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Pole flips tied to plate tectonics

Lopsided arrangement of continents could lead to reversals in Earth's magnetic field

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1:38pm, September 23, 2011
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Continents moving around over millions of years in the slow-motion geologic jigsaw puzzle known as plate tectonics could trigger the occasional swapping of the north and south magnetic poles.

Assumed to be caused by random fluctuations in the circulation of the molten iron core, the flips may actually be tied to what’s going on at Earth’s surface. At times in the geologic past when landmasses have bunched together on one side of the equator, the Earth’s magnetic field has begun flipping soon thereafter, suggests a study to appear in Geophysical Research Letters.

“What we see clearly is that the surface positions of the continents are linked with the frequency of the reversals,” says group member François Pétrélis, a geophysicist at the French research agency CNRS in Paris.

Other scientists aren’t so sure, cautioning that more work needs to be done to confirm any such link.

Computer simulations have shown how molten iron in the spinning core can generate magnetic fields that spontaneously flip direction every so often, so that a compass that once pointed north would suddenly begin pointing south. The last reversal happened around 780,000 years ago. Before that there were times when no flips happened for tens of millions of years, followed by periods with lots of flips.

Curious about what drove these changes, Pétrélis and his colleagues mathematically probed the way liquid iron flows in the core. In earlier work, they found that if the flow is interrupted so that it no longer moves symmetrically with respect to the equator, the planet’s magnetic field soon reverses.

Now the researchers have calculated where the continents sat at various points in the past, to see when they had equatorial symmetry. When the distribution of landmasses was lopsided, the team found, magnetic field flips followed soon after. And when continents were more evenly arranged — such as between about 80 million and 120 million years ago, and again between 260 million and 310 million years ago — there were long stretches without geomagnetic field reversals.

But a third reversal-free period between around 460 million and 490 million years ago doesn’t fit the proposed pattern, because the continents were bunched in the southern hemisphere then, says Trond Torsvik, a geophysicist at the University of Oslo. “I am not terribly convinced with their analysis,” he says.

Pétrélis says scientists don’t know enough about continental arrangements that far back to say anything conclusive about how they might have affected the magnetic field.

What connects seemingly isolated happenings at the surface and the core remains a mystery. One possibility is that the middle layer of the Earth, the mantle, can wiggle around separately from the Earth’s overall turn, so that changes in the crust that influence the mantle can also affect flow in the core. Testing this idea is not easy, though, as “it requires a very accurate description of what is going on in the mantle,” Pétrélis says.

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