Tricky topics #2 - Earth's Early Atmosphere

We’ve looked through the Tassomai data, and found some of the topics that our students find the hardest! We’ll post a brief explainer on each of these topics over the next few weeks in the run up to exam season. Give this series a read if you, your child or your students are struggling with any part of Tassomai - even if these particular topics aren’t problematic, we’ll also touch on some common errors students make when answering questions on Tassomai!

Understanding how the Earth’s Early Atmosphere changed

Lots of our students seem to struggle with questions about how the Earth’s early atmosphere changed, so we’ve gone over that here.

Here’s an example Tassomai Question:

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When the Earth’s atmosphere first formed, it consisted of approximately 95% carbon dioxide, 3% nitrogen and 0.05% oxygen. Now, it is approximately 78% nitrogen, 20% oxygen and only 0.05% carbon dioxide, and it has been like this for approximately 200 million years - but how did these change happen?

Scientists believe that the majority of the gases which made up the Earth’s early atmosphere, a few billion years ago, were formed by volcanic activity - causing it to be primarily carbon dioxide. It was only after the volcanic activity had subsided that the atmosphere began to change.


What caused these changes?

Decrease in amount of Carbon Dioxide

 

CO2 went from making up 95% of the atmosphere to only 0.5% - a drastic change. There were three main causes for this reduction in carbon dioxide levels:

  • Oceans - carbon dioxide is highly soluble, so as the oceans formed it dissolved, removing it from the atmosphere;
  • Sequestration as fossil fuels - the carbon cycle trapped CO2 in organic matter, which was then stored underground as fossil fuels;
  • Sequestration as sedimentary rock - the carbon cycle trapped CO2 in organic matter, which was then stored underground as limestone.

The Earth’s Early Atmosphere - mainly carbon dioxide, but with some oxygen and nitrogen

The Earth’s Early Atmosphere - mainly carbon dioxide, but with some oxygen and nitrogen

Increase in amount of Oxygen

The level of oxygen increased from 0.05% of the atmosphere to its current levels of around 20%. The main reason for this is photosynthesis; as plants began to grow, they converted carbon dioxide into oxygen via photosynthesis (this is another reason for the drop in CO2 levels).

The Earth’s Atmosphere now - oxygen levels have increased slightly due to photosynthesis, while carbon dioxide levels have decreased drastically as it dissolved in the sea. Nitrogen levels have remained basically the same

The Earth’s Atmosphere now - oxygen levels have increased slightly due to photosynthesis, while carbon dioxide levels have decreased drastically as it dissolved in the sea. Nitrogen levels have remained basically the same

No change in amount of Nitrogen

This seems counterintuitive - the atmosphere has gone from being 3% nitrogen to being 78% nitrogen, so how can the amount of nitrogen not have changed? What has actually happened is that, while the amount of nitrogen hasn’t significantly changed, the decrease in carbon dioxide means that there is relatively more nitrogen compared to the other gases. Therefore the proportion of the atmosphere made up of nitrogen has increased - even though there is no more nitrogen than before.

The answer to this question

Therefore the answer to this question is A - the proportion of nitrogen has increased as carbon dioxide dissolved into the oceans, but it has not been created by photosynthesis (B) or by human activity (C). A lot of students choose D for this question, because they assume “All of these” is always right when it is one of the answers - so make sure you read the entire question! Examiner’s reports always mention students not reading questions properly, so make sure you practice doing that on Tassomai now.

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