Life › The microbial world › Great Oxidation Event
Banded iron formations
mostly 2.8–1.8 billion years ago· Fossil or site· Earth, climate & extinctions
Well establishedStrong fossil, genetic or geological evidence.
Striped rocks of rusty iron oxide and silica that formed on ancient seafloors and now supply most of the world's iron ore.
See it on the Life map →What happened
Early oceans, short of oxygen, held dissolved iron from vents. When that iron met oxygen from cyanobacteria, or was oxidized by microbes or sunlight, it formed rust that sank, building layers that alternate with silica. Some deposits are hundreds of meters thick and run for hundreds of kilometers. Most formed 2.8–1.8 billion years ago, with a brief return during Snowball Earth. Major deposits include Western Australia's Hamersley Range and Minnesota's Mesabi Range.
Why it matters
They record oxygen's rise and the oceans' changing chemistry, and they are the source of most iron mined today: much of the steel in modern buildings began as ancient seafloor rust.
The evidence (fossils, genes, rocks)
The rocks themselves, dated radioactively; iron isotopes; their timing relative to other oxygen signals. Geologist Preston Cloud linked them to oxygen in the 1960s–70s.
Still debated
Exactly how the iron was oxidized, and what set the fine, regular banding, perhaps seasonal or other natural cycles.
Connections (3)
▸Oxygen-making photosynthesis rusted dissolved iron into Banded iron formationsStrong evidence
The connection
Early oceans held large amounts of dissolved iron from vents. Oxygen released by cyanobacteria reacted with it, forming insoluble iron oxide, essentially rust, that settled to the seafloor. Alternating with silica layers, it built the striped banded iron formations, which soaked up oxygen before it could reach the air.
When and how
Mostly 2.8–1.8 billion years ago, in basins now in Western Australia, Minnesota, Brazil and South Africa; geologist Preston Cloud argued the link in the 1960s–70s.
How sure are we?
The rust layers need an oxidizer, and oxygen is the simplest one. But some bands could have formed through iron-using microbes or ultraviolet light, without oxygen.
Debates and open questions
How much of the iron was rusted by free oxygen versus oxygen-free microbes, and what caused the regular banding.
▸Banded iron formations record the build-up to Great Oxidation EventStrong evidence
The connection
Banded iron formations peaked as oxygen first appeared and mostly stopped around 1.85 billion years ago. They record the build-up: dissolved iron absorbed oxygen until the supply was used up, after which oxygen could accumulate in air and water.
When and how
The largest deposits formed about 2.6–2.4 billion years ago, such as Western Australia's Hamersley Range; they returned briefly during Snowball Earth about 700 million years ago.
How sure are we?
Dating shows the biggest deposits cluster around the oxidation event, and their iron chemistry tracks changing ocean oxygen.
Debates and open questions
Why they stopped around 1.85 billion years ago: an oxygenated deep ocean, sulfur-rich seas trapping iron, or less iron from vents.
▸Banded iron formations later supplied most of the iron ore for Industrial Revolution & fossil fuelsStrong evidence
The connection
Britain's early industry mostly smelted local ironstones found alongside coal. As steelmaking boomed from the late 1800s, it turned to banded iron formations: huge, rich deposits such as Minnesota's Mesabi Range (from the 1890s), then Australia's Pilbara and Brazil's Carajás. Today these ancient seafloor rocks supply most of the iron ore behind modern steel.
When and how
Lake Superior region (Michigan, Minnesota) from the mid-1800s; Western Australia and Brazil from the 1960s–80s. The rocks formed mostly 2.8–1.8 billion years ago.
How sure are we?
Mining and trade records show these deposits dominate world iron-ore output; geology ties the ore to banded iron layers laid down on ancient seafloors.
Debates and open questions
Whether microbes or oxygen from cyanobacteria did most of the iron rusting that formed them, and how much later weathering enriched the richest ores.