The Lava That Baked a 1.7-Billion-Year-Old Secret: Australia's Hart Dolerite and the McArthur Basin

Deep in the Northern Territory's McArthur Basin, a 300-metre-thick slab of dolerite intruded into layers of black shale 1.7 billion years ago. The magma baked the surrounding sediment so intensely that fragments of organic matter were cooked into charcoal — and then preserved for over a billion years. These are the oldest known traces of wildfire on Earth.

The Fire That Left No Flame

Before the Devonian period, before the first tree, fire existed only where magma met rock. The Hart Dolerite, a vast sill emplaced during the Proterozoic, heated the Barney Creek Formation shale to hundreds of degrees Celsius. Organic material in the shale — the remains of ancient microbial mats — did not burn in open air. It pyrolysed under pressure, turning to inert carbon.

Geologists call these fragments inertinite. They are indistinguishable from modern charcoal except for their age. In 2022, a team from the University of Melbourne and the University of New South Wales identified inertinite grains in drill cores from the McArthur Basin, dating them to 1.7 billion years ago. The previous record for the oldest wildfire evidence was 430 million years.

The McArthur Basin charcoal pushes the known history of fire back by more than a billion years — to a time when the only oxygen came from cyanobacteria.

A World Without Wood

The landscape that burned was alien. No trees, no leaves, no grasses. The continents were barren rock and water. Oxygen levels were perhaps 1 percent of today's atmosphere — far too low to sustain an open flame. Yet the charcoal exists.

The paradox resolves when you consider the source of heat. The Hart Dolerite did not ignite a forest. It cooked buried organic sediment that had never seen the sky. This was fire without atmosphere, combustion without oxygen. The charcoal records not a wildfire but a geological baking — a preview of the heat that would later, when land plants evolved, become true wildfire.

The Sill That Preserved a Basin

The McArthur Basin itself is a geological treasure. Its sediments accumulated in a shallow sea between 1.8 and 1.6 billion years ago, capturing the rise of oxygen and the early diversification of microbial life. The Barney Creek Formation is famous for its exquisitely preserved microfossils and organic biomarkers.

The Hart Dolerite intruded into these sediments around 1.7 billion years ago, during a period of continental stretching that preceded the breakup of the supercontinent Nuna. The magma sheet is part of a larger igneous province that extends across northern Australia. Where it contacted organic-rich shale, it created a natural laboratory for studying the effects of extreme heat on ancient carbon.

What the Charcoal Tells Us

The inertinite grains are more than a curiosity. Their abundance and distribution allow geologists to estimate the temperature and duration of the heating event. The charcoal formed at temperatures between 300 and 500 degrees Celsius, consistent with the outer margin of a cooling sill.

More broadly, the discovery changes how we think about fire in deep time. Fire is not exclusively a surface phenomenon. It can occur kilometres underground, wherever heat and organic matter meet. The McArthur Basin charcoal shows that the ingredients for combustion — carbon, heat, and pressure — have existed since the Proterozoic.

Today the Hart Dolerite forms a low ridge in the rugged country south of Borroloola. The charcoal it created lies hidden in drill cores, a billion-year-old ember that never reached the sky.