Australia's youngest volcano could be sitting off the coast of a South Australian fishing town — if a decades-old geological hunch survives contact with a hammer. As ABC South East SA reports, Mount Gambier hydrogeologist Ian Lewis says geologists have long wondered whether a volcano lies off Beachport, on the Limestone Coast, ever since a 1960s oil and gas survey revealed three irregular reefs running at right angles to the coast.
"One way to interpret them is: are they not limestone reefs at all? Is there some other rock?" Mr Lewis said. "Geologists putting two and two together have said: 'Is it possible that the largest earthquake in South Australia [in 1897] may have been a sub-sea eruption, and the three reefs are lava flows?' Those of us who are geologists have been aware of this for a long time, but it's really been in the realm of dreamland." The 1897 Beachport earthquake — 10 May 1897 — is recorded as South Australia's largest, damaging Kingston, Robe and Beachport and rattling Adelaide.
Now Mr Lewis hopes to drag the question out of dreamland: he wants to lead a dive at the site, about 20 kilometres offshore and at depths of up to 45 metres, to collect rock samples for testing. "The idea of a geologist under the water in the middle of the ocean going tap, tap, tap on a rock is pretty bizarre," he said. "But I'm one of the few diving geologists, so it would be a real privilege to do it."
The setting makes the theory less exotic than it sounds. The Limestone Coast already hosts some of Australia's youngest volcanoes — Mount Gambier and Mount Schank erupted only about 5,000 years ago — and Mr Lewis says the volcanic field of south-west Victoria and south-east SA generally gets younger the further west you go, part of a state with a shakier history than its reputation suggests. "Roughly every 10,000 years or so it appears an eruption has occurred," he said. "If they're erupting progressively westward, Beachport would be in line as another spot." There is even a tantalising clue from local fishers: anecdotal reports of little streams of "pin-prick bubbles" rising near the reefs. "Even if we got there and trapped the gas in a container and tested, we might find out if it's volcanic gases, even before we do any diving," Mr Lewis said.
If the theory linking an underwater eruption to the 1897 quake were ever proven, it would stand as the most recent volcanic eruption on Australian soil, excluding overseas territories. Diver Josh Richards — who discovered cave extensions on the Limestone Coast and, the ABC notes, took part in efforts to rescue people trapped in a Laos mine earlier this year — is keen to join. "If [the rocks] are [volcanic], then that's pretty conclusive proof that there's a volcano there," he said, drawing parallels with the limestone-cave systems he is preparing to explore on Christmas Island.
For now, the volcano remains a question mark 45 metres down. Any dive off Beachport, Mr Lewis says, will need significant planning and ideal sea conditions — but after decades of wondering, a geologist with a hammer may finally get to knock on the answer.