BIF

A banded iron formation from Canada shows that the atmosphere and ocean once had no oxygen and that was nearly to 3 billions years old .

 Iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert . They can be up to several hundred meters in thickness and extend laterally for several hundred kilometers. Almost all of these formations are of Precambrian age and are thought to record the oxygenation of the earth's ocean.

Photosynthetic organisms were making oxygen, but it reacted with the iron dissolved in seawater to form iron oxide minerals on the ocean floor, creating banded iron formations. Each band is similar to a varve, resulting from cyclic variations in oxygen production. The dark layers in this boulder are mainly composed of magnetite (Fe3O4) while the red layers are chalcedony, a form of silica (SiO2) that is colored red by tiny iron oxide particles. Some geologists suggest that the layers formed annually with the changing seasons.

Some of the Earth's oldest rock formations, which formed about 3,700 million years ago (Ma), are associated with banded iron formations. They were first discovered in northern Michigan  in 1844. Banded iron formations account for more than 60% of global iron reserves and provide most of the iron ore presently mined. Most formations can be found in Australia, Brazil, Canada, India, Russia, South Africa, Ukraine, and the United States .