Evolution of the Atmosphere: Five Phases

🌋 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