Memory Anchor:

• Super Challenge:

  Stretch:

  Challenge:

Key Questions:

1. What is the composition of the earths atmosphere?
   • about four-fifths (approximately 80 %) nitrogen ? about one-fifth (approximately 20 %) oxygen ? small proportions of various other gases, including carbon dioxide, water vapour and noble gases.
2. How has the Earths atmosphere evolved?
   • One theory suggests that during the first billion years of the Earth?s existence there was intense volcanic activity that released gases that formed the early atmosphere and water vapour that condensed to form the oceans. At the start of this period the Earth?s atmosphere may have been like the atmospheres of Mars and Venus today, consisting of mainly carbon dioxide with little or no oxygen gas.
3. How has the Earths atmosphere evolved?
   • One theory suggests that during the first billion years of the Earth?s existence there was intense volcanic activity that released gases that formed the early atmosphere and water vapour that condensed to form the oceans. At the start of this period the Earth?s atmosphere may have been like the atmospheres of Mars and Venus today, consisting of mainly carbon dioxide with little or no oxygen gas.
4. What types of gases did volcanoes produce?
   • Volcanoes produced nitrogen which gradually built up in the atmosphere and there may have been small proportions of methane and ammonia.
5. What happened to the atmopshere when the oceans formed?
   • When the oceans formed carbon dioxide dissolved in the water and carbonates were precipitated producing sediments, reducing the amount of carbon dioxide in the atmosphere.
6. What process has caused oxygen levels in our atmosphere to increase since its formation?
   • Photosynthesis, carried out by organisms such as algae and plants converted carbon dioxide into the oxygen now found in the atmosphere.
7. When did Algae first produce oxygen?
   • Algae first produced oxygen about 2.7 billion years ago and soon after this oxygen appeared in the atmosphere. Over the next billion years plants evolved and the percentage of oxygen gradually increased to a level that enabled animals to evolve.
8. How did Algae and plants affect the atmosphere?
   • Algae and plants decreased the percentage of carbon dioxide in the atmosphere by photosynthesis.
9. Apart from through the process of photosynthesis, what else has caused carbon dioxide levels in the atmosphere to decrease since its formation?
   • Carbon dioxide levels also decreased due to some dissolving into the oceans, the formation of sedimentary rocks and the formation of fossil fuels.
10. How did the Earth's atmosphere change over time? Why did these changes happen?
    • Volcanic activity caused high levels of carbon dioxide, as the volancos died out the Earth's temperature cooled. The water vapour produced by volcanos condensed to form oceans. The carbon dioxide levels decreased as it dissolved into the oceans and formed part of the shells of marine animals.
11. How and why did limestone, coal, crude oil and natural gas form?
    • Limestone, crude oil and natural gas all contain carbon. Limestone was formed by the calcium carbonate from sea organisms and their shells. Crude oil and natural gas form from the decomposition of living animals and plants.