AUSTRALIA: Indian Ocean seamounts mystery solved

Scientists have unexpectedly found traces of the supercontinent Gondwana in the Indian Ocean - in the process solving a mystery behind a large group of ocean 'mountains' known as seamounts that include Christmas Island.

The German-Australian team of marine geologists set out on the German research vessel Sonne to map and sample about 60 seamounts - ranging in height from one to three kilometres - in one the world's largest volcanic seamount provinces off the north-west Australian coast.

"These particular seamounts lie in a 200 kilometre-thick band almost parallel to the equator," said Dietmar Müller, a professor of geophysics from the University of Sydney and senior member of the team.

"In contrast to other seamount zones, such as Hawaii or the Canary Islands, their existence was something of a mystery because there was no known hotspot nearby. The seamounts' ages also didn't show a progression in any particular direction - something we'd normally expect to see in volcanoes formed over hotspots."

When the researchers dated and analysed rocks from the seamounts they were surprised to find their composition must have had some influence from continental rocks. "Finding residue continental rock all the way out into the Indian Ocean was completely unexpected," Müller said.

"It turns out that when continents break-up they don't just split nicely down the middle and merrily go their separate ways," explained Ana Gibbons, a PhD student at Sydney and one of the co-authors of a Nature Geoscience paper on the subject.

"When Gondwana was splitting apart millions of years ago, small fragments of deep, gooey continental rocks managed to get separated, lost, buried and then drawn out underneath the Indian Ocean while India and Australia drifted apart.

"Since these continental leftovers were incubated in the depths of the earth's crust for about 200 million years, they were still quite warm and buoyant. They gradually floated up when their thick continental blankets were replaced with the Indian Ocean's younger and thinner crust."

These bits of continental rock, with their varied chemical and structural make-up, were then free to mix with the young oceanic crust, Gibbons said. "It was like a big seafloor cocktail party which left behind the party-hats - or seamounts - which we can see today."

Gibbons calls this exotic alliance of oceanic and continental ingredients a 'Lava Flow Cocktail' - an Indian Ocean version of the popular Hawaiian cocktail famous for its contrasting colours, flavours and temperatures.

* The findings were published in Nature Geoscience.