forms a layer of rock that we know as coal.
Coal is black or brownish-black in appearance. Coal is a rock that burns easily. Most forms
of coal aresedimentaryrock. But the hardest type of coal, anthracite, is ametamorphic
rock, because it is exposed to higher temperature and pressure as it forms. Coal is mostly
carbon, but some other elements can be found in coal, including sulfur.
Around the world, coal is the largest source of energy for electricity. The United States is
rich in coal, which is used for electricity. California once had a number of small coal mines
but the state no longer produces coal.
A common way of turning coal into a useful form to make electricity starts with crushing
the coal into powder. Then, a power plant burns the powder in a furnace that has a boiler.
Like other fuels, coal releases most of its energy as heat when it burns. The heat that the
burning coal releases in the furnace is enough to boil the water in the boiler, making steam.
The power plant uses this steam to spin turbines, and the spinning turbines make generators
turn to create electricity.
For people to use coal as an energy source, they need to get it out of the ground. The
process of removing coal from the ground is known as coal mining. Coal mining can take
place underground or at the surface. The process of coal mining, especially surface mining,
affects the environment. Surface mining exposes minerals from underground to air and water
from the surface. These minerals contain the chemical element sulfur, and sulfur mixes with
air and water to make sulfuric acid, which is a highly corrosive chemical. The sulfuric acid
gets into nearby streams and can kill fish, plants, and animals that live in or near the water.
The process of burning coal causes other problems for the environment. A little later, we
will look at these other pollution problems, when we explore problems with fossil fuels in
general.
Oil
Oil is a thick liquid that is usually dark brown or black in appearance. It is found mostly in
formations of porous rock in the upper layers of the Earth’s crust. Oil is currently the single
largest source of energy in the world. How does oil form? The process of making oil is similar
in many ways to the process of making coal. The main difference is in the size of the living
things—the organisms—whose remains turn into these fossil fuels. The organisms that die
and became the material for making oil are much smaller than the plants that turned into
coal. These organisms are called plankton and algae. When the plankton and algae die,
their remains settle to the bottom of the sea. There, they were buried away from oxygen,
just as the plants did in the process of becoming coal. As layers of sediment pile on top of
these decaying organisms, heat and pressure increase. Over a period of millions of years, the
heat and pressure turn the material into liquid oil.
The United States produces oil, although only about one-quarter as much as it uses. The