Australia used to be quite a hotbed of volcanic activity. The east coast is littered with the remnants of hundreds of eruptions, the most recent just a few thousand years old.
Many were one-off events. Some are now notable structures such as Cradle Mountain in Tasmania, the Organ Pipes in Victoria, or the Undara Lava Tubes in Queensland, while others just look like regular hills.
But volcanic they were, and geoscientists have been at a loss to explain why there was so much action over 80 million years in parts of an otherwise stable continent.
Now we may be closer to an answer. “Under our east coast we find a special volatile mix of molten rock that bubbles up to the surface through the younger, thinner east coast Australian crust,” says Ben Mather, lead author of a new paper in Science Advances.
He and colleagues from the University of Sydney (USyd) worked with Monash University and New Zealand’s GNS Science to analyse hundreds of eruptions along the coast from North Queensland to Tasmania and over the Tasman to the largely submerged continent Zealandia.
They were particularly interested in peaks of activity 20 million and two million years ago.
“Most of these eruptions are not caused by Australia’s tectonic plate moving over hot plumes in the mantle under the Earth’s crust,” says USyd’s Maria Seton. “Instead, there is a fairly consistent pattern of activity, with a few notable peaks.”
This became evident because these peaks coincided with an increased volume of seafloor material being pushed under the Australian continental shelf, starting at the Tonga-Kermadec Trench east and north of New Zealand.
“From there it is being slammed into the transition zone between the crust and the magma at depths of about 400 to 500 kilometres,” says Mather. “This material is then re-emerging as a series of volcanic eruptions along Australia’s east coast, which is thinner and younger than the centre and west of the continent.”
This process, known as subduction, is not unique to this area, but what sets the east Australia-Zealandia region apart, Mather says, is that the seafloor being moved is highly concentrated with hydrous materials and carbon-rich rocks. “This creates a transition zone right under the east coast of Australia that is enriched with volatile materials.”
Previous models have suggested volcanoes in Victoria were due to convection eddies in the mantle from being near the trailing edge of the tectonic plate or have relied on the plate passing over hot spots in the mantle.
“Neither of these gave us the full picture,” Mather says. “But our new approach can explain the volcanic pattern up and down the Australian east coast.”
It also could also explain intraplate volcanic regions in the Western US, eastern China and around Bermuda, he adds. It can’t, however, predict when the next volcano will emerge.
Nick Carne is the editor of Cosmos Online and editorial manager for The Royal Institution of Australia.
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