Mud and acidic water prevented the plant matter from coming into contact with oxygen. As the plant debris sifted deeper under Earth’s surface, it encountered increased temperatures and higher pressure. Over time, the plants (mostly mosses) and algae were buried and compressed under the weight of overlying mud and vegetation. The seas occasionally flooded the forested areas, trapping plants and algae at the bottom of a swampy wetland. During this time, Earth was covered in wide, shallow seas and dense forests. The conditions that would eventually create coal began to develop about 300 million years ago, during the Carboniferous period. Because coal takes millions of years to develop and there is a limited amount of it, it is a nonrenewable resource. Fossil fuels are formed from the remains of ancient organisms. Coal is the largest source of energy for generating electricity in the world, and the most abundant fossil fuel in the United States. It is composed mostly of carbon and hydrocarbons, which contain energy that can be released through combustion (burning). Coal is a black or brownish-black sedimentary rock that can be burned for fuel and used to generate electricity.
