The Lava That Built a 65-Million-Year-Old Plateau of Basalt: Victoria's Great Western Volcanic Province

About 65 million years ago, not long after the dinosaurs vanished, lava began to pour across what is now western Victoria. It kept coming—episode after episode, vent after vent—for longer than most mountain ranges have existed. The result is a 15,000-square-kilometre plain of dark basalt, dotted with over 400 volcanic centres, that stretches from Melbourne almost to the South Australian border.

The Longest Eruption

Victoria's Newer Volcanics Province is one of the most recent volcanic fields on the Australian continent, but its story reaches back into the age of mammals. The earliest eruptions began around 65 million years ago, during the Palaeocene, when Australia was still connected to Antarctica. The most recent was only about 5,000 years ago—a blink in geological time.

That last eruption built what is now called Mount Gambier, a maar volcano whose crater lakes are famous for their deep blue water. But Mount Gambier is just one of hundreds. The province contains scoria cones, lava shields, explosion craters, and extensive lava flows that buried ancient river valleys and created the flat, fertile plains that now support Victoria's dairy and sheep industries.

Most of the vents are small—mere bumps on the landscape—but together they record a remarkable fact: the Australian plate has been drifting north over a stationary hotspot for tens of millions of years. The Newer Volcanics Province marks the most recent expression of that hotspot, whose track extends northward through the Cosgrove hotspot chain all the way to Queensland.

Basalt and the Shape of the Land

Basalt is not a dramatic rock. It is dark, fine-grained, and unremarkable to the untrained eye. But it shapes landscapes in distinctive ways. When basalt lava flows cool, they contract into polygonal columns—the famous columnar jointing seen at places like the Organ Pipes near Melbourne. These columns can be metres tall, forming natural pillars that look as if they were carved by hand.

The lava also created the conditions for some of Australia's most unusual caves. In the lava flows near Byaduk, underground tubes formed when the surface of a flow solidified while the interior continued to move. When the lava drained away, it left behind long, tunnel-like caves—lava tubes—that can be walked through for hundreds of metres.

The Byaduk Caves are not carved by water, but by the movement of molten rock itself—a rare case where the cave and the rock that contains it are the same age.

Weathering of the basalt over millions of years produced the deep red soils that give the region its characteristic colour. These soils are rich in iron and nutrients, which is why the volcanic plains are among Australia's most productive agricultural lands.

A Window into the Mantle

Some of the scoria cones in the province contain small fragments of a very different kind of rock: peridotite, the rock of the Earth's mantle. These fragments, called xenoliths, were ripped from the mantle as magma rose through the crust. They are typically green because they contain the mineral olivine, and they sometimes include small crystals of spinel or garnet.

For geologists, these xenoliths are a rare gift. Basaltic magma normally brings mantle material to the surface, but it usually melts or alters it along the way. The xenoliths in Victoria's volcanic cones are unusually fresh, preserving the original mineral composition of the mantle beneath southeastern Australia. They tell us that the mantle at a depth of about 50 kilometres is composed mostly of olivine and pyroxene, with traces of water and carbon dioxide.

The presence of these xenoliths also confirms that the magma rose rapidly from the mantle to the surface—within days or weeks—cooling too quickly for the xenoliths to be destroyed.

The Youngest Volcano

The youngest eruption in the province produced Mount Gambier. It was a maar volcano, formed when rising magma encountered groundwater and exploded. The explosion blasted a crater into the existing landscape, which later filled with water to form two lakes: the Blue Lake and the Valley Lake.

The Blue Lake is famous for its colour change. Each year, in late spring, it turns from a dull grey-blue to a vivid cobalt blue. The cause is still debated, but the leading theory involves the precipitation of calcium carbonate crystals that scatter sunlight. The lake is also one of Australia's most important sources of drinking water for the city of Mount Gambier.

A maar volcano is a violent end to a story that began with quiet, steady flows. But that is the nature of the Newer Volcanics Province: it has never followed a predictable pattern. The most recent eruption was 5,000 years ago, which means the province is not extinct—only dormant. At some point in the future, somewhere in western Victoria, the lava will flow again.