Before free oxygen gas became abundant in the oceans and atmosphere, it reacted with a metal and produced banded iron formations.
Explanation;
Cyanobacteria are thought to have introduced oxygen into the early atmosphere. This was important because oxygen played a key role in the evolution of eukaryotic organisms creating an ozone layer to shield life from UV rays.
Banded iron layers were formed in sea water as the result of oxygen released by the photosynthetic cyanobacteria. As the photosynthetic organisms released oxygen, the available iron in the Earth's oceans precipitated out as iron oxides.
