The 250-Million-Year-Old Caldera That Became a Lake of Glass

The Lava Lake That Crystallised Itself

From the air, south-west Tasmania looks like a wound. A dark, scalloped gash 1.2 kilometres across, its walls a chaos of black rock, its floor a flat, pale green plain of button-grass and tea-tree. It is called Lake St Clair, and it is not a lake at all. It is the exposed heart of a 250-million-year-old volcano that drained its own magma chamber and then collapsed into the void.

The Caldera That Filled with Water

The depression is a caldera, formed when the Permian-Triassic volcanic centre of what geologists call the Parmeener Supergroup erupted so violently that the roof of the magma chamber fell in. The eruption was not a single event but a series of pulses, each one emptying more of the chamber and deepening the collapse. When the volcano finally quietened, the basin filled with water—the deepest natural freshwater lake in Australia, 167 metres at its deepest point.

What survives today is the plumbing, not the volcano. The cone that once rose above this spot—perhaps several thousand metres high—was eroded away long ago, during the 180 million years since the last eruptions. What remains is the subterranean structure: a ring of dolerite, the frozen magma that never made it to the surface.

The Columns That Record the Cooling

Tasmanian dolerite has a habit of forming columns as it cools, and the walls of Lake St Clair are a textbook example. The hexagonal and pentagonal pillars, some over 100 metres tall, form the towering cliffs known as the Labyrinth and the Acropolis to bushwalkers. The columns are not perfectly straight; they curve and buckle, recording the stresses of the cooling magma as it contracted and cracked.

The columns lean in toward the lake like the ribs of a half-buried ship, each one a record of the precise rate at which the magma lost heat 250 million years ago.

This is the same dolerite that caps Tasmania's highlands and forms the famous sea cliffs of the Tasman Peninsula. But at Lake St Clair, the magma never reached the surface—it stalled in the crust and cooled slowly, allowing the crystals to grow large enough to see with the naked eye. The rock is coarse-grained, a gabbro rather than a basalt, and it contains pyroxene and plagioclase feldspar that give the cliffs their dark, blue-grey colour.

The Landscape That the Ice Refused to Erase

During the Pleistocene, glaciers flowed through the Tasmanian highlands, grinding valleys into U-shapes and scouring the landscape. But the dolerite of Lake St Clair resisted. The walls of the caldera are too steep and the rock too hard for the ice to widen the basin significantly. The glacier that occupied the lake during the last glacial maximum, around 20,000 years ago, merely deepened the existing depression and left behind moraines at its southern end.

Today, the lake is fed by the Cuvier River and drained by the Derwent, one of Tasmania's major river systems. The water is oligotrophic—almost devoid of nutrients—and so clear that the lake's colour shifts from deep blue to emerald depending on the light and the angle of the sun. It is a landscape of inversion: a volcano that became a hole, a lake that sits inside a mountain's skeleton, a scar that has healed into the most peaceful place in Tasmania.