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Rare rocks provide clues on Indian Ocean formation

The first rocks ever collected from a remote part of the seafloor are helping IMAS researchers to better understand the formation of the Indian Ocean.

The unique samples are from an area 1000 kilometres west of Perth which is believed to have been the plate boundary between India and Australia when they separated around 136 million years ago.

The rocks were dredged from a depth of around 3000 metres from Dirck Hartog Ridge, a 600 kilometre long feature which rises 2000m from the seafloor of the Perth Abyssal Plain, by the research vessel Southern Surveyor in 2011.

Along with collaborators from the University of Tasmania and three New South Wales universities, IMAS PhD candidate Sally Watson (pictured, below) has for the first time analysed the rocks and documented their significance.

"If this ridge was on land it would be mammoth, it would be very significant, but because it's under the ocean no one can see it and little is known about it," she said.Sally Watson

"The fact that we don't often get an opportunity to dredge up rocks from 3,000 metres water depth makes these samples pretty exciting.

"It's hard to get out there and it's often bad weather in the southern Indian Ocean, but these rocks are really important for understanding how the Australian and Indian landmasses separated over millions of years."

Ms Watson said new bathymetric profiles and analysis of the rocks showed the seafloor off Western Australia was likely subjected to extensive upwelling from the mantle (the inner part of the Earth) as well as major tectonic activity.

When the two landmasses first separated the seafloor between India and Australia formed at a rate of around 70mm each year but the rate slowed and then ceased around 102 million years ago due to a complex interaction of volcanism and tectonic plate reorganisation.

"The composition of the rocks, the sort of trace elements that are in them, can tell us what sort of conditions existed when they were formed, and that can give us an indication of where they were formed and how they were formed.

"It also enables us to understand the fundamental solid Earth processes that have been forming our planet for billions of years and will continue to do so."

Ms Watson said piecing together Earth's history from such rare and ancient rock material was an exciting challenge.

"I really like that no one's looked at it before because there haven't been any samples from this part of the seafloor.

"Essentially I feel a little bit like a detective. 

"As people get more information they piece together parts of the story over time - it's just like solving a massive jigsaw puzzle," she said.

The findings have been published in the journal Gondwana Research and can be found here.

Authorised by the Executive Director, Institute for Marine and Antarctic Studies
28 October, 2022