The Earth's early atmosphere contained very little oxygen gas.
Today, the Earth's atmosphere contains about 21% oxygen gas.
Which process was primarily responsible for increasing the concentration of oxygen gas in the Earth's atmosphere?
Volcanic eruptions
Photosynthesis by primitive plants and algae
Nitrogen fixing by bacteria
Combustion of volcanic sulfur
32 exam-style questions on OCR GCSE Chemistry Interpreting and interacting with earth systems. Each one has a worked solution and a mark scheme showing where the marks go.