Banded iron formations are units of sedimentary rock usually of Precambrian age (1.8 to 2.5 billion years old)
They tells us that there was little free oxygen, and Iron-rich oceans, before these rocks were formed. This translates to the Earth's atmosphere when the iron in the oceans became used up, resulting in the build up of oxygen in the atmosphere.
Dec 27th, 2014
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