Formation of fossil fuels

How a swamp forest becomes coal

Tap each step to see why it matters.

1. 1Swamp forests grow and dieAbout 300 million years ago, in the Carboniferous period, much of the Earth was covered in swampy forests. Trees and other plants lived there, and died there.+
2. 2Buried under mud and more plantsThe dead plants were covered by layers of mud and more dead plant material. Layer after layer built up on top.+
3. 3No oxygen, so rotting slows downThe conditions were anaerobic, which means without oxygen. That slowed down decomposition, the breakdown of dead and decaying matter.+
4. 4Peat formsThose anaerobic conditions, together with high temperatures and high pressure, turned the dead plants into peat.+
5. 5Peat becomes coalThe layers kept being squeezed by the weight of the material above and heated by the earth. Over millions of years, that continued heat and pressure turned the peat into hard, carbon-rich coal.+

Where oil and gas collect

Switch between the three views, then tap each part of the rock to find out what it does.

The drill goes down through the impermeable rock to the reservoir below.

Switch between the three views, then tap each part of the rock.

Hydrocarbon or not?

Each example describes the atoms in one molecule. Decide: is it a hydrocarbon?

5 of 5 still to sort.

Why does this matter? The dead plants and plankton that formed coal, crude oil and natural gas decomposed into hydrocarbons, so all three fossil fuels contain them.

What you need to know

• Fossil fuels are made from dead plants and animals over millions of years. Coal, crude oil and natural gas are all fossil fuels.
• Coal formed from swamp plants: buried under mud and more plants, in anaerobic conditions that slowed decomposition, then turned into peat and finally coal by heat and pressure.
• Crude oil and natural gas formed from dead plankton on the ocean floor, buried under sediment and changed by high pressure and high temperature.
• Crude oil and natural gas collect in reservoirs below impermeable rock, and companies drill through that rock to reach them.
• Fossil fuels contain hydrocarbons, which release large amounts of energy in combustion. They are used faster than they form, so they are non-renewable.

The big picture

Fossil fuels are made from dead plants and animals over millions of years. Coal formed from swamp plants: buried, kept away from oxygen so they rotted only slowly, then heated and squeezed into peat and finally coal. Crude oil and natural gas formed from plankton on the ocean floor, and collect in reservoirs under impermeable rock. All three contain hydrocarbons, which release lots of energy when they burn. We use them faster than they form, so they are non-renewable and will eventually run out.

⚠ Watch out

Thinking all fossil fuels formed the same way from the same thing. Coal formed from plants in swampy forests. Crude oil and natural gas formed from plankton on the ocean floor.

Memory hook

Coal is P → P → C: Plants → Peat → Coal. Oil and gas start with a different P: Plankton.

Check yourself

Cover the page. Tell the coal story in four steps, from swamp forest to hard black rock. At which step did the missing oxygen matter, and what did it do?