January 20. Banded Iron Formation: 1.8 billion years ago

By Richard I. Gibson

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Transcript:

Yesterday we touched on the role of iron in holding off the growing oxygen crisis for some hundreds of millions of years. By about 1.8 billion years ago, the last of the abundant reactive iron in the oceans had combined with oxygen to form iron-rich rocks known as banded iron formations. There are some younger examples, but by far most of the banded iron formations on earth are older than 1.8 billion years.

These rocks are excellent ore bodies, and the Mesabi Iron Ranges of northern Minnesota are great examples. They continue into northern Wisconsin and upper Michigan, and they supply about 97% of the iron ore produced in the United States, worth about $6 billion in 2013. That’s enough to make the U.S. a net exporter of iron. Western Australia’s Pilbara region contains similar iron ores of similar age, and other banded iron formations are found around the world. Iron was clearly precipitating out of the oceans all over the place.

The bands are alternating layers of hematite and magnetite, two iron oxide minerals, plus silica in the form of cryptocrystalline chert. A lot of this chert is jasper, colored red by iron. Banded iron formations are pretty rocks.

On this day, January 20, in 1875, Geologist Charles Kenneth Leith was born in La Crosse, Wisconsin. He was the head of the geology department at the University of Wisconsin for 31 years, and his specialty was the iron-bearing rocks of the Lake Superior region. 

Photo by André Karwath, from Wikipedia, licensed under the Creative Commons Attribution-Share Alike 2.5 Generic license.

Read more:  James St. John’s page • Genetic modeling banded iron formation