the atmosphere

• 78% nitrogen
• 21% oxygen
• small proportions of gases eg C02 water vapour and noble gases such as argon
• in the first billion years, there was intense volcanic activity, releasing gases that formed the early atmosphere eg water vapour

  -as the earth cooled, the water vapour condensed to form oceans

  -volcanoes also released huge amounts of C02 so the atmosphere consisted of mainly C02 with little to no oxygen

  -volcanoes also released other gases eg nitrogen which gradually build up in the atmosphere and small amounts of methane and ammonia
• -in the oceans, some CO2 dissolved to form a weak acid reacting with minerals in the sea to form precipitates

  -over time this formed sediment of carbonate rock on the sea bed

  -some CO2 in the sea was used to make corals and shell organisms so when they died they formed the sedimentary rock, limestone removing CO2 from the atmosphere
• oxygen:

  -photosynthetic aglae evolved in oceans producing oxygen

  -plants evolved increasing the amount of oxygen in the atmosphere

  -the carbon from photosynthesis can also be trapped in fossil fuels and caused the levels of CO2 to fall
• coal: formed from the remains of ferns and trees

  if they die in marshy wetlands, they dont decompose due to lack of oxygen or acidic conditions

  preventing bacteria from carrying out decomposition

  over time plant remains are covered with sediment and compressed

  high temperature and pressure creates coal
• crude oil: formed from plankton which are tiny plants and animals found in the sea

  when they die they settle on mud in the seabed

  if oxygen is not present they dont decompose

  over time they are compressed by sediment

  heat and pressure convert them into crude oil
• natural gas: mainly the hydrocarbon methane