The Earth’s oldest reliably dated rocks are located in western Greenland and are about 3.8 billion years old.
They are a series of volcanic rocks which have been subjected to metamorphic effect brought about by heat and pressure. In other areas of the world there are similar metamorphic rocks somewhat younger than those in Greenland.
At about 3.2 billion years ago there first appear rocks which are called banded ironstones. They consist of alternate layers of silica (SiO) and ferric oxide (Fe O). Since they contain no silt it has to be assumed that they originated by precipitation on the ocean floor.
Prior to this date it is deduced from the absence from the rocks of iron in the ferric state that there was virtually no oxygen available to produce it. The first occurrence of banded ironstones marks the point where biological activity released enough oxygen to produce ferric oxide in significant quantities.
This does not mean that there was enough oxygen for it to be a free component of the atmosphere. That did not happen until much later at about 1.2 billion years ago. At that time, we find ‘red beds’ deposited. The fact that these contain much ferric iron and were clearly laid down in oxygen-rich conditions substantiates the fact that there must have been free oxygen in the atmosphere.