Changes to the Earth and Atmosphere

4.5 billion years ago

- When the Earth first formed, its surface was molten.

- As it cooled surface rocks formed.

- An atmosphere formed from volcanic gases: carbon dioxide, steam, ammonia and methane.

3 billion years ago

- The Earth cooled enough for the steam to condense and form the oceans.

- The main gas in the atmosphere was carbon dioxide.

- Simple plants evolved.

2 billion years ago

- The first plant produced oxygen, this killed off many other life forms.

1 billion years ago

- The excess oxygen in the atmosphere reacted in the presence of UV light to form ozone.

- This ozone filtered out most of the damaging UV light and allowed more complicated life forms to evolve.

The present time

- The atmosphere is approximately 1/5 oxygen and 4/5 nitrogen.

- carbon dioxide is absorbed by plants, shellfish and dissolved in the oceans.

- It is given out by burning fuels, the decay of organic waste and released from volcanoes.

- ammonia in the air reacted with oxygen to form nitrogen. More nitrogen was formed by bacteria in the soil.