What the early atmosphere was made of, how the oceans formed, how oxygen appeared and carbon dioxide fell, and why the evidence is uncertain.
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1.What is the composition of the atmosphere today?
About four fifths nitrogen and one fifth oxygen, with small amounts of carbon dioxide, water vapour and noble gases.
2.How long has the atmosphere been roughly as it is now?
For about the last 200 million years.
3.What was the main source of gases in the early atmosphere?
Intense volcanic activity during the first billion years of the history of the Earth.
4.What was the early atmosphere mainly made of?
Carbon dioxide, with little or no oxygen.
5.Which planets today have an atmosphere similar to the early Earth?
Mars and Venus.
6.What other gases did volcanoes release?
Water vapour, nitrogen, and small proportions of methane and ammonia.
7.How did the oceans form?
The water vapour released by volcanoes condensed as the Earth cooled.
8.What happened to carbon dioxide when the oceans formed?
Much of it dissolved in the water, and carbonates were precipitated to form sediments.
9.How did oxygen first appear in the atmosphere?
Algae and later plants produced it by photosynthesis.
10.Roughly when did algae first appear?
About 2.7 billion years ago.
11.Write the word equation for photosynthesis.
carbon dioxide + water gives glucose + oxygen
12.Why did the level of oxygen rise gradually rather than suddenly?
Plants evolved and spread over hundreds of millions of years, so the amount of photosynthesis increased slowly.
13.Why did animals only appear once oxygen levels had risen?
Animals need oxygen for respiration, so they could not survive until enough had accumulated.
14.State four processes that reduced carbon dioxide levels.
Dissolving in the oceans, photosynthesis by algae and plants, the formation of sedimentary rocks, and the formation of fossil fuels.
15.How were limestone and other carbonate rocks formed?
Carbon dioxide dissolved in the oceans and carbonates were precipitated, or the shells and skeletons of marine organisms were deposited as sediment.
16.How did coal form?
From thick plant deposits buried and compressed by sediment over millions of years.
17.How did crude oil and natural gas form?
From the remains of plankton deposited on the sea bed and covered by layers of sediment.
18.Why is carbon in fossil fuels described as locked away?
It was removed from the atmosphere by living organisms millions of years ago and stored underground, out of circulation.
19.Why is evidence about the early atmosphere limited?
The changes happened billions of years ago, so very little direct evidence has survived.
20.What does that mean for theories about the early atmosphere?
They are based on limited evidence, so they may change as new evidence is found and different scientists interpret it differently.
21.Where did the nitrogen in the atmosphere come from?
It was released by volcanoes and gradually built up, being unreactive so it was not removed.
22.Why has nitrogen remained in the atmosphere?
It is very unreactive, so it does not readily take part in reactions that would remove it.
23.What effect did burning fossil fuels have on the atmosphere?
It released carbon dioxide that had been locked away for millions of years back into the atmosphere.
24.Why is the exact proportion of oxygen today about 20 per cent?
It reached a balance between the oxygen produced by photosynthesis and the oxygen used by respiration and combustion.
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