Explain how fossil fuels found in the Earth's crust are ultimately the result of biological processes.

Fossil fuels are hydrocarbons, primarily coal, fuel oil or natural gas, formed from the remains of dead plants and animals. Fossil fuel is a general term for buried combustible geologic deposits of organic materials, formed from decayed plants and animals that have been converted to crude oil, coal, natural gas, or heavy oils by exposure to heat and pressure in the earth's crust over hundreds of millions of years by the result of biological processes. The organic compounds break down and convert into other compounds via biological processes.