9.1 The composition and evolution of the earth's atmosphere

• What are the main gases in the atmosphere today?


  Approximately 80 % nitrogen and 20 % oxygen
• Why is evidence of the Earth's early atmosphere limited?


  Due to the 4.6 billion year time scale
• What was one theory about the Earth's early atmosphere?


  Intense volcanic activity released gases
• How might the early atmosphere have resembled Mars and Venus?


  It was mainly carbon dioxide with little or no oxygen
• What gases did volcanoes produce during the early atmosphere?


  Nitrogen, methane, and ammonia
• What happened to water vapor in the early atmosphere?


  It condensed to form the oceans
• What effect did dissolved carbon dioxide have in the oceans?


  It produced sediments and reduced atmospheric carbon dioxide
• How did algae and plants contribute to oxygen levels?


  They produced oxygen through photosynthesis
• When did algae first produce oxygen?


  About 2.7 billion years ago
• What happened to oxygen levels over the next billion years?


  Oxygen levels gradually increased for animal evolution
• How did algae and plants decrease atmospheric carbon dioxide?


  By photosynthesis, reducing carbon dioxide levels
• What contributed to the formation of sedimentary rocks containing carbon?


  Decayed remains of dead plants and animals
• What are examples of sedimentary rocks that contain carbon?


  Limestone and coal
• How are fossil fuels produced?


  From the remains of decayed plants and animals