9.1 - Composition & Evolution of the Earth's Atmosphere

Describe the proportions of gases in the modern atmosphere.

• Around 80% of N₂.

• 20% of O₂.

• Small quantities of CO₂, H₂O, and noble gases.

Describe the Earth’s early atmosphere and how it was formed.

1. The Earth was hot when it was formed.

2. There was significant volcanic activity.

3. Gases have been released from these volcanoes.

4. These gases included CO₂, N₂, CH₄, NH₃, H₂O.

How did the amount of oxygen in the atmosphere increase?

• Algae and plants produced the atmospheric O by photosynthesis, which can be represented by the equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

• As the amount of gradually O₂ increased, there were more plants that produced more oxygen.

How did the amount of oxygen in the atmosphere decrease?

1. Some CO₂ was dissolved in the oceans, where it reacted with metal ions to form insoluble carbonates, such as CaCO₃, or was taken in by animals to become a part of skeleton. When the animal dies, this becomes a sedimentary rock.

2. Some CO₂ was used in photosynthesis to make oxygen.