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Earth Evolution
Earth Materials
Earth Evolution
Geological History
Solar System
Archean Proterozoic Early
Late Paleozoic Mesozoic Cenozoic

Archean Beginnings 3800-2500 Ma

O2 - Free Atmosphere - Banded Iron formation

Banded Iron Formations (BIF) are a distinct rock type found dominantly in Precambrian rocks between 2500 and 1800 million year old. They consist of repeated thin layers of iron oxide, magnetite (Fe3O4, grey) or hematite (Fe2O3, red), alternating with bands of iron-poor chert.

During most of Archean time the Earth's surface water and atmosphere were oxygen-free so that iron was stable in a reduced (ferrous or Fe2+) form which is water-soluble. Vast amounts of ferrous iron entered the oceans through volcanic activity, erosion and run-off.

Eventually, as the first photosynthetic organisms generated free oxygen in the oceans, the available dissolved ferrous iron was oxidized and precipitated as less soluble red ferric iron (Fe3+) oxides forming the BIF’s red Fe- rich bands. Biological cycles, probably seasonal, resulted in intervening periods when iron or oxygen may have not been as available, resulting in the interlayered chert.

Direct Evidence for an early Hydrosphere