The Earth and its Atmosphere
Oxygen started to appear on Earth due to blue-green algae which formed colonies called stromatolites.

The Earth and its Atmosphere

The Earth's inner and outer layers and their composition is one of the topics covered in GCSE Chemistry. In this quiz we look at the gases which make up the Earth's atmosphere and how they got there.

When the Earth was formed about 4,500 million years ago, it was a very different place. The surface was extremely hot, possibly even molten, there were no oceans and little or no atmosphere. It is believed that the early atmosphere was formed from volcanic gases and was highly toxic. It is not known for sure but one theory suggests that during the first billion years of its existence, the Earth’s atmosphere was mainly carbon dioxide and there would have been little or no oxygen, rather like the atmospheres of Mars and Venus today. There may also have been water vapour which eventually condensed to form the oceans, and small proportions of methane and ammonia.

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Somewhere around 3.5 and 2.7 billion years ago, bluish-green microscopic organisms called cyanobacteria (commonly called blue-green algae) appeared in Earth’s oceans. They made oxygen from carbon dioxide, water, and sunlight by photosynthesis. As cyanobacteria created more free oxygen, the amount of oxygen in the atmosphere slowly increased. Earth scientists (geologists) have discovered rocks in Canada that are about 2,500 million years old and contain iron oxide. This is good evidence that there was definitely free oxygen in the atmosphere at that time.

For the last 200 million years, the proportions of different gases in the atmosphere have been about the same as they are today (about four-fifths nitrogen, about one-fifth oxygen with small proportions of various other gases, including carbon dioxide, water vapour and noble gases). Since the industrial revolution began, human activity has started to change the composition of the atmosphere - burning fossil fuels releases carbon dioxide into the air. This comes from carbon that was locked away in rocks hundreds of millions of years ago.

No one knows how life began on Earth but there are several theories. The scientist Charles Darwin suggested that it had arisen in warm ponds, hundreds of millions of years ago. That was in the 19th century and science was not advanced enough to be able to offer any possible mechanism. It is still not possible to prove or disprove theories about how life started on Earth, but we know much more about it now. In the 1950s, two scientists (Miller and Urey) carried out an experiment and managed to produce some simple organic chemicals that could possibly eventually have given rise to life by using a mixture of gases similar to what they thought were in the early atmosphere. Since then, other scientists have come up with other theories including one that suggests that life developed around hydrothermal vents on the sea floor and that the first food chains and webs were based on chemosynthesis instead of photosynthesis. In your exams, you may be given information or data about these theories and asked to interpret it.

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  1. Which was thought to be the most abundant gas in the Earth's early atmosphere?
    Oxygen didn't appear until the cyanobacteria had evolved. These microorganisms were able to photosynthesise
  2. Carbon dioxide entered the early atmosphere when it was released from...
    Volcanoes released other gases into the atmosphere too, including water vapour
  3. The percentage of water on the planet did NOT increase due to...
    Plants give out oxygen as a waste product of the process of photosynthesis
  4. Which of the following gases is thought NOT to have been present in the early atmosphere?
    Oxygen did not appear until the evolution of photosynthesising organisms
  5. Oxygen started to appear on Earth due to what organisms?
    More accurately called cyanobacteria
  6. What process produces oxygen as a waste product?
    During photosynthesis, the useful product glucose is made from water and carbon dioxide. The process gets its energy from sunlight
  7. Which of the following is NOT a reason for the small percentage of carbon dioxide in the Earth's atmosphere today?
    The carbon cycle locks away carbon dioxide. The Earth needs some carbon dioxide so that plants can photosynthesise and also to help to keep the average temperature of the Earth at about 15°C, but if there is too much, there would be a runaway greenhouse effect and the Earth would become uninhabitable
  8. At one time in the Earth's formation, some of the surface iron was oxidised. What gas in the atmosphere could NOT have caused this change?
    Nitrogen is almost inert, it is very hard to get it to react with anything
  9. Which of the following developments is NOT as a result of concerns about carbon dioxide levels and global warming?
    Diesel is also a hydrocarbon obtained from fossil fuels and produces carbon dioxide when burnt
  10. The change in carbon dioxide concentration in the last 50 years is thought to be a result of...
    More and more people have cars and the developing countries have a greater need for energy as they become more industrialised

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