Fossil fuels are non-renewable because they take millions of years to form. Fossil fuels make up most sources of non-renewable energy, and they were created millions of years ago as a result of marine creatures decaying under immense pressure and heat.
Coal, petroleum and natural gas are fossil fuels. They are ready-made fuels that are relatively inexpensive to extract. Carbon is the main element in fossil fuels, and fossil fuels are available in limited supplies.
Hundreds of millions of years ago, Earth was covered with shallow seas and swamps. The plants, algae and plankton that grew in the wetlands created energy through photosynthesis, and when they died, energy was still stored in them. As rocks and sediment accumulated on these organisms, high heat and pressure turned them into fossils. Reservoirs of these sources of non-renewable energy exist throughout the world.
Not all non-renewable energy sources are fossil fuels. Uranium ore is used as fuel in nuclear power plants. Uranium is classified as a non-renewable fuel even though it is not a fossil fuel.
Biomass can be considered renewable and non-renewable because biomass energy uses the energy found in plants. Once people were able to extract energy from fossilized organisms, fossil fuels replaced renewable energy sources like wood, wind, solar and water as the main sources of fuel.