Extinct volcanoes have been discovered by accident in deep water off the coast of Sydney

The volcano cluster off Sydney. Image: Marine National Facility.

Extinct volcanoes have been found in 4,900 metres of water about 250 kilometres off the coast of Sydney by Australia’s new ocean-going research vessel Investigator.

They were discovered during a search for nursery grounds for larval lobsters. At the same time the ship was routinely mapping the seafloor.

“The voyage was enormously successful, not only did we discover a cluster of volcanoes on Sydney’s doorstep, we were amazed to find that an eddy off Sydney was a hotspot for lobster larvae at a time of the year when we were not expecting them,” says marine biologist Iain Suthers, the chief scientist for the voyage.

The four extinct volcanoes in the cluster are calderas, which form after a volcano erupts and the land around them collapses, forming a crater.

The largest is 1.5 km across the rim and it rises 700 metres from the sea floor. The cluster, 20 kilometres long and six kilometres wide, is likely to be 50 million years old.

These type of volcanoes tell part of the story of how New Zealand and Australia separated around 40 to 80 million years ago.

They haven’t been found before because the sonar on the previous Marine National Facility research vessel, Southern Surveyor, could only map the seafloor to 3,000 metres, which left half of Australia’s ocean territory out of reach.

Investigator, a $120 million vessel launched in December last year, has sonar which can map the seafloor to any depth.

The latest voyage, which started in Brisbane on June 3 and ended in Sydney on June 18, involved 28 scientists from The University of NSW, Latrobe University, the University of British Columbia, the University of Sydney, the University of Auckland, the University of Technology Sydney and Southern Cross University.

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