The Magma That Kindled a Reef: New South Wales' Warrumbungle Volcanoes

Seventeen million years ago, magma rose through a crack in eastern Australia and encountered something unexpected: mantle rock that had been metasomatised—chemically transformed—by fluids released from a subducted slab half a billion years earlier. The result was not basalt but something rarer: a suite of alkaline lavas that built one of the most chemically unusual volcanic fields on the continent.

The Alkaline Anomaly

Most of eastern Australia's Cenozoic volcanoes erupted basalt—runny, iron-rich lava that spread in flat sheets. But the Warrumbungle Volcanoes, in north-central New South Wales, produced trachyte, hawaiite, and mugearite: lavas high in sodium and potassium, low in silica. These are the products of low-degree partial melting, where only a small fraction of the mantle source melts, concentrating the incompatible elements.

The trigger was the passage of the Australian plate over the Cosgrove hotspot track—the same mantle plume that built the Tweed Volcano (now Mount Warning) and the Canobolas volcanoes further south. But at Warrumbungle, the plume tapped a mantle source that had been enriched by ancient subduction fluids during the Delamerian Orogeny, around 500 million years ago. The magma inherited a chemical signature unlike any other volcanic province in Australia.

The Spires That Remain

Erosion has gutted the original shield volcano. What remains is the plumbing: volcanic plugs, dykes, and domes that cooled inside the cone and now stand as spires. The Breadknife, a 90-metre-high trachyte dyke only a few metres wide, cuts across the landscape like a blade. Belougery Spire, a volcanic plug of coarse-grained trachyte, rises above the surrounding bushland.

The Warrumbungles contain the only known occurrence in Australia of analeime occurring as a primary mineral in volcanic bombs. These bombs—blobs of magma ejected during Strombolian eruptions—crystallised mid-flight, trapping the alkaline chemistry of the source magma in glassy, zeolite-rich nodules. They are small time capsules of a mantle that no longer exists.

The lava that built the Breadknife travelled 60 kilometres from its source through a crustal fracture, cooling so slowly that the crystals grew large enough to see with the naked eye.

A Landscape That Breathed Fire

The volcanic field produced at least four distinct eruption phases over two million years. Early eruptions built a broad shield of basalt and hawaiite. Later eruptions, more explosive, deposited pyroclastic flows and built the trachyte domes that now form the highest peaks. The final phase produced cinder cones and lava flows that spilled down the flanks.

At Siding Spring Mountain, a trachyte plug hosts the Siding Spring Observatory—not by coincidence. The same alkaline magma that cooled to form the mountain also produced a landscape free of light pollution and stable enough for a major astronomical facility. The mountain itself is a volcanic neck: the frozen core of an ancient vent.

What the Bombs Remember

The chemistry of Warrumbungle lavas tells a story of deep time. The high concentrations of incompatible elements—niobium, zirconium, rare earth elements—indicate that the mantle source had been enriched long before the plume arrived. The most likely explanation is that fluids from a Neoproterozoic subduction zone, active when eastern Australia was part of the Rodinia supercontinent, metasomatised the mantle wedge. When the Cosgrove hotspot passed through 500 million years later, that enriched mantle melted preferentially.

The result is a volcanic field that looks like a standard continental basalt province but tastes different in the geochemical data. The Warrumbungles are not just a landscape of spires and bombs. They are a record of two separate tectonic events separated by half a billion years, brought together by the slow drift of a continent over a stationary plume. The magma that built the Breadknife carried the memory of an ocean that closed before animals appeared on Earth.