The Earth's early atmosphere

The Earth formed about 4.5 billion years ago. Scientists believe that its early atmosphere was produced by volcanic activity.

An erupting volcano
A volcano in Iceland

Composition

The early atmosphere probably contained:

  • little or no oxygen
  • a large amount of carbon dioxide
  • water vapour
  • small amounts of other gases, such as ammonia and methane

Scientists cannot be sure about the early atmosphere, since it formed so long ago. They can only analyse evidence from other sources.

Oxygen

Some ancient rocks contain iron compounds that break down in the presence of oxygen. They could only have formed if there was little or no oxygen at the time.

Carbon dioxide and water vapour

A volcano releases large volumes of carbon dioxide and water vapour. Since the early atmosphere was produced by volcanic activity, it was likely to have contained a large amount of carbon dioxide and water vapour.