Most iron ore deposits are the result of anaerobic bacterial action, mostly more than 2.5 billion years ago. Early photosynthesizing life (cyanobacteria) in the seas gave off oxygen; it did not initially enter the low-oxygen atmosphere because there was too much iron in the sea water. This iron reacted with the newly produced oxygen to form iron oxides, known today as banded iron formations. These deposits contain the vast majority of the earth’s iron ores. After most of the iron in the oceans had reacted, then oxygen created by the early plants could enter the atmosphere, which it did. Not until 2.0 or 1.8 billion years ago or later (after more than half the life of the earth had passed) did there begin to be enough oxygen in the atmosphere to support life as we see it today.
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