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What was the early Earth atmosphere mostly made of?
Carbon dioxide & water vapour (from volcanoes)
What is today's atmosphere mostly made of?
78% nitrogen, 21% oxygen
How did oceans form?
Earth cooled → water vapour condensed → fell as rain → oceans formed
Why did CO₂ levels decrease? (2 reasons)
1) Dissolved in oceans 2) Used by plants/algae for photosynthesis 3) Locked up in rocks & fossil fuels
Why did oxygen levels increase?
Plants/algae evolved → photosynthesis released oxygen
Why did nitrogen levels increase?
Nitrogen is unreactive → built up slowly in the atmosphere