Question

1. how and when did the earths atmosphere become rich with oxygen?

volcanic ash outgassing; 4.6 billion years ago
photosynthesizing cyanobacteria; about 2 billion years ago
asteroid and meteor impact accretion; 3.4 billion years ago

Explanation:

Cyanobacteria are capable of photosynthesis, which releases oxygen. The graph shows a significant rise in oxygen levels around the time associated with the activity of photosynthesizing cyanobacteria. Volcanic ash outgassing and asteroid - meteor impact accretion do not directly contribute to oxygen - rich atmosphere formation as described in the context of oxygen production.

Answer:

photosynthesizing cyanobacteria; about 2 billion years ago