Fossil Fuel
A fossil fuel is a carbon-rich energy source, such as coal, oil, or natural gas, formed over millions of years from buried ancient organisms.
Definition
Fossil fuels are coal, oil, and natural gas, formed from plants and tiny sea creatures that died millions of years ago, were buried, and were transformed by heat and pressure over geologic time, coal mostly from Carboniferous swamp plants and oil and gas mostly from marine plankton buried in fine sediment. Formation requires organic-rich source rock buried into the oil window, roughly $60$ to $120^\circ C$ at $2$ to $4\ \text{km}$ depth, where kerogen cracks into liquid hydrocarbons or, deeper still, into gas, followed by migration into porous reservoir rock and trapping beneath an impermeable seal, while coal itself progresses in rank from peat through lignite and bituminous coal to anthracite as carbon content rises. Fossil fuels supply about $80\%$ of world energy, and burning them adds roughly $37$ billion tonnes of $CO_2$ to the atmosphere each year, identifiable by its depleted $^{13}C$ and absent $^{14}C$.
Example
The gasoline in a car, the natural gas that heats many homes, and the coal burned in some power plants are all fossil fuels, and small coal deposits in the Deep River basin near Sanford were mined into the 1950s while the state's Triassic basins hold some natural gas in shale. Hydraulic fracturing of the Marcellus Shale in Pennsylvania and West Virginia made the United States the world's largest natural gas producer, though methane leaks from wells and pipelines offset part of gas's climate advantage over coal, and the Carboniferous coal beds of the eastern United States preserve $300$-million-year-old swamp forests of lycopsid trees.
Key Insight
Burning a fossil fuel releases sunlight that plants captured millions of years ago, essentially ancient solar energy stored as carbon, and a single gallon of gasoline holds about as much energy as a person could produce through weeks of hard physical labor, which is why fossil fuels took over the world so completely. In effect, fossil fuels run the carbon cycle in reverse: carbon that photosynthesis buried over hundreds of millions of years is being returned to the atmosphere in only a few centuries, far faster than any natural process could remove it again.