They found that large-scale volcanic activity increased atmospheric ... which in turn temporarily increased atmospheric oxygen. The increase in oxygen was not very steady, though, and came and ...

Chemical composition of ancient rocks Iron sulfide can only exist in rocks that were formed before there was oxygen in the atmosphere. Rocks containing iron oxide can only form if oxygen is present.

They found that large-scale volcanic activity increased atmospheric ... which in turn temporarily increased atmospheric oxygen. The increase in oxygen was not very steady, though, and came and ...