The Volcano That Erupted Under Ice: New South Wales' Barrington Tops Lava Tubes

When a volcano erupts under a glacier, the result is a landscape with no Earthly parallel. In New South Wales, a chain of such eruptions 18 million years ago carved a labyrinth of lava tubes, caves, and gorges that still hold the shape of the moment the ice melted.

The Deep Plumbing

The Barrington Volcano, now extinct, sat atop a plateau 1,500 metres high in what is today Barrington Tops National Park. For perhaps a million years, it erupted not into open air but beneath a thick ice cap. The heat melted the ice from below; the meltwater flash-cooled the lava into columns, pipes, and tunnels. Geologists call the result a subglacial volcanic complex, and it preserves the internal anatomy of a volcano as few places on Earth do.

Today, the Barrington Tops shield volcano is gone—eroded to its roots. What remains are the frozen conduits: vertical pipes of basalt that fed the eruptions, now standing as cliffs. The main lava tube system runs for more than 30 kilometres, making it one of the longest known in the Southern Hemisphere. Its walls are lined with basalt columns so regular they look cut by a mason.

The Glass That Preserved a River

One of the most striking features is the "rhyolite flow" at Gloucester Tops. Rhyolite is the volcanic equivalent of granite—thick, viscous, slow-moving. When it erupted under ice, it cooled so rapidly that it formed a glassy rock called obsidian. Blocks of black obsidian, some the size of cars, litter the valley floor. They are unweathered, sharp-edged, as if broken yesterday.

The obsidian at Gloucester Tops is among the purest natural glass on the continent—so dark it absorbs 99 percent of visible light.

Beneath the glass lies a fossil river channel. The lava, chilled by the overlying ice, flowed along a pre-existing valley, preserving its course. Today, that ancient riverbed sits 200 metres above the modern drainage. The landscape has been inverted: what was once a low point is now a ridge.

A Climate in the Rock

The Barrington subglacial eruptions belong to a broader episode of volcanic activity that swept eastern Australia between 30 and 10 million years ago, as the continent drifted north over a hot spot in the mantle. But the ice that capped the volcano tells a different story. At 18 million years ago, Australia was not yet the arid continent it is today. The ice cap on the Barrington plateau implies that the highlands of New South Wales were cold enough to support permanent glaciers—a rare condition in a continent that otherwise escaped the Pleistocene glaciations.

Evidence of this ice cap survives in the form of tillite: a sedimentary rock made of glacial debris, found sandwiched between basalt flows. The tillite contains striated pebbles, scratched by the grinding of ice against rock. It is one of the few records of pre-Pleistocene glaciation in Australia.

The Living Cave

The lava tubes have become a refuge for species that survived the drying of the continent. The Barrington Tops caves host colonies of bent-wing bats, whose guano supports a food chain of invertebrates found nowhere else. The cool, stable microclimate of the tubes—maintained at 10 degrees Celsius year-round—mimics the conditions of the Ice Age. In a sense, the caves are a living fossil: a fragment of Pleistocene Australia preserved beneath the skin of the plateau.

Walking through the tubes, you step on floors of broken basalt. The walls are coated with a thin film of moisture, the breath of the volcano still condensing after 18 million years. The only sound is the drip of water and the distant roar of a creek that flows through the lowest passage, carving deeper into the rock with each millennium.