gas forms at higher temperatures than crude oil, the two are often found together.
The formation of a minable oil and gas deposit is seen in this animation: http://www.nature.nps.gov/GEOLOGY/usgsnps/oilgas/PETSYS_3.MPG.
The largest natural gas reserves in the United States are in the Appalachian Basin, Texas, and the Gulf of Mexico region (Figure below). California also has natural gas, mostly in the Central Valley. In the northern Sacramento Valley and the Sacramento Delta, a sediment-filled trough formed along a location where crust was being pushed together (an ancient convergent margin).
Map showing gas production in the Lower 48 United States.
Map showing gas production in the Lower 48 United States.

Gas production in the Lower 48 United States.

Natural Gas Use

Like crude oil, natural gas must be processed before it can be used as a fuel. Some of the chemicals in unprocessed natural gas are poisonous to humans. Other chemicals, such as water, make the gas a less-useful fuel. Processing the natural gas removes almost everything except the methane. Once the gas is processed, it is ready to be delivered and used. Natural gas is delivered to homes for uses such as cooking and heating. Like coal and oil, natural gas is also burned to make heat for powering turbines to make electricity. The spinning turbines turn generators, and the generators create electricity.

Consequences of Natural Gas Use

Like other fossil fuels, processing and using natural gas has harmful environmental effects. However, natural gas burns much cleaner than other fossil fuels, meaning that it causes less air pollution. Natural gas also produces less carbon dioxide than the other fossil fuels for the same amount of energy (Figure below).
Gas drill rig in Alvarado, Texas.
Gas drill rig in Alvarado, Texas.

A natural gas drill rig in Texas.

Fossil Fuel Reserves

Fossil fuels provide about 85% of the world’s energy at this time. World fossil fuel use has increased many times over in the past half century (coal – 2.6x, oil – 8x, natural gas – 14x) because of population increases, the increase in number of cars, televisions, and other fuel uses in the developed world, and because of lifestyle improvements in the developing world.
Worldwide oil reserves.
Worldwide oil reserves.

Worldwide oil reserves.
The amount of fossil fuels that remain untapped is unknown but can likely be measured in decades for oil and natural gas and a few centuries for coal (Figure above). Alternative sources of fossil fuels, such as oil shales and tar sands, are increasingly being exploited (Figure below).
A satellite image of an oil-sands mine in Canada.
A satellite image of an oil-sands mine in Canada.

A satellite image of an oil-sands mine in Canada.
The environmental consequences of mining these fuels, and of fossil fuel use in general, along with the fact that these fuels do not have a limitless supply, are prompting the development of alternative energy sources.

Nuclear Energy

When the nucleus of an atom is split, it releases a huge amount of energy, called nuclear energy. For nuclear energy to be used as a power source, scientists and engineers have learned to split nuclei and to control the release of energy (Figure below).
Illustration of nuclear fission.
Illustration of nuclear fission.

When struck by a tiny particle, Uranium-235 brakes apart and releases energy.

Nuclear Energy Use

Nuclear power plants, such as the one seen in Figure below, use uranium, which is mined, processed, and then concentrated into fuel rods. When the uranium atoms in the fuel rods are hit by other extremely tiny particles, they split apart. The number of tiny particles allowed to hit the fuel rods needs to be controlled or they would cause a dangerous explosion. The energy from a nuclear power plant heats water, which creates steam and causes a turbine to spin. The spinning turbine turns a generator, which in turn produces electricity.
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Nuclear power plants like this one provide France with almost 80% of its electricity.
Many countries around the world use nuclear energy as a source of electricity. In the United States, a little less than 20% of electricity comes from nuclear energy.

Consequences of Nuclear Power

Nuclear power is clean. It does not pollute the air. However the use of nuclear energy does create other environmental problems. Uranium must be mined (Figure below). The process of splitting atoms creates radioactive waste, which remains dangerous for thousands or hundreds of thousands of years. As yet, there is no long-term solution for storing this waste.
Uranium mine near Moab, Utah.
Uranium mine near Moab, Utah.

Uranium mine near Moab, Utah.
The development of nuclear power plants has been on hold for three decades. Accidents at Three Mile Island and Chernobyl, Ukraine, verified people’s worst fears about the dangers of harnessing nuclear power (Figure below). But recently, nuclear power may be making a comeback as society looks for alternatives to fossil fuels and the problems caused by the greenhouse gases they release.
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Damaged building near the site of the Chernobyl disaster.

Lesson Summary

  • Coal, oil, and natural gas are fossil fuels formed from the remains of living organisms.
  • Coal is the largest source of energy for producing electricity.
  • Oil and natural gas are important energy sources for vehicles and electricity generation.
  • Nuclear energy is produced by splitting atoms. It also produces radioactive wastes that are very dangerous for many years.
  • Fossil fuels are non-renewable sources of energy that produce environmental damage.

Review Questions

  1. What is a hydrocarbon?
  2. How do fossil fuels form?
  3. Why is anthracite harder and cleaner than other kinds of coal?
  4. What byproduct of nuclear energy has caused concerns about the use of this resource and why?
  5. What are two important fuels that comes out of the oil refining process?
  6. Which chemical element exposed in surface coal mining can cause environmental problems in nearby bodies of water?
  7. Why does natural gas need to be processed before it can be used as a fuel?
  8. What characteristic of gasoline is most important in making it a useful fuel for transportation? Explain.
  9. Since nuclear power is clean, why is it not used more extensively in the United States?

Further Reading / Supplemental Links


oil A liquid fossil fuel from ancient dead organisms used for transportation and other products. nuclear energy Energy that is released from the nucleus of an atom when it is changed into another atom. natural gas A fossil fuel composed of the hydrocarbon methane. hydrocarbon A chemical compound containing carbon and hydrogen that is used for energy. crude oil Unrefined oil as it is taken from the ground; a fossil fuel. coal A solid fossil fuel from ancient dead organisms used for electricity.

Points to Consider

  • What are the main categories of non-renewable energy discussed in this chapter?
  • Why is nuclear energy considered non-renewable?
  • Are non-renewable energy sources equally harmful? What are the advantages to them?
  • Are renewable energy sources harmful or beneficial for the environment?