GCSE Chemistry - Evolution of the Atmosphere

Evolution of Earth’s Atmosphere

The video explains how Earth’s atmosphere changed over roughly 4.6 billion years, from one rich in carbon dioxide to one made mostly of nitrogen and oxygen. This account is presented as a theory: it is supported by evidence, but the evidence is incomplete because of the immense timescale involved.

How the atmosphere changed

• Early Earth: During the first billion years, intense volcanic activity released large amounts of carbon dioxide, water vapour and nitrogen, along with smaller amounts of methane and ammonia. The atmosphere was mostly carbon dioxide.
• Formation of the oceans: As Earth cooled, water vapour condensed to form the oceans. Carbon dioxide dissolved in the water, and some eventually became carbon-containing deposits and seabed sediments.
• Photosynthesis: Algae appeared around 2.7 billion years ago, followed over time by green plants. Through photosynthesis, they absorbed carbon dioxide and released oxygen. Over billions of years, this reduced atmospheric carbon dioxide and increased oxygen, helping make complex life possible.
• Long-term carbon storage: Some organisms were buried beneath sediment after they died. Over millions of years, burial and compression stored their carbon in materials such as fossil fuels and sedimentary rocks, preventing it from returning to the atmosphere.

Examples of carbon-containing materials

• Crude oil and natural gas form mainly from buried plankton.
• Coal forms from thick deposits of dead plant material.
• Limestone is made largely from calcium carbonate in the shells and skeletons of marine organisms.

Overall takeaway

Volcanic activity helped create the early atmosphere, while the formation of oceans and the growth of photosynthetic organisms gradually changed its composition. Carbon was also locked away in seabed deposits, rocks and fossil fuels. The video describes today’s atmosphere as about 80% nitrogen and 20% oxygen, with other gases making up less than 1%.

Speakers and sources

• One unnamed narrator/presenter from the Cognito educational video.