Evolution of the Atmosphere

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. 

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. 

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.