Fossil fuels are the remains of organic matter that, over hundreds of millions of years, have undergone substantial physical changes induced by pressure and chemical changes caused by the action of bacteria. The fossil fuels are coal, oil, and natural gas. They provide the United States with about 90 percent of the energy it uses. This energy was originally provided by the sun, which made it possible for the plants to grow.
Coal was formed by vegetation that fell to the ground. We have already described how plants use the sun's energy to sustain their own metabolism and combine carbon dioxide and water to generate the sugars and starches which are the basis of our food. As plants grow, they remove carbon dioxide from the atmosphere and release oxygen. If the plants which had fallen to the ground had not been protected somehow from interacting with oxygen, they would ultimately have decayed into their original constituents: carbon dioxide and water. The presumption is that the fallen vegetation was shielded from the atmosphere by water, which was possible either because it was close to bodies of water to begin with, or because there was a sudden flooding which killed the vegetation and covered it. This permitted the process of fuel formation to begin.
One of the precursors of coal is peat, which is compacted vegetation. Peat formed at the edges of deep lakes or in swampy areas where the vegetation fell into water, thus shielding the deposits from