atmosphere

• What are the main gases in the atmosphere today?

  ~80% nitrogen and ~20% oxygen
• What does ∼ mean in the context of gas proportions?

  It means around or approximately
• How long have the proportions of gases in the atmosphere remained stable?

  For 200 million years
• Why is evidence about the Earth's early atmosphere limited?

  Due to the time scale of 4.6 billion years
• What was one theory about the early atmosphere of Earth?

  Intense volcanic activity released gases
• What gases may have been present in the early atmosphere?

  Mainly CO2 with little or no O2
• What gases did volcanoes produce that built up in the atmosphere?

  Nitrogen (N2)
• What happened to water vapor in the early atmosphere?

  It condensed to form the oceans
• How did CO2 levels decrease in the early atmosphere?

  CO2 dissolved in water and formed sediments
• What is the only theory of the early atmosphere that needs to be known?

  The theory of volcanic activity releasing gases
• What process do algae and plants use to produce oxygen?

  Photosynthesis
• What is the equation for photosynthesis?

  6CO2 + 6H2O -> C6H12O6 + 6O2
• When did algae first produce oxygen?

  About 2.7 billion years ago
• How did the percentage of oxygen change over the next billion years?

  It gradually increased to support animal evolution
• How did algae and plants decrease CO2 levels in the atmosphere?

  By photosynthesis
• What contributed to the formation of sedimentary rocks that decreased CO2?

  Formation of limestone and coal
• How are fossil fuels produced in relation to CO2 levels?

  From the remains of decayed plants and animals
• What are the two main processes that decreased CO2 in the atmosphere?

  Photosynthesis and sedimentary rock formation