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Study plugs holes in island creation theory

Island chains A study of hotspot trails in the South Atlantic Ocean supports the theory that volcanic island chains are formed by plumes of magma breaking through the Earth's crust.

Dr Oliver Nebel of the Australian National University and colleagues report their findings today in Nature Geoscience.

Most scientists think that island chains like Hawaii were formed by plumes of hot magma, originating from thousands of kilometres below the surface near the Earth's core.

It is believed that as a tectonic plate moves across the Earth's surface, this stationary plume punches holes through the plate, forming a chain-like pattern of islands.

But it's not possible to observe plumes directly and the "plume" hypothesis is challenged by the fact that many island chains have "disturbed tracks," says Nebel.

"Some chains disappear at one point," he says. "That shouldn't be the case if the hypothesis is correct."

Nebel and colleague now believe they can explain the anomaly.

"The hypothesis is correct - it's just more complicated," says Nebel.

Underwater study

Nebel and colleagues looked closely at three underwater "hotspot trails" in the southeast Atlantic Ocean.

The researchers took samples of rocks and dated them using argon isotopes.

They then related these ages to the movement of the African Plate, the date the hotspot erupted and the plate thickness in the different locations.

Nebel and colleagues also carried out a detailed geochemical analysis to show that the rocks had come from a source deep in the Earth.

They found that the pattern of the islands in each chain could be explained by the plume hypothesis, as long as the thickness of the plate was taken into account.

"We showed that the thickness of crust has a major effect on whether these mantle plumes can burst through," says Nebel.

He likens the situation to putting a candle beneath moving cardboard. If the cardboard is moving slowly enough, the flame will burn through but only if the cardboard is not too thick.

"If you put a couple of pieces of cardboard on top of each other, the time may not be sufficient to burn through," says Nebel.

"Because the Earth's crust is very diverse there are parts where you can cut through and others not."

Nebel says the same findings in the three different island chains provide serious support for the plume hypothesis.

"That cannot be a coincidence," he says. "We are pretty sure there is a plume underneath."

The great plume debate

What is sometimes called "the great plume debate" has been going on for decades, ever since the plume hypothesis was introduced in the 1980s.

Some scientists think the patterns we see in island chains can be better explained by shallow cracks in tectonic plates that allow magma to break through, and there are other theories too.

While Nebel hopes his research will settle the debate as to whether mantle plumes exist, some scientists are not so sure.

Associate Professor Weronika Gorczyk, of the Centre for Exploration Targeting is one scientist who is not completely convinced plumes are the only explanation for island chains.

"Other geological process can produce the pattern of islands we see. I'm open to other alternatives," says Gorczyk, who introduced a debate at her Centre earlier this year titled "To plume or not to plume".

She says given the controversial nature of the plume hypothesis, Nebel and colleagues could have done more tests to confirm the source of the rock was deep in the Earth.

"By combining many techniques you can get a better picture."