The first banded iron formations
Alternating layers of iron and silica settle on the seafloor, recording an ocean full of dissolved iron.
- When
- Known to
- ± 60 million years
- Era
- Eoarchean, Archean
Banded iron formations begin appearing at Isua around 3.7 to 3.8 billion years ago. They can only form in oceans where iron stays dissolved — which requires water essentially free of oxygen — and they precipitate when something oxidises it. Whether the oxidant was early photosynthesis, ultraviolet light or iron-oxidising bacteria is unresolved, but these layers are the beginning of a record that runs for two billion years and supplies most of the iron ore mined today.