The 165-Million-Year-Old Scar That Split a Continent: Western Australia's Wallowa Craton Margin

Along the southern coast of Western Australia, from Esperance to the South Australian border, the land tells a story of separation. Not the slow kind, but a violent tearing apart that happened 165 million years ago, when Antarctica decided it had spent long enough attached to Australia and began to drift south.

The evidence is written in the rocks of the Wallowa craton margin, a 1,000-kilometre-long zone where the Earth's crust was stretched, thinned, and eventually broken. It is one of the most complete records of a continental rift anywhere on the planet.

The Moment the Cracking Began

In the Jurassic period, Australia and Antarctica were joined as part of the supercontinent Gondwana. The boundary between them ran roughly along what is now the southern coast of Western Australia, from the Bight Basin in the east to the Perth Basin in the west.

Around 165 million years ago, the crust beneath this region began to stretch. As it thinned, the underlying mantle rose, decompressed, and began to melt. The molten rock forced its way upward through fractures, erupting as flood basalts that spread across the landscape.

Today, these basalts are exposed along the coast as the Bunbury Basalt Group, a series of lava flows that erupted between 165 and 130 million years ago. They are the physical record of the moment a continent began to tear.

The Lava That Marked the Split

The Bunbury Basalts are not just any basalts. They are chemically distinct from the younger volcanic rocks that dot the Australian landscape, bearing the signature of a mantle plume that assisted the rifting process. The basalts contain high concentrations of titanium and other incompatible elements, indicating they came from a deep mantle source.

The lava that poured out along this ancient rift zone still holds the chemical fingerprint of the mantle plume that helped push Australia and Antarctica apart.

At Cape Naturaliste, near the southwestern tip of Australia, the basalt forms a series of cliffs that drop directly into the Southern Ocean. The rock is dark, fine-grained, and peppered with small gas cavities—vesicles—that formed as the lava degassed during eruption. Each bubble is a tiny record of the pressure and temperature conditions at the moment the lava reached the surface.

The Scar That Remains

As the rifting continued, the crust became so thin that it eventually broke completely, and a new ocean basin began to form between Australia and Antarctica. The separation was not clean; fragments of continental crust were left behind as submarine plateaus, and the rift margins were tilted and faulted.

Along the Wallowa craton margin, the effects of this stretching are still visible. The land slopes gently southward toward the coast, a reflection of the crustal thinning that occurred during rifting. Inland, the terrain is flat and ancient, part of the Yilgarn Craton, which has remained stable for over 2.5 billion years. But at the coast, the story changes: the rocks are younger, fractured, and tilted, marking the edge of the rift zone.

The Bunbury Basalts themselves are now found in scattered outcrops along a 200-kilometre stretch of coast, from Cape Naturaliste to Bunbury. They are the only surface expression of a rift that ultimately created an ocean basin over 4,000 kilometres wide.

A Quiet Record of Violence

What makes the Wallowa craton margin remarkable is not its drama—the cliffs are modest, the basalts weathered—but its completeness. The rift sequence is exposed in a way that allows geologists to trace the entire process, from initial stretching to final breakup.

At Point D'Entrecasteaux, a headland south of Walpole, the contact between the ancient Yilgarn Craton and the younger rift basalts is exposed in the sea cliffs. The boundary is sharp: on one side, gneisses that have remained unchanged for billions of years; on the other, basalt that erupted when dinosaurs still walked the Earth.

The rocks here record a moment of planetary rearrangement: the breakup of Gondwana, the birth of the Southern Ocean, and the beginning of Australia's long journey northward toward its present position. It is a quiet monument to violence, preserved in the cliffs of a coastline that most travellers pass without a second glance.