NationMaster - Encyclopedia: Banded iron formation

Banded iron formations (also known as banded ironstone formations) are a distinctive type of rock often found in old sedimentary rocks. The structures consist of repeated thin layers of iron oxides, either magnetite or hematite, alternating with bands of shale and chert. Some of the oldest known rock formations, formed around three thousand million years before present (3000MA), include banded iron layers, and the banded layers are a common feature in sediments for much of the Earth's early history. Banded iron beds are less common after 1800MA, although some are known that are much younger. Image File history File links Download high resolution version (2692x1936, 1704 KB) File links The following pages link to this file: Banded iron formation Ironstone ... Image File history File links Download high resolution version (2692x1936, 1704 KB) File links The following pages link to this file: Banded iron formation Ironstone ... Two types of sedimentary rock: limey shale overlaid by limestone. ... Iron oxide pigment There are a number of iron oxides: Iron oxides Iron(II) oxide or ferrous oxide (FeO) The black-coloured powder in particular can cause explosions as it readily ignites. ... Magnetite is a ferromagnetic mineral form of iron(II,III) oxide, with chemical formula , one of several iron oxides and a member of the spinel group. ... Hematite (AE) or haematite (BE) is the mineral form of Iron (III) oxide, (Fe2O3), one of several iron oxides. ... Shale Shale is a fine-grained sedimentary rock whose original constituents were clays or muds. ... Chert Chert is a fine-grained silica-rich cryptocrystalline sedimentary rock that may contain small fossils. ...

The conventional concept is that the banded iron layers were formed in water as the result of oxygen released by photosynthetic cyanobacteria, combining with dissolved iron in Earth's oceans to form insoluble iron oxides, which precipitated out, forming a thin layer on the substrate, which may have been anoxic mud (forming shale and chert). Each band is similar to a varve. The banding is assumed to result from cyclic variations in available oxygen. It is unclear whether these banded formations were seasonal or followed some other cycle. It is assumed that initially the Earth started out with vast amounts of iron dissolved in the world's acidic seas. Eventually, as photosynthetic organisms generated oxygen, the available iron in the Earth's oceans was precipitated out as iron oxides. At the tipping point where the atmosphere became permanently oxygenated, small variations in oxygen production produced pulses of free oxygen in the surface waters. General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless Atomic mass 15. ... Leaf. ... Orders The taxonomy of the Cyanobacteria is currently under revision. ... A varve is an annual layer of sediment or sedimentary rock. ... The phrase tipping point or angle of repose is a sociology term that refers to that dramatic moment when something unique becomes common. ...

Water flowing over iron-rich beds

Water colored by Iron. ... Water colored by Iron. ...

Later banded iron formations

Until fairly recently, it was assumed that the rare later banded iron deposits represent unusual conditions where oxygen was depleted locally and iron-rich waters could form then come into contact with oxygenated water. An alternate explanation of these later rare deposits is undergoing much research as part of the Snowball Earth hypothesis — wherein it is believed that an early equatorial supercontinent (Rodinia) was totally covered in an ice age (implying the whole planet was frozen at the surface to a depth of several kilometers) which corresponds to evidence that the earth's free oxygen may have been nearly or totally depleted during a severe ice age circa 750 to 580 million years ago (mya) (See Cryogenian period, from 800 million years ago (mya, boundary defined chronometrically) to approximately 635 mya) prior to the Ediacaran wherein the earliest multicellular lifeforms appear. Alternatively, some geochemists suggest that BIFs could form by direct oxydation of iron by autotrophic (non-photosynthetic) microbes. The Snowball Earth hypothesis attempts to explain a number of phenomena noted in the geological record by proposing that an ice age that took place in the Neoproterozoic was so severe that the Earths oceans froze over completely, with only heat from the Earths planetary core causing some... In geology, a supercontinent is a land mass comprising more than one continental core, or craton. ... Depiction of Rodinia at time of initial breakup. ... Variations in CO2, temperature and dust from the Vostok ice core over the last 400 000 years For the animated movie, see Ice Age (movie). ... General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless Atomic mass 15. ... In astronomy, geology, and paleontology, mya is an acronym for million years ago and is used as a unit of time to denote length of time before the present. ... The Cryogenian Period (from Greek cryos ice and genesis birth) is the second geologic period of the Neoproterozoic Era followed by the Ediacaran Period. ... The Ediacaran Period is the last geological period of the Neoproterozoic Era, just before the Cambrian. ... An autotroph (in Greek eauton = self and trophe = nutrition) is an organism that produces its own cell mass and organic compounds from carbon dioxide as sole carbon source, using either light or chemical compounds as a source of energy. ... Leaf. ...

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. Banded iron beds are an important commercial source of iron ore. This heap of iron ore pellets will be used in steel production. ...

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Later banded iron formations