Evolution of the Atmosphere: Five Phases
Part of Composition of Atmosphere — GCSE Chemistry
This deep dive covers Evolution of the Atmosphere: Five Phases within Composition of Atmosphere for GCSE Chemistry. Revise Composition of Atmosphere in Atmosphere for GCSE Chemistry with 20 exam-style questions and 12 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 4 of 13 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 4 of 13
Practice
20 questions
Recall
12 flashcards
🌋 Evolution of the Atmosphere: Five Phases
Phase 1: Formation (4.6 billion years ago)
Earth's surface was covered by molten rock and volcanoes. Volcanic activity released gases to form the early atmosphere:
- Mainly carbon dioxide (CO₂) and water vapour
- Also nitrogen (N₂), methane (CH₄), and ammonia (NH₃)
- Little or no oxygen
- Similar to Venus and Mars today
Phase 2: Ocean Formation (4 billion years ago)
As the Earth cooled, water vapour condensed to form the oceans. CO₂ levels in the atmosphere decreased because:
- CO₂ dissolved in the oceans
- Chemical reactions formed carbonate rocks (limestone): CO₂ + H₂O + CaCO₃ reactions
- Marine organisms used dissolved CO₂ to make calcium carbonate shells
Phase 3: Oxygen Production (3.5–2.5 billion years ago)
Cyanobacteria (blue-green algae) evolved — the first photosynthesising organisms:
- 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (using light energy)
- Gradually increased oxygen levels from near-zero to ~21% over billions of years
- The ozone layer (O₃) gradually formed, protecting the surface from UV radiation
Phase 4: Complex Life (2–1 billion years ago)
Rising oxygen enabled more complex organisms to evolve:
- Plants and algae continued photosynthesis, removing CO₂ and releasing O₂
- Dead organisms buried under sediment → coal, oil, and natural gas (fossil fuels)
- Carbon locked away underground, preventing it from re-entering the atmosphere
Phase 5: Stable Atmosphere (200 million years ago to present)
- Balance between photosynthesis and respiration kept O₂ and CO₂ relatively constant
- Human activity is now disrupting this balance by burning fossil fuels
Quick Check: Name the first organisms that produced oxygen through photosynthesis.
Cyanobacteria (blue-green algae) were the first photosynthetic organisms. They evolved approximately 3.5 billion years ago and were responsible for gradually increasing atmospheric oxygen levels.