Evolution of the Atmosphere

- Surface of the Earth was covered in volcanoes that erupted and released lots of gases

- Early atmosphere was probably mostly made up of carbon dioxide, with very little oxygen, much like the atmosphere of Mars and Venus today

- Volcanic activity also released nitrogen, which built up in the atmosphere over time, as well as water vapour ans small amounts of methane and ammonia

- Water vapour in the atmosphere condensed and formed oceans

- Lots of carbon dioxide was removed from the early atmosphere as it dissolved in the oceans

- Dissolved carbon dioxide went through reactions to form carbon precipitates that formed sediments on the seabed

- Marine animals evolved - shells and skeletons contained some carbonates from the ocean

- Green plants and algae evolved and absorbed some of the carbon dioxide so they could carry out phtosynthesis 

- Green plants also produced oxygen by photsynthesis - use light to convert carbon dioxide and water into sugars

- Algae evolved first - 2.7 billion years ago

- Green plants evolved over the next billion years 

- As oxygen levels built up in the atmosphere over time, more complex life, like animals, could evolve

- Eventually, about 200 million years ago, the atmosphere reached a composition similar to how it is today - 80% nitrogen - 20% oxygen - small amounts of gases (each making up less than 1% of the atmosphere), mainly carbon dioxide, noble gases and water vapour

- Some of the carbon that organisms took in from the atmosphere and oceans became locked up in rocks and fossil fuels after the organisms died

- When plants, plankton and marine animals die, they fall to the seabed and get buried by layers of sediment

- They become compressed and form sedimentary rocks, oils and gas - trapping the carbon within them and helping keep it out of the atmosphere

- Coal, crude oil and natural gase are made by the same process

- Crude oil and natural gas are formed by deposits of plankton - these form resevoirs under the seabed when they get trapped in rocks

- Coal is a sedimentary rock made from thick plant deposits 

- Limestone is a sedimentary rock that is mostly made of calcium carbonate deposits from shells and skeletons of marine organisms