Australian Iron FormationThis image shows a rock type known as a Banded Iron Formation (BIF). Hopefu

Australian Iron FormationThis image shows a rock type known as a Banded Iron Formation (BIF). Hopefully the “banded” part makes sense just from the photo.These rocks are about 2.5 billion years old and come from Dales Gorge in the Pilbara Craton of Western Australia. This is a key time in Earth’s history as it’s when oxygen first began building up in the atmosphere. There are hints from some locations on Earth that bacteria evolved the ability to generate oxygen through photosynthesis hundreds of millions of years earlier, but at around 2.5 billion years ago oxygen started rising.Before there was oxygen in the atmosphere, iron could exist as “ferrous” iron, with a 2+ charge. When iron rusts today, it becomes “ferric” iron with a 3+ charge – extra oxygen causes the change in charges.Ferrous iron can dissolve in ocean waters, ferric iron cannot. Prior to oxygen in the atmosphere, iron could dissolve in the oceans, but once oxygen started being generated, the iron dissolved in the world’s oceans reacted with that oxygen and formed sedimentary iron rocks. Many of the world’s economic iron deposits were produced in this way.The other distinct characteristic of these rocks is their banding. The layers are produced by alternating patterns of iron-rich sediments and silica-rich sediments. The silica rich layers are produced by other sediments which are washed into the areas producing iron formations, possibly by storms, river inputs, or maybe other processes as well.-JBBImage credit: Pete Hill (Creative Commons licensed)http://www.flickr.com/photos/blundershot/5992386207Read more:http://www.newark.osu.edu/facultystaff/personal/jstjohn/Documents/Cool-Rocks/Banded-iron-formations.htmhttp://www.sciencedirect.com/science/article/pii/0301926893900513 -- source link

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