The creation of fossil fuels involves a slow chemical change driven by underground heat. Dead organic matter gets trapped deep within sedimentary rock formations without any oxygen. Land-based swamp plants undergo aggressive crushing and heating to produce solid coal.
Coal forms a complex molecular network made mostly of carbon, moisture, and ash. Meanwhile, microscopic marine life settling in ocean basins transforms into liquids and gases. Heat and pressure crack these marine organisms into liquids through a process called catagenesis.
This produces fluid hydrocarbons like liquid petroleum and natural gas. The chemical makeup of these fluid hydrocarbon mixtures remains remarkably diverse. They range from lightweight, easily evaporated short-chain alkanes like methane and ethane.
They also include the thick, long-chain carbon structures found in heavy crude oil. We extract energy from these fuels through a rapid, heat-releasing combustion process. This high-temperature oxidation reaction breaks the densely energetic carbon-carbon and carbon-hydrogen bonds.
The breaking chemical bonds release usable heat along with carbon dioxide and water vapor. The Earth requires millions of years to form new fossil fuel deposits. Since we extract them much faster than they form, they remain nonrenewable resources.
Burning them acts as the largest human-made source of atmospheric carbon dioxide emissions. This extra carbon dioxide traps heat in the atmosphere and directly drives climate change.