Massive volcanoes, such as those in the Pacific Ocean’s “Ring of Fire”, usually occur near the edges of tectonic plates as one plate slips beneath another.
But Australia is bang in the middle of a tectonic plate.
“Australia’s volcanoes are not really tied to any plate boundaries … and most of them don’t form part of a larger island chain,” said Ben Mather, a geologist at the University of Sydney.
Chains of smaller volcanoes also can pop up away from the edges of tectonic plates if the plate slides over a hotspot.
And in fact, Australia is home to three ancient volcano chains, created as the continent moved north-east over the top of the Pacific plate after splitting from Antarctica.
The Cosgrove Track, which stretches more than 2,000 kilometres from Cape Hillsborough in Queensland to Cosgrove in Victoria, is the world’s longest chain of ancient volcanoes.
There are two other chains off the east coast in the Tasman and Coral Seas.
But most volcanoes in Australia were not created this way, Dr Mather said.
“You would normally expect that volcanoes [created by hotspots] would be quite old in the north of the continent and get younger towards the south,” he said.
Yet most volcanoes across eastern Australia and Zealandia — a piece of continental crust that includes New Zealand and is mostly submerged underneath the Tasman and Coral Seas — are of random ages.
“The volcanoes we’ve studied are much smaller eruptions, and they are far more frequent.”
Previously, scientists have proposed that some volcanic regions such as those around Mount Gambier in South Australia were formed by magma eddies left in the wake of the edge of the continent — much in the same way as a boat leaves an eddy of water behind it as it glides over a lake — or by multiple plumes of molten rock cracking through the crust.
But these mechanisms also fail to explain how all Australia’s volcanoes formed.
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