The 2.5-Billion-Year-Old Banded Iron That Built an Industry

Some rocks look like they were painted. The banded iron formations of Western Australia's Hamersley Range run in alternating stripes of red and black—chert and hematite, laid down in rhythms so regular they resemble tree rings or the pages of a book. They are the largest known accumulation of iron on Earth, and they tell the story of a planet learning to breathe.

The Chemistry of a Poisoned Ocean

The Hamersley Group was deposited between 2.6 and 2.4 billion years ago, during the Great Oxidation Event. Before that, Earth's oceans were rich in dissolved ferrous iron—iron in its reduced, soluble form—because there was almost no free oxygen in the atmosphere or seawater. Cyanobacteria had already invented photosynthesis, but any oxygen they released was immediately consumed by iron, rusting it out of solution.

The rust fell to the seafloor in fine layers. Each band of red hematite represents a pulse of oxygen; each band of black chert represents a pause. Over tens of millions of years, the rising and falling of microbial activity, seasonal blooms, or changes in ocean circulation wrote the rock in stripes. The formations are up to 2.5 kilometres thick, covering an area the size of England.

The iron in a single tonne of Hamersley ore once circulated through the global ocean as dissolved metal, waiting for a microbe to exhale.

The Mountain That Became a Mine

The Hamersley Range today is a series of flat-topped ridges and deep gorges, weathered from what was once a continuous plateau. The banded iron formations—the Brockman Iron Formation, the Marra Mamba Iron Formation—are hard and resistant, so they form the highest ground. The softer shales between them erode into valleys.

Mining began in earnest in the 1960s. The iron content of the best ore exceeds 64 percent, and almost no processing is needed beyond crushing and washing. The ore is shipped from Port Hedland and Dampier to steel mills in China, Japan, and South Korea. In 2023, Western Australia produced 854 million tonnes of iron ore, most of it from the Hamersley and the adjacent Chichester Range. The industry employs tens of thousands of people and generates more export revenue than any other Australian commodity.

But the scale of the mining is incidental to the geology. The formations were already a wonder before anyone drilled a blast hole.

Why the Iron Never Disappeared

Most ancient banded iron formations were destroyed by subduction—dragged into the mantle when the plates that carried them were recycled. The Hamersley Basin survived because it sat on the Pilbara Craton, a fragment of Archaean crust so thick and buoyant that it never got pulled under. The craton has remained stable for 3.5 billion years, a raft that carried the banded iron formations through every subsequent episode of mountain-building and continental drift.

The formations also escaped metamorphism. They were buried but never deeply enough to recrystallise, so the original sedimentary textures remain: the fine laminations, the ripple marks, even the trace fossils of microbial mats. A core sample from the Brockman Iron Formation looks almost exactly as it did when it settled out of the Proterozoic sea.

The Archive of a World Before Oxygen

The banded iron formations are not just an economic resource. They are the most detailed record we have of the transition from an anoxic Earth to one with free oxygen. Each band is a geochemical snapshot—a measurement of how much iron the ocean held, how much oxygen the microbes produced, and how the two interacted over hundreds of millions of years.

The formations stop appearing in the rock record around 1.8 billion years ago. By then, the oceans had lost their dissolved iron. The atmosphere had stabilised at roughly modern oxygen levels. The rust had all fallen out, and the sea was clear. The Hamersley formations remain as the residue of that chemical transformation—2.5 billion tonnes of iron, still waiting to be put to use.