The 4.5-Million-Year-Old Volcanoes That Never Saw the Sky

When a volcano erupts under ice, the result is not a mountain but a flat-topped mesa. Australia holds some of the oldest examples on Earth — volcanic mountains that were born beneath glaciers, their shapes frozen in time.

The Shape of Ice-Capped Fire

The flat-topped mountains of western Victoria and eastern New South Wales are not ordinary volcanoes. They are tuyas — a rare landform that forms when lava erupts beneath a glacier. The ice acts as a lid, trapping the heat and forcing the lava to pile up vertically rather than spread out. The result is a steep-sided, flat-topped mound that looks like a table rising from the landscape.

These Australian tuyas are between 1.8 and 4.5 million years old, from the Pliocene epoch. At that time, the Kosciuszko region and the Western Volcanic Plains were covered by ice caps. When the volcanoes erupted, they melted vertical shafts through the ice, building columns of basalt that hardened into rock as the glaciers later retreated.

Where the Oldest Tuyas Stand

The best examples lie in the Monaro Volcanic Province of southern New South Wales. Mount Taylor, near the Queanbeyan River, is a classic tuya — a flat cap of basalt resting on a pedestal of older sedimentary rock. Its summit is level because the lava ponded against the ice ceiling and cooled into a horizontal sheet.

A tuya is a volcano that never saw the sky — it grew into ice, not air.

Further south, in the Western Volcanic Plains of Victoria, the tuya at Mount Eccles is younger but built by the same process. Here, the lava left behind a distinctive pattern of columnar jointing — hexagonal pillars that formed as the basalt cooled slowly under the insulating ice. The columns stand like the pipes of an organ, but one that was built in darkness, beneath a glacier that has long since vanished.

A Climate Archive in Basalt

Tuyas are valuable to geologists because they record two things at once: the fact of a volcanic eruption and the presence of ice at that exact moment. By dating the basalt in an Australian tuya, researchers can determine when ice covered the highlands of southeastern Australia. The dates cluster around 2 to 3 million years ago, during the late Pliocene, when global temperatures were slightly warmer than today but ice still clung to the highest peaks.

The geometry of a tuya also tells us the minimum thickness of the glacier that confined it. The height of the flat cap above the surrounding landscape equals the thickness of the ice at the time of eruption. For Mount Taylor, that means the glacier was at least 200 metres thick — a sobering image for a landscape that today sees snow only rarely.

The Continent's Buried Glacial Past

Australia is not known for glaciers today, but the tuya fields prove that ice once shaped the southeastern highlands. The same volcanic province that produced the Newer Volcanics — those 5,000-year-old scoria cones and lava flows — also produced these older, ice-confined volcanoes. The contrast is striking: the same magma chamber, erupting first beneath ice, then beneath open air, as the climate warmed and the glaciers melted.

Other tuyas exist in Antarctica and Iceland, but the Australian examples are among the oldest known. They preserve a rare record of Pliocene glaciation on a continent that is now mostly ice-free. The flat tops of these mountains are not eroded plateaus; they are the original cooling surfaces of lava that never broke through its frozen roof.