What is Banded iron formation?
[[File:Banded iron formation Dales Gorge.jpg|thumb|right|250px|Banded iron formation, [[Karijini National Park]], [[Western Australia]]]] [[File:Black-band ironstone (aka).jpg|thumb|2.1 billion year old banded iron formation]] '''Banded iron formations''' (or '''BIF'''s) are a distinctive type of rock often found in [[Precambrian]] [[sedimentary rock]]s. They consist of repeated thin layers of [[iron oxide]]s, either [[magnetite]] (Fe<sub>3</sub>O<sub>4</sub>) or [[hematite]] (Fe<sub>2</sub>O<sub>3</sub>), alternating with bands of iron-poor [[shale]] and [[chert]]. Some of the oldest known rock formations, formed over 3,700 million years ago (mya), include banded iron layers.<ref>Minik T. Rosing ''et al.'' 1996. Earliest part of Earth's stratigraphic record: A reappraisal of the >3.7 Ga Isua (Greenland) supracrustal sequence. ''Geology'' '''24''' #1, 43-46</ref> The banded layers are a common feature in [[sediment]]s for much of the Earth's early history.<ref>Knoll, Andrew H. 2004. ''Life on a young planet: the first three billion years of evolution on Earth''. Princeton, N.J. {{ISBN|0-691-12029-3}}</ref><ref>Faces of Earth - Building the Planet. [[YouTube]] account of early Earth history. [https://www.youtube.com/watch?v=f-jD7kQvyPs] American Geosciences Institute.</ref> == Relation to atmospheric oxygenation == [[Image:MichiganBIF.jpg|thumb|Close-up of banded iron formation specimen from Upper Michigan. [[Scale (ratio)|Scale bar]] is 5.0 mm.]] The formations are abundant around the time of the [[Great Oxygenation Event]], 2,400 mya.<ref name=Cloud>Cloud, Preston 1973. Paleoecological significance of the Banded Iron-Formation. ''Economic Geology'' '''68''' (7): 1135. doi:10.2113/gsecongeo.68.7.1135.</ref> They become less common after 1,800 mya. The total amount of oxygen locked up in the banded iron beds is estimated to be perhaps twenty times the volume of oxygen present in the modern [[atmosphere of Earth]]. Banded iron beds are an important commercial source of [[iron]] [[ore]], such as the [[Pilbara]]