The composition and evolution of the Earth’s atmosphere

9.1.1 The proportions of different gases in the atmosphere

  • Nitrogen – 80%
  • Oxygen – 20%
  • Small proportions of other gases, including carbon dioxide, water vapour & noble gases

There is not much O2 in the atmosphere of Mars. Suggest why (1)

  • Mars has no plants

The percentage of argon in Earth’s atmosphere today is same as it was in early atmosphere. Why? (2)

  • Argon is a noble gas
  • Unreactive coz have full & stable outer shell

9.1.2 The Earth’s early atmosphere

  1. Intense volcanic activity that released gases that formed the early atmosphere
  2. Water vapour that condensed to form the oceans.
  3. At the start of this period the Earth’s atmosphere may have been like the atmospheres of Mars & Venus today, consisting mainly CO2 with little or no O2 gas
  4. Volcanoes also produced N2 which gradually built up in the atmosphere & there may have been small proportions of CH4 & NH3
  5. When oceans formed, CO2 dissolves in the water & carbonates were precipitated producing sediments, reducing amount of CO2 in atmosphere
  6. Earth cooled further, reducing no of volcanoes and form crust. Soon continents are formed.
  7. Algae & plants produced O2 that is now in the atmosphere by photosynthesis, which can be represented by the equation:     

Describe what happened to CO2 in Earth’s early atmosphere (3)

  • CO2 is absorbed by plants for photosynthesis
  • Dissolves in ocean
  • Used to form shells & skeletons of marine organisms
  • Locked up as limestone & fossil fuels/coal/crude oil

Describe & explain how composition of Earth’s atmosphere was changed by formation of coal. (3)

  • Plants absorb CO2 & releases O2 by photosynthesis. This causes CO2 to decrease & O2 to increase in atm
  • CO2 is also locked up as coal so CO2 decreases

 Explain how CO2 is released from limestone (1)

  • Thermal decomposition of calcium carbonate
  • CaCO3(s) → CaO(s) + CO2(g)

Where did the carbon that is locked up in fossil fuels come from? (1)

  • CO2 in Earth’s early atmosphere

The burning of fossil fuels has caused the percentage of carbon dioxide in the atmosphere to increase to above 0.03%. Explain why (2)

  • Increasing in burning fossil fuels
  • Coz CO2 that is locked up as fossil fuels reacts with O2 & produce CO2

In this question you will be assessed on using good English, organising information clearly and using specialist terms where appropriate.

This Earth and its atmosphere today are not like the early Earth and its atmosphere.

Describe and explain how the surface of the early Earth and its atmosphere have changed to form the surface of the Earth and its atmosphere today. (6)

Main changes

  • Oxygen increased because plants / algae developed and used carbon dioxide for photosynthesis / growth producing oxygen; carbon dioxide decreased because of this
  • Carbon dioxide decreased because oceans formed and dissolved / absorbed carbon dioxide; carbon dioxide became locked up in sedimentary / carbonate rocks and / or fossil fuels
  • Oceans formed because the Earth / water vapour cooled and water vapour in the atmosphere condensed
  • Continents formed because the Earth cooled forming a supercontinent / Pangaea which formed the separate continents
  • Volcanoes reduced because the Earth cooled forming a crust

Other changes

Nitrogen has formed because ammonia in the Earth’s early atmosphere reacted with oxygen / denitrifying bacteria

Why don’t scientists know accurate composition of Earth’s early atmosphere? (1)

  • Too long ago (time scale of 4.6 billion years)

In 1915, Alfred Wegener had an idea that the change shown in the diagram was caused by continental drift. Most scientists could not accept his idea.

Suggest why most scientists in 1915 could not accept Wegener’s idea of continental drift (1)

Wegener had no evidence

9.1.3 How oxygen increased

Algae & plants produced O2 by photosynthesis

Plants evolved & percentage of O2 increased → able animals to evolve

9.1.4 How carbon dioxide decreased

For the last 200 million years the amount of carbon dioxide in the atmosphere has remained almost the same.

Describe the natural processes which remove carbon dioxide from the atmosphere.

To gain full marks in this question you should write your ideas in good English. Put them into a sensible order and use the correct scientific words. (4)

  • Plants take in (CO2)
  • Converted to glucose / starch / carbohydrates
  • CO2 locked up in fossil fuels
  • CO2 reacts with / dissolves (sea)water
  • Producing hydrogencarbonates / carbonates
  • Marine animals use carbonates to make shells
  • Forms sedimentary rocks / limestone / chalk

Carbon dioxide has decreased due to:

  • Plants / microorganisms / bacteria / vegetation / trees
  • Photosynthesis
  • ‘Locked up’ in (sedimentary) rocks / carbonates / fossil fuels
  • Dissolved in oceans

Oxygen has increased due to:

  • Plants / bacteria / microorganisms / vegetation / trees
  • Photosynthesis

Nitrogen increased due to:

  • Ammonia reacted with oxygen
  • Bacteria / micro organisms