Banded Iron Formation
“Ancient rust from Earth's first oxygen”
Banded iron formations (BIFs) are striking sedimentary rocks with alternating layers of iron-rich and silica-rich bands, mostly deposited between 2.5 and 1.8 billion years ago. They record one of Earth's most transformative events — the rise of photosynthetic organisms that began pumping oxygen into an atmosphere that previously had almost none.
hematite, magnetite, chert (microcrystalline quartz), siderite, iron silicates
Pilbara and Hamersley Basin (Western Australia), Minas Gerais (Brazil), Lake Superior region (USA/Canada), Transvaal (South Africa), Krivoy Rog (Ukraine)
5-7
BIFs are the source of roughly 90% of all iron ore mined globally — the steel in your car, buildings, and bridges almost certainly originated as rust that formed in ancient Precambrian seas.
Test Your Knowledge
What biological event is primarily responsible for the formation of banded iron formations?
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