Composition and Evolution of Earth's Atmosphere

• proportions of gases in the atmosphere (approximately)

  • 80% nitrogen
  • 20% oxygen
  • small proportions of other gases (carbon dioxide, water vapour and noble gases)
• theory of the earth's early atmosphere

  first billion years of earth's existence was intense volcanic activity that released gases that formed early atmosphere and water vapour that condensed to form oceans - at start of this period, earth's atmosphere may have been like that of mars and Venus today - consists of mainly carbon dioxide with little to no oxygen
• what gas did volcanoes produce

  nitrogen which gradually built up in atmosphere and may have been small proportions of methane and ammonia
• what happened when the oceans formed

  carbon dioxide dissolved in the water and carbonates were precipitated producing sediments - reduced amount of carbon dioxide in atmosphere
• what produced oxygen

  algae and plants via photosynthesis
• when did algae first produce oxygen

  2.7 billion years ago and soon after, oxygen appeared in atmosphere - over next billion years, plants evolved and percentage of oxygen gradually increased to a level that enabled animals to evolve
• how did carbon dioxide levels decrease

  algae and plants decreased it due to photosynthesis - also decreased by formation of sedimentary rocks and fossil fuels containing carbon