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Sedimentary rocks are layers of accumulated sediments that can contain coal, petroleum, and natural gas, which are preserved in rocks over millions of years. Sedimentary rocks, which include chalk, shale, clay, sandstone, and limestone, cover approximately 75 percent of the Earth's surface.[1][2]
Sediments accumulate in layers at the bottom of seas and valleys over millions of years, and new layers increase pressure on the older layers. The combination of pressure and temperature transforms decaying animal and plant matter found in sedimentary rocks into fossil fuels, which are composed of hydrocarbons in solid, liquid, and gaseous forms.[3]
Three main types of fossil fuels are coal, oil, and natural gas. These fuels are formed depending the amount of animal and plant matter, how long it has been underground, and the temperature and pressure applied to the matter over time. Fossil fuels are combustible and release energy when they are burned. They are reformulated into gasoline, diesel fuel, and heating oil. Additionally, they are burned to generate electricity. From 1900 to 2015, fossil fuels accounted for at least 80 percent of total U.S. energy consumption, according to the U.S. Energy Information Administration.[4]
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