The 2.6-Billion-Year-Old Seafloor That Still Holds the Sky's Blueprint

The Hamersley Range in northwest Western Australia is a spine of rust-coloured ridges that runs for 400 kilometres. From the air, the escarpments look like a stack of red and grey pages—a book written in iron, silica, and silence. These are the banded iron formations of the Brockman Iron Formation, laid down 2.6 billion years ago, when the oceans were full of dissolved iron and the sky held almost no free oxygen.

The Chemistry of a Rusting World

Before the Great Oxidation Event, Earth's oceans were an iron soup. Hydrothermal vents on the Archaean seafloor pumped dissolved ferrous iron into the water, where it accumulated for hundreds of millions of years because there was nothing to make it rust. The atmosphere lacked oxygen; the seas lacked oxygen. The iron just stayed dissolved.

Then something changed. Cyanobacteria—microscopic, photosynthetic, unremarkable—began releasing oxygen as a waste product. That oxygen reacted with the dissolved iron, turning it into insoluble ferric iron—rust—that sank to the seafloor in thin, periodic layers. Each band in the Hamersley formations is a pulse of that planetary chemistry: a dark band of iron oxide, a light band of chert, a rhythm that repeated for tens of thousands of years.

The result is a stack of sediment 2.5 kilometres thick, covering an area the size of Tasmania. The Hamersley Basin holds the largest banded iron formation on Earth, and it contains more than 80 percent of Australia's iron ore.

The iron in a single skyscraper's steel frame began as a molecule dissolved in Archaean seawater, waiting for the first breath of oxygen.

The Dales Gorge Section

At Dales Gorge in Karijini National Park, the banded iron formations are exposed in a narrow chasm cut by perennial springs. The walls rise sheer and striped, each band a few millimetres to several centimetres thick. Geologists have counted more than 200 individual iron-oxide bands in the Dales Gorge member alone, each one representing a seasonal or decadal bloom of oxygen-producing microbes.

What makes the Hamersley formations remarkable is their preservation. Most Archaean rocks have been deformed, heated, and recrystallised by later tectonic events. But the Hamersley Basin sat undisturbed on the Pilbara Craton for two and a half billion years, protected by the craton's stability. The bands are still flat, still crisp, still readable.

The Rock That Built Modern Australia

Banded iron formations are not rare—they exist on every continent. But the Hamersley deposits are uniquely thick and rich, with iron grades averaging 60 percent or higher. When the steel boom of the twentieth century demanded ore, Western Australia's Pilbara region became the world's largest supplier. The town of Newman, built on the southern edge of the range, ships tens of millions of tonnes of iron ore each year to ports at Port Hedland and Dampier.

The irony is plain: the same rust that poisoned Archaean oceans—oxygen was toxic to most early life—now holds up the modern world's infrastructure. Every beam, bridge, and railway line that contains Australian steel carries a trace of that ancient bacterial waste.

What the Bands Still Tell Us

Geologists continue to read the Hamersley bands as a climate archive. The thickness and spacing of the layers record shifts in ocean chemistry, volcanic activity, and the pace of biological evolution. Some bands contain traces of glacial debris, suggesting that the planet was still swinging between greenhouse and icehouse states even as the oxygen revolution unfolded.

One puzzle remains: why did the banded iron formations stop forming? After 1.8 billion years ago, they largely disappear from the geological record. The answer is that the oceans had finally rusted through. All the dissolved iron had precipitated out, leaving the seas clear and oxygenated—and leaving the atmosphere free to accumulate the gas that would make animal life possible.