Why the early atmosphere is hard to be sure about
- Earth formed about 4.6 billion years ago, and almost no direct evidence survives from that time.
- Because the evidence is limited, scientists build theories rather than proven facts about the early air.
- These theories have changed and developed as new evidence is found and old evidence is re-interpreted.
- You need to know one theory, the volcanic theory below, but you may be asked to interpret or evaluate another one from information you are given.
Note
You do not need to recall any theory other than the volcanic one; if a question gives you a different theory, judge it using only the information provided.
Volcanoes may have released a carbon-dioxide-rich atmosphere
Definition
Volcanic activity
Eruptions and other processes through which molten rock, ash and gases are released from a volcano onto the surface.
- One theory says the first billion years of Earth's history saw intense volcanic activity.
- Erupting volcanoes released gases that built up to form the early atmosphere.
- This early air may have been mainly carbon dioxide, CO2\text{CO}_2CO2, with little or no oxygen, O2\text{O}_2O2.
- That makes it similar to the atmospheres of Mars and Venus today, which are still rich in carbon dioxide.
- Volcanoes also released nitrogen, N2\text{N}_2N2, which is unreactive and slowly built up in the atmosphere.
- Smaller proportions of water vapour, H2O\text{H}_2\text{O}H2O, methane, CH4\text{CH}_4CH4, and ammonia, NH3\text{NH}_3NH3, may also have been released.
Common Mistake
- Do not describe the early atmosphere as containing much oxygen; the theory says there was little or no oxygen.
- Use cautious wording such as may have contained, because the limited evidence cannot prove the exact composition.
Cooling oceans pulled carbon dioxide out of the air
Definition
Water vapour
Water in the gaseous state, present in a small proportion of Earth's atmosphere.
Definition
Carbonate
A compound that contains the carbonate ion, CO32−\text{CO}_3^{2-}CO32−.
- As the young Earth cooled, the water vapour in the air condensed into liquid water.
- The liquid water collected on the surface and formed the oceans.
- Carbon dioxide is soluble, so a large amount dissolved into the new oceans.
- Dissolved carbon dioxide formed carbonates, which came out of solution as sediments on the sea floor.
- Locking carbon into oceans and sediments lowered the amount of carbon dioxide left in the atmosphere.
Key Idea
The long-term shift was from an early atmosphere rich in carbon dioxide to one with far less, as oceans formed and carbon became locked in carbonate sediments.
Interpreting and evaluating theories from the evidence
- To evaluate a theory, weigh how well the evidence you are given fits its claims.
- For each piece of evidence, decide whether it supports, contradicts or has no clear effect on the theory.
- Check whether evidence shows what the air contained, or only that a process such as volcanic activity happened.
- If two theories are given, compare how well each one explains all the evidence, not just one.
- Finish with a cautious judgement, because evidence from 4.6 billion years ago is limited and cannot give complete certainty.
Exam technique
- Signal a fair evaluation with phrases like the evidence supports and the evidence does not prove; these separate a reasoned judgement from an opinion.
- Where the evidence is thin, say so, because recognising the limits of the evidence is itself a marking point on 'evaluate' questions.
Self review
- Why is the evidence about Earth's early atmosphere so limited?
- Which process may have released the gases of the early atmosphere?
- Which gas may have made up most of the early atmosphere, and how much oxygen was present?
- How did the formation of the oceans reduce the amount of carbon dioxide in the air?
- What two other gases may have been present in small proportions in the early atmosphere?
