Evolution and Composition of the Earth's Atmosphere — AQA GCSE Chemistry
The Earth's atmosphere has changed enormously over billions of years to become the one we have today.
Today's atmosphere
For about the last 200 million years the proportions have been roughly stable:
- Nitrogen — about 78%
- Oxygen — about 21%
- Small amounts of carbon dioxide, water vapour and noble gases (e.g. argon).
How the atmosphere evolved
The evidence is limited because it is from billions of years ago, but the widely accepted account is:
- The early atmosphere came from intense volcanic activity, releasing mainly carbon dioxide, with water vapour, nitrogen and small amounts of methane and ammonia — similar to today's Mars and Venus.
- As the Earth cooled, water vapour condensed to form the oceans.
- Much carbon dioxide dissolved in the oceans, and carbonates were precipitated as sediments, reducing CO₂ levels.
- Algae and plants evolved and, through photosynthesis, removed CO₂ and produced oxygen. Over about two billion years, oxygen levels rose enough for animals to evolve.
- Carbon became locked up in sedimentary rocks (limestone) and in fossil fuels.
Exam tips
- Learn today's composition: ~78% nitrogen, ~21% oxygen.
- Oxygen increased due to photosynthesis by algae and plants.
- Carbon dioxide decreased because it dissolved in oceans, was used in photosynthesis, and got locked in rocks and fossil fuels.
- Early atmosphere came from volcanic activity (mostly CO₂).