The Lava That Tore a Continent Apart: Tasmania's Jurassic Dolerite

On the slopes of Mount Wellington, above the city of Hobart, the mountain is not made of mountain. It is made of columns. Hundreds of five-sided basalt pillars stand vertically, like a petrified pipe organ left by some forgotten deity. They rise in perfect hexagonal symmetry, each face pressed tight against its neighbour, a geometry so precise it looks artificial. It is not.

The Organ Pipes are one of the most striking exposures of the Jurassic dolerite that covers nearly a quarter of Tasmania. This is not a local curiosity. The dolerite forms a province that once stretched across the entire southern supercontinent of Gondwana, from Antarctica to South Africa. In Tasmania, it still covers roughly 40,000 square kilometres, in places more than 600 metres thick.

The Melt That Came From Below

About 183 million years ago, in the early Jurassic, something stirred beneath the crust of Gondwana. A mantle plume—a column of superheated rock rising from deep within the Earth—began to melt the base of the continent. The magma was basaltic, rich in iron and magnesium, and it did not erupt. Instead, it forced its way into horizontal cracks between layers of sedimentary rock, spreading laterally for hundreds of kilometres.

These intrusions are called sills. They are like sheets of molten rock injected between the pages of a book, pushing the older layers apart without breaking the surface. In Tasmania, the Karoo-Ferrar large igneous province—of which the dolerite is a part—represents one of the largest such events in Earth history. The volume of magma emplaced in Tasmania alone exceeds 50,000 cubic kilometres.

As the magma cooled slowly, buried beneath kilometres of overlying rock, it contracted. The shrinkage was not random. The cooling front advanced inward from the edges, and the tensile stress organised itself into a pattern of evenly spaced joints. The result was columnar jointing: five- and six-sided pillars, each one metre to two metres across, that run perpendicular to the cooling surface.

The Organ Pipes exist because the magma was patient. If it had erupted, it would have become featureless basalt. Instead, it cooled underground, and in cooling, it organised itself into architecture.

The Rift That Became an Ocean

The dolerite is not merely a geological curiosity. It is a record of the moment Gondwana began to die.

The same mantle plume that fed the sills also weakened the continent. The heat softened the crust, and the magma injected along pre-existing weaknesses, creating a network of fractures. These fractures grew into rifts. The rifts widened into seas. Within 50 million years of the dolerite's emplacement, the separation was complete: Australia, Antarctica, India, and Africa had begun to drift apart.

Tasmania sits at the junction of two of these rifts. To the south, the Southern Ocean opened between Australia and Antarctica. To the east, the Tasman Sea opened between Australia and Zealandia. The dolerite marks the hinge point, the place where the continent tore.

Exposures like the Organ Pipes on Mount Wellington, the columns at Cape Raoul on the Tasman Peninsula, and the cliffs of the Huon Valley are all windows into this event. They show the plumbing of a breakup frozen in stone.

The Landscape the Lava Left Behind

The dolerite did not stop shaping Tasmania after it cooled. It has been doing so for the last 180 million years.

Because the dolerite is harder than the sedimentary rocks it intruded, it resists erosion. When the softer surrounding rock weathered away, the dolerite sills remained as caprock, forming the flat-topped mountains called plateaus that dominate Tasmania's central highlands. Mount Wellington, Mount Field, and the Central Plateau are all dolerite-capped. The Organ Pipes themselves were only exposed when the overlying sedimentary rock was stripped away by glacial and fluvial erosion during the Pleistocene.

The columns are not permanent. Frost wedging prises them apart. Water seeps into the joints, freezes, expands, and topples the pillars one by one. The scree slopes below the Organ Pipes are littered with fallen columns, a slow-motion demolition that began when the ice ages arrived.

Tasmania's dolerite is a reminder that the most enduring landscapes are also the most temporary. The same rock that records the breakup of a supercontinent is now being dismantled by rain and frost, grain by grain. In another 100 million years, the Organ Pipes will be gone. But the story they tell—of a plume, a rift, and a continent torn apart—will remain written in the rocks of the places that were left behind.