How carbon dioxide decreased: describe and explain the formation of deposits of limestone, coal, crude oil and natural gas.

The idea

These deposits are carbon stores formed over millions of years. Limestone is mostly calcium carbonate, CaCO₃, precipitated or compacted from the shells and skeletons of marine organisms such as corals and molluscs, so it forms in warm shallow seas. Coal, crude oil and natural gas are fossil fuels formed from the remains of living organisms. Coal formed from plant material that was buried in swampy conditions where there was little oxygen, so decay was incomplete; heat and pressure over geological time drove off water and gases, raising the carbon content. Crude oil and natural gas formed mainly from marine plankton buried in mud on the seabed. Absence of oxygen prevented full decay, and heat and pressure converted the organic material into liquid and gaseous hydrocarbons. These hydrocarbons then migrated through porous rock and were trapped beneath impermeable caps, forming reservoirs.

What earns the marks

• Limestone is mainly calcium carbonate, CaCO₃, formed from the shells and skeletons of marine organisms such as corals and molluscs.
• Coal formed from plant material buried in swampy, low-oxygen conditions, so decay was incomplete.
• Heat and pressure acting over millions of years drove off water and gases from buried plant remains, increasing the carbon content to form coal.
• Crude oil and natural gas formed mainly from marine plankton buried in mud on the seabed in the absence of oxygen.
• Heat and pressure over geological time converted the buried organic remains into liquid and gaseous hydrocarbons.
• Hydrocarbons migrated through porous rock and accumulated beneath impermeable traps, forming crude oil and natural gas reservoirs.
• All four deposits are carbon stores formed over millions of years, which is why fossil fuels are described as finite.

Common mistakes and how to put them right

• Saying coal formed from dinosaurs or from marine organisms.

  Put it right: Coal formed from plant material, while crude oil and natural gas formed mainly from marine plankton.

• Stating that the organisms simply rotted away.

  Put it right: Decay was incomplete because oxygen was limited, which allowed carbon-rich remains to be preserved.

• Claiming that heat and pressure alone created the deposits without burial or time.

  Put it right: Burial, low oxygen and millions of years are all needed.

Exam tips

• Link each deposit to its source organism and to the conditions of burial, then add the role of heat and pressure over geological time.
• Use the word 'hydrocarbons' for crude oil and natural gas, and 'calcium carbonate' for limestone, to show precise scientific vocabulary.
• When asked to explain, give a cause-and-effect chain such as burial → little oxygen → incomplete decay → heat and pressure → deposit.