9.1 The Earth's early atmosphere and how it changed
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What was the main gas in the Earth's early atmosphere?
Choose one answer.
Correct Not quite: the answer is Carbon dioxide
Mark scheme
B
Model answer
B: the early atmosphere was mainly carbon dioxide from volcanoes.
Describe how the oceans formed.
Mark scheme
- the Earth cooled (1)
- water vapour (from volcanoes) condensed (1)
Model answer
As the Earth cooled, water vapour released by volcanoes condensed to form the oceans.
Explain how the amount of carbon dioxide in the atmosphere decreased over time.
Mark scheme
- carbon dioxide dissolved in the oceans (1)
- formed carbonates that were precipitated as sediments / sedimentary rocks (1)
- algae and plants used carbon dioxide in photosynthesis (1)
- carbon locked in fossil fuels / sedimentary rocks (1)
Model answer
Carbon dioxide dissolved in the oceans, forming carbonates that settled as sediments. Algae and then plants removed carbon dioxide by photosynthesis. Carbon from living things became locked up in sedimentary rocks such as limestone and in fossil fuels.
Explain how the amount of oxygen in the atmosphere increased.
Mark scheme
- algae and plants photosynthesised (1)
- photosynthesis releases oxygen (1)
Model answer
Algae, and later plants, carried out photosynthesis, which releases oxygen.
Give the approximate percentages of nitrogen and oxygen in the atmosphere today.
Mark scheme
- nitrogen about 78% / four-fifths (1)
- oxygen about 21% / one-fifth (1)
Model answer
About 78% nitrogen and 21% oxygen.
Suggest why scientists cannot be certain about the composition of the early atmosphere.
Mark scheme
- it was billions of years ago, so there is little evidence (1)
Model answer
It was billions of years ago, so there is very little direct evidence.
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