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CognitoEarth's atmosphere — evolution and composition
Key facts & methods
Current composition of the atmosphere
- Nitrogen (N2): approximately 78%.
- Oxygen (O2): approximately 21%.
- Argon (Ar): approximately 1%.
- Carbon dioxide (CO2): approximately 0.04% (but increasing).
- Water vapour: variable (up to ~2%).
- Noble gases (He, Ne, Kr, Xe): trace amounts.
Evolution of the atmosphere
- 4.6 billion years ago: Earth formed. Early atmosphere mainly CO2, water vapour and ammonia (from volcanic activity). Very little or no oxygen.
- Volcanic activity: released CO2, H2O vapour, N2, SO2 into atmosphere.
- Water vapour condensed: formed oceans. CO2 dissolved into oceans and precipitated as calcium carbonate.
- Photosynthesis: algae and early plants produced O2 by photosynthesis — O2 levels rose over 2.7 billion years.
- Nitrogen: increased as ammonia was converted by bacteria and by lightning.
Evidence for atmosphere changes
- Rocks: carbonate rocks (limestone) = evidence that CO2 was removed from atmosphere.
- Iron-banded formations: formed before O2 rose (iron oxidised as O2 appeared).
- Fossils of early photosynthetic organisms (stromatolites, cyanobacteria).
- Gas bubbles in ancient ice cores show past atmospheric composition.
- Mathematical models based on volcanic activity, weathering and biological processes.
Role of key processes
- Photosynthesis: removes CO2, produces O2. Vital for rising oxygen levels.
- Respiration and combustion: consume O2, release CO2.
- Dissolution in oceans: CO2 dissolves — critical for reducing early CO2 levels.
- Formation of fossil fuels: ancient organisms absorbed CO2 — now released by combustion.
- Maintaining current balance: photosynthesis and respiration broadly in balance (before human activity).
Exam questions — 4 questions · 10 marks · AQA 8462 style
⚡ Show all working — the AI marks against the AQA mark scheme. Method marks awarded for correct working even if the final answer is wrong.
1 markAtmosphere compositionAQA 8462 style
Approximately what percentage of the current atmosphere is nitrogen?
3 marksEvolution of atmosphereAQA 8462 P1 style
Explain how the level of oxygen in the atmosphere increased over billions of years. (3 marks)
Hint: Start with: early atmosphere had no O2. Explain: photosynthesis by early organisms produced O2. Over billions of years O2 accumulated.
+30 XP
1 markCO2 decreaseAQA 8462 style
Which of the following is NOT a reason why CO2 levels decreased in the early atmosphere?
A CO2 dissolved in oceans
B Photosynthesis removed CO2
C CO2 was converted to oxygen by volcanic activity
D Formation of carbonate rocks (limestone) locked up carbon
3 marksRole of oceansAQA 8462 style
Explain how the formation of oceans helped reduce the level of CO2 in the early atmosphere. (3 marks)
Hint: Chain: water vapour condenses → oceans form → CO2 dissolves in ocean water → carbonate rocks form → CO2 removed from atmosphere.
+30 XP
Module complete! 🎉
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