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AQA GCSE Chemistry · 8462
AQA 8462 · 4.9.1.2 Check the specification (PDF) (opens in a new tab)
Earth formed about 4.6 billion years ago. Its atmosphere has changed greatly since then, but no sample of air from the first billion years has survived for us to measure directly. Evidence from such a distant time is limited, so scientists develop and revise theories as they interpret the clues available. The volcanic account below is one theory, not a complete record known with certainty.
One theory suggests that intense volcanic activity during Earth's first billion years released the gases that formed its early atmosphere. At the start of that period, the atmosphere may have resembled those of Mars and Venus today: mainly carbon dioxide, with little or no oxygen gas. Volcanoes also released nitrogen, which gradually built up in the atmosphere. There may have been small proportions of methane and ammonia. Comparing Earth with Mars and Venus offers a clue, but their present-day atmospheres do not directly measure what Earth's air contained billions of years ago.
Volcanoes released water vapour as well as other gases. As Earth cooled, the vapour condensed into liquid water, which collected to form oceans. Carbon dioxide in the atmosphere then dissolved in the ocean water. Carbonates precipitated from the water as solids and accumulated as sediments. In this way, carbon that had been in atmospheric carbon dioxide became part of material deposited in the oceans, reducing the amount of carbon dioxide left in the air. Dissolving the gas and forming carbonate sediments are connected steps, not two unrelated explanations.
In one theory, volcanic gases formed the early atmosphere. Cooling formed oceans, which helped remove carbon dioxide from the air.
Because direct evidence is scarce, an evaluation should ask how well a theory fits the evidence provided, what that evidence can actually show, and where uncertainty remains. For instance, the present-day carbon-dioxide-rich atmospheres of Mars and Venus are consistent with the proposed early composition of Earth's air, but a resemblance alone cannot prove that Earth followed the same history. New evidence may favour one explanation over another or lead scientists to change a theory. You may be given information about a different theory to interpret and evaluate; you do not need to memorise other theories.
Earth formed about 4.6 billion years ago. Evidence for its early atmosphere is limited, so theories have changed over time. One theory proposes that intense volcanism in the first billion years produced an atmosphere mainly of , with little or no . Nitrogen released by volcanoes gradually built up; small amounts of methane and ammonia may also have been present.
Volcanic water vapour condensed as Earth cooled, forming oceans. Atmospheric dissolved in the water; carbonates then precipitated as sediments. These processes reduced atmospheric .
Use the evidence supplied to compare explanations. Distinguish evidence consistent with a theory from proof, and recognise the uncertainty caused by the immense time scale.
Describe the volcanic account as a theory, not a certain record of events. Evidence from 4.6 billion years ago is limited.
Keep the sequence clear: volcanic water vapour condensed to form oceans; carbon dioxide then dissolved in the water and carbonates precipitated as sediments.
When given evidence for competing theories, weigh how well each explains it and acknowledge limitations. You do not need to learn the details of other theories.
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Scientific theory
A proposed explanation of events or observations that can be assessed using evidence.
Condensation
A change from a gas to a liquid, such as water vapour forming liquid water as it cools.
Precipitation
To form a solid from substances in a solution; early oceans produced solid carbonates that settled as sediments.
Sediment
Particles of solid material that settle and accumulate, often on the floor of a body of water.
Put your knowledge into practice — try past paper questions for Chemistry
Scientific theory
A proposed explanation of events or observations that can be assessed using evidence.
Condensation
A change from a gas to a liquid, such as water vapour forming liquid water as it cools.
Precipitation
To form a solid from substances in a solution; early oceans produced solid carbonates that settled as sediments.
Sediment
Particles of solid material that settle and accumulate, often on the floor of a body of water.