Striped rocks recording the 'rusting' of Earth's oceans. Whenever oxygen spiked; dissolved iron precipitated out. Creating layers that track atmospheric change.
Graeme Churchard from Bristol, UK, CC BY 2.0, via Wikimedia Commons
A planet-wide chemical reaction
The dramatic striped cliffs of the Hamersley Range are a record of a deep planetary transformation. These rocks, known as Banded Iron Formations (BIFs), document the period when Earth’s oceans began to rust. For over a billion years, early oceans were full of dissolved iron and almost completely devoid of free oxygen. Life was exclusively anaerobic. Around 2.7 to 2.5 billion years ago, photosynthetic cyanobacteria evolved and began to release oxygen as a waste product.
This new, highly reactive gas immediately combined with the abundant dissolved ferrous iron (Fe²⁺) in the seawater. The reaction formed insoluble iron oxides, primarily magnetite (Fe₃O₄) and hematite (Fe₂O₃), which precipitated and settled on the seafloor. This process created the distinct layering visible today. The dark, iron-rich bands represent periods of oxygen production and iron oxide deposition, while the lighter-colored layers are iron-poor chert, a form of silica. These alternating bands, some just millimeters thick, are thought to reflect cyclic or seasonal variations in microbial activity and oxygen output. The extensive BIFs in this region, such as the Brockman and Marra Mamba Iron Formations, can be several hundred meters thick.
The Great Oxidation Event
These local formations are the most compelling physical evidence for a global atmospheric shift called the Great Oxidation Event (GOE). This event, which unfolded between roughly 2.4 and 2.1 billion years ago, is the interval when oxygen produced by microbes finally overwhelmed the planet's chemical "sinks," like dissolved iron. For hundreds of millions of years, the oceans absorbed the new oxygen, locking it away in these vast iron formations. Estimates suggest the total amount of oxygen captured in the world's BIFs is as much as twenty times the volume present in our modern atmosphere.
Once the dissolved iron in the oceans was largely used up, oxygen began to escape into the atmosphere for the first time. This permanently altered the planet's chemistry, allowing for the evolution of aerobic respiration and, eventually, complex multicellular life. Before the GOE, atmospheric oxygen was less than 0.001% of its current level; afterward, it rose significantly, though it would take another billion years to reach modern concentrations. The deposition of most BIFs ceased around 1.8 billion years ago, signaling that the deep oceans had also become oxygenated. Today, these ancient chemical precipitates form the economic backbone of the Pilbara region, accounting for over 60% of global iron reserves and making it one of the most significant iron ore mining regions on Earth.
💡Fun Facts
The iron formations of the Pilbara are so vast they are the largest single source of seaborne iron ore in the world.
The total quantity of oxygen locked away in all of Earth's banded iron formations is estimated to be twenty times the amount of oxygen in the atmosphere today.
The appearance of "red beds"—rust-colored sandstones on land—after about 2.2 billion years ago signals that oxygen had finally saturated the oceans and started oxidizing minerals on the continents.
Some of the oldest evidence of life on Earth, 3.5-billion-year-old fossil stromatolites built by microbes, are also found in the Pilbara region, not far from the BIFs.
The gorges and lookouts of Karijini National Park are generally accessible year-round, but roads may close after heavy rain. The Karijini Visitor Centre is open daily from 9 am to 4 pm, with a seasonal closure for 6-9 weeks starting in mid-December.
Admission
A National Park entry fee of $17 per vehicle applies, which can be paid online or at the visitor center. Multi-day and annual passes are available.
Accessibility
The terrain is rugged. While the Karijini Visitor Centre is accessible with ramps and accessible toilets, access into the gorges involves steep steps, ladders, and uneven, rocky paths. Some lookouts, like the Hamersley Gorge Lookout, offer views from a short distance from the car park.
