Precambian Banded Iron Formation
Banded Iron Formation, Upper Peninsula, Michigan, USA. Precambrian, ~ 2.5 billion years old. Banded Iron Formations, also called BIFs, represent a time when Earth's atmosphere contained significant amounts of oxygen produced for the first time from marine organisms as a byproduct of photosynthesis. Bands consist of repeated layers of silver to black iron oxides (such as hematite or magnetite), alternating with bands of iron-poor shales and silica (cherts or jasper) often red in color.
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