A Planetary Poisoning
These striped rocks are a crime scene written on a geological scale. The evidence records a mass extinction, the rusting of an entire planet, and the rise of a new atmospheric order. For the first billion years of Earth's history, the oceans were full of dissolved iron. Then, around 3.4 billion years ago, a new form of life evolved: cyanobacteria. These microbes developed a novel way to make energy from sunlight called oxygenic photosynthesis. Its waste product, oxygen, was toxic to the anaerobic life that dominated the planet.
This led to the Great Oxidation Event, a period starting around 2.4 billion years ago when oxygen produced by cyanobacteria began to accumulate in the oceans and atmosphere. The free oxygen immediately reacted with the abundant dissolved iron, which precipitated out of the seawater as insoluble iron oxides. These particles of what was effectively rust settled on the ocean floor, forming vast layers of iron-rich sediment. This process, which locked away immense quantities of iron, happened on a global scale but the formations in Western Australia's Hamersley Range are among the largest and best-preserved.