1) - Volcanoes gave out gases.
- The Earth's surface was originally molten for many millions of years - it was so hot due to volcanic activity that any atmosphere just 'boiled away' into space.
- Eventually, the heat cooled down and a thin crust formed, yet volcanoes kept erupting.
- These volcanoes gave out lots of gases, including: carbon dioxide, water vapour annd nitrogen.
- According to this theory, the early atmosphere was mostly carbon dioxide and water vapour wth hardly any oxygen. We think this was how the atmosphere was created.
- The oceans were formed by the water vapour condensing, due to the Earth cooling down.
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2) _- Green plants evolved and produced Oxygen
- Green plants evolved over most of the Earth, because they were quite happy in the carbon dioxide atmosphere.
- A lot of early carbon dioxide dissolved into oceans. The green plants also removed carbon dioxide from the atmosphere and produced oxygen by photosynthesis, causing the carbon dioxide count to minimise.
- When plants died, they were buried under many layers of sediment, and the carbon they had removed got 'locked up' in sedimentary rocks as insoluble carbonates and fossil fuels.
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3) _- Complex Organisms evolved.
- The buildup of oxygen killed off some early organisms that couldn't tolerate it, but allowed more complex organisms to evolve.
- There is virtually no carbon dioxide left now.
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