9.1 The composition & evolution of the Earth's atmosphere

Question 1

Describe the proportions of gases in
the modern atmosphere

Answer 1

● 78% nitrogen

● 21% oxygen

● 1% carbon dioxide & noble gases

Question 2

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

Answer 2

● The Earth was hot when it was formed

● There was significant volcanic activity

● Gases have been released from these volcanoes

● These gases included:
CO2, N2, CH4, NH3, H2O

Question 3

How did the amount of oxygen
in the atmosphere increase?

Answer 3

Algae and plants produced the atmospheric O2 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.

Question 4

How did the amount of carbon
dioxide in the atmosphere
decrease?

Answer 4

● 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 an animal dies, this becomes a sedimentary rock.

● Some CO2
was used in photosynthesis to make oxygen.