The Evolution of the Atmosphere Notes
AQA GCSE Chemistry: Revision notes
Key facts
- The early atmosphere came from volcanic outgassing about 4.6 billion years ago: mainly carbon dioxide and water vapour, with little or no oxygen.
- Water vapour condensed to form the oceans.
- Carbon dioxide dissolved in the oceans, formed carbonates in sedimentary rocks and was locked in fossil fuels; photosynthesis also removed it.
- Algae and plants made oxygen by photosynthesis, which built up over billions of years.
- Today the atmosphere is about 78% nitrogen and 21% oxygen, and has been broadly stable for about 200 million years.
| Early atmosphere | Today | |
|---|---|---|
| Main gas | Mainly carbon dioxide | Nitrogen (about 78%) |
| Oxygen | Little or none | About 21% |
| Water vapour | Large amounts, then condensed | Variable, small amount |
Early atmosphere
- Main gas:
- Mainly carbon dioxide
- Oxygen:
- Little or none
- Water vapour:
- Large amounts, then condensed
Today
- Main gas:
- Nitrogen (about 78%)
- Oxygen:
- About 21%
- Water vapour:
- Variable, small amount
The early atmosphere
It came from volcanoes and was mainly carbon dioxide and water vapour, with little or no oxygen.
About 4.6 billion years ago, outgassing from volcanoes released gases from the Earth's interior.
- Carbon dioxide: the main gas
- Water vapour: large amounts
- Ammonia and methane: small amounts
- Oxygen: very little or none
This is very different from today.
- 1
Volcanoes were very active
about 4.6 billion years ago
- 2
Outgassing released gases
from the Earth's interior
- 3
The atmosphere formed
mostly CO₂ and water vapour, small amounts of ammonia and methane, almost no oxygen
- Early atmospheremainly and water vapour, little or no
Which process released the gases that formed Earth's early atmosphere?
Losing water and CO₂
The Earth cooled so water condensed into oceans, and carbon dioxide dissolved and was locked away.
As the Earth cooled, water vapour condensed to form the oceans.
Carbon dioxide then dissolved in the oceans and reacted to form carbonates, which became sedimentary rocks such as limestone. Some carbon was trapped in fossil fuels from dead organisms. Photosynthesis by algae and plants also removed carbon dioxide.
- 1
In the atmosphere
the main gas
- 2
Dissolves in the oceans
once the water had condensed
- 3
Forms carbonates
locked in sedimentary rocks such as limestone
- 4
Trapped in fossil fuels
from dead organisms on the ocean floor
What happened to the water vapour as the Earth cooled?
Photosynthesis and oxygen
Algae and later plants used light to make glucose and released oxygen, which slowly built up.
Photosynthesis by algae, and later plants, used carbon dioxide and water to make glucose, releasing oxygen as a waste product. Oxygen built up slowly over hundreds of millions of years, from nearly 0% to about 21%.
The oxygen allowed complex life and formed the ozone layer, which absorbs harmful ultraviolet radiation.
- 1
Algae and plants evolve
in the oceans and on land
- 2
Photosynthesis
removes carbon dioxide, makes glucose
- 3
Oxygen released
as a waste product
- 4
Oxygen accumulates
over hundreds of millions of years
Which gas, produced by photosynthesis, led to the formation of the ozone layer?
Ammonia and nitrogen
Ammonia was broken down, and nitrogen, which is unreactive, built up in the atmosphere.
The early atmosphere had small amounts of ammonia. It is no longer a significant gas: it was broken down and replaced by nitrogen.
Denitrifying bacteria also released nitrogen back to the air. Nitrogen is unreactive, so it stays in the atmosphere and now makes up about 78%.
- 1
Early atmosphere: a little ammonia
released by volcanoes
- 2
Ammonia was broken down
forming nitrogen
- 3
Denitrifying bacteria added nitrogen
returning it to the air
- 4
Nitrogen built up
because it is unreactive, so it stays in the atmosphere
- Ammoniabroken down, so nitrogen builds up
Which gas is the most abundant in today's atmosphere?
Today's atmosphere
About 78% nitrogen and 21% oxygen, with a little argon and carbon dioxide.
The atmosphere has been broadly stable for about 200 million years. Nitrogen and oxygen make up about 99%. Carbon dioxide and water vapour are present in much smaller amounts than in the early atmosphere.
Summary: early = mainly carbon dioxide and water vapour, little oxygen; today = mainly nitrogen and oxygen.
Which two gases make up about 99% of today's atmosphere?
Try an exam question
Describe how the amount of carbon dioxide in the atmosphere decreased and how the amount of oxygen increased over billions of years.
[4 marks]
- [1]Carbon dioxide dissolved in the oceans.
- [1]Carbonates formed sedimentary rocks, and carbon was locked in fossil fuels.
- [1]Algae and plants used carbon dioxide for photosynthesis.
- [1]Photosynthesis released oxygen, which built up in the atmosphere.
That's the notes covered.
Carry on to the next subtopic.