EARTH × LIFE
Banded Iron Formations
~3.5 to 1.8 billion years ago
Banded iron formations are sedimentary rocks made of alternating beds of iron oxide and silica, deposited between 3.5 and 1.8 billion years ago in an ocean where iron stayed dissolved because the water had no oxygen.
Banded iron formations are sedimentary rocks, formed from material that settled on the sea floor, in which beds of iron oxide alternate with beds of silica. Iron oxide is iron combined with oxygen, and silica is the substance of quartz. The formations were deposited between 3.5 and 1.8 billion years ago, in an ocean that contained dissolved iron. In water without oxygen, iron is in a soluble form. Oxidation, a reaction that takes electrons from the iron, converts it into an insoluble form. The iron then precipitates: it leaves the water as solid particles that sink. Each episode of oxidation therefore deposited a red bed of iron oxide, and silica settled between two episodes. The resulting banded rock is hundreds of metres thick and still supplies most of the iron ore mined in the world.
Why it matters
Geologists reconstruct the chemistry of the early ocean, layer by layer, from banded iron formations. These formations also explain the low cost of steel, because most of the great iron mines on the planet extract their ore from one of them.
Dating & uncertainty
Deposition peaked around 2.5 billion years ago and ceased almost entirely 1.8 billion years ago. The oxidising agent, the substance or process that oxidised the iron, remains debated. It may have been oxygen produced by cyanobacteria, bacteria that carry out photosynthesis. It may have been photoferrotrophy, a form of photosynthesis that takes its electrons directly from dissolved iron and releases no oxygen. Both mechanisms may have operated successively or together.
Earth history as one day
05:29:5218 h 30 min before midnight
if Earth’s 4.54 billion years were compressed into a single day