Banded Iron Formation Age: Archean (approximately 3.1 billion years ago) Location: Cleaverville Formation, Ord Range, West Australia
Banded Iron formations consist of thin laminated bands of chert and hematite, frequently in association with tiger eye. They are always very ancient, ranging from 3.8 billion years ago until 1.2 billion years ago (with a brief reoccurrence between 800-600 million years ago during snowball earth). The iron banding in these formations are due to the precipitation of iron oxide, which occurred when dissolved iron came in contact with oxygenated seawater. The oxygenation of the ocean was due to the arrival of cyanobacetia, which release oxygen as a byproduct of photosynthesis.
Eventually so much oxygen was released by cyanobacteria that the Earth's atmosphere changed. This led to the great oxygenation event which occurred between 2.4 and 2.2 billion years ago. Up to this point all life on Earth had occurred in an anaerobic environment. The change in oxygen levels probably lead to the Earth's first mass extinction event and changed the nature of the Earth forever. Life remade the world and banded iron formations mark this miraculous moment.