The Craters That Became Blue Lakes: South Australia's Mount Gambier Maars

The last volcanic eruption in Australia happened 5,600 years ago, and it created a string of craters that still hold water in a landscape that has not seen lava since.

Mount Gambier's Volcanic Field

South Australia's Mount Gambier volcanic field sits on the edge of the continent, a cluster of vents and cones that erupted during a period geologists call the Late Quaternary. The youngest of them, Mount Gambier itself, erupted around 5,600 years ago—recent enough that Aboriginal oral traditions may record it. The eruption was a phreatomagmatic explosion: rising magma hit groundwater, flashed it to steam, and blasted a series of craters through the limestone bedrock.

The craters are called maars. They are wide, shallow, and rimmed by tuff—volcanic ash compacted into rock. Today, three of them hold lakes: Blue Lake, Valley Lake, and Leg of Mutton Lake. Blue Lake is the most famous, a deep-blue body of water that sits in a crater 75 metres deep. Its colour shifts from grey to vivid blue each November, a phenomenon linked to calcium carbonate crystals that form in the warm summer water.

The Limestone Basement

What makes Mount Gambier unusual is not the magma—it is what the magma erupted through. The entire region is underlain by the Gambier Limestone, a 40-million-year-old marine deposit full of fossil shells and bryozoans. When the rising basalt hit this water-saturated limestone, the result was explosive. The same process that creates steam explosions in a kitchen created these craters.

The limestone also controls what happens next. It is porous, so rainwater drains through it quickly. But the crater floors are lined with volcanic ash and clay—impermeable layers that seal the basins. The lakes hold because the volcanoes made their own waterproof liners.

The youngest volcano on the Australian mainland produced lakes, not lava.

Blue Lake's Hidden Plumbing

Blue Lake is not a simple rain-filled crater. It has a complex hydrology that scientists have only recently mapped. The lake is 77 metres deep at its centre, but its water level stays remarkably constant year-round. Underground springs feed it from the surrounding limestone aquifer, and the same porous rock drains overflow through submerged outlets.

Each year between November and March, the lake turns a deep cobalt blue. The colour comes from calcium carbonate microparticles that form in the warm, still surface waters. These particles scatter sunlight in a way that makes the water appear blue—a natural version of the same effect that gives tropical seas their colour. In winter, the particles settle, and the lake turns grey-green again.

The Youngest Volcanoes on the Continent

Australia is not a volcanically active continent in the way Iceland or Hawaii are. The last eruption on the mainland happened here, at Mount Gambier. The most recent eruption in Australia overall was on Kangaroo Island, around 4,800 years ago—a single vent that produced a small lava flow. Both are part of the Newer Volcanics Province, a field of about 400 volcanic centres stretching from Melbourne to Mount Gambier.

This province is not driven by a hotspot or a subduction zone. It sits above a region of thinned lithosphere where the mantle is hot enough to melt without any obvious trigger. The eruptions are sporadic—separated by tens of thousands of years—and the province is considered dormant, not extinct. Another eruption is statistically unlikely in the next few thousand years, but not impossible.

A Landscape of Inversion

The Mount Gambier maars are a lesson in how volcanic landscapes age. What looks like a hole in the ground is actually a construction: the tuff ring that surrounds each crater is built from ejected debris, and the crater floor sits below the original land surface only because the explosion removed material. Over time, the softer tuff erodes faster than the surrounding limestone, and the craters become more pronounced.

At Blue Lake, the tuff ring rises 30 metres above the surrounding plain. From a distance, the lake is invisible—you see only a rim of grassy hills. Walk to the edge, and the ground drops away into a deep bowl of water. The effect is the same at each maar: a hidden lake, held in a volcanic cup, waiting inside a landscape that looks like nothing more than gentle pasture.