Of ocean weather and volcanoes

Kerr, Richard

'The job of testing hot-spot fixity has fallen largely to paleomagnetists. In principle, they can decipher where on Earth-or at least at what latitude-a volcanic rock formed: They measure the orientation of Earth's magnetic field frozen into a rock when it solidified from lava. Because Earth's field is horizontal at the equator, vertical at the magnetic pole, and tilted in proportion to its latitude in between, determination of the inclination of a rock's locked-in field can yield its latitude at the time it was formed. That means a core of rock drilled from one of the now-submerged seamounts-built one at a time in a string across the Pacific plate as it moved over the Hawaiian hot spot-should tell a paleomagnetist whether that volcano formed over the present location of the hot spot 19°N. Researchers did just that in the 1970s and 1990s at two seamounts in the northern North Pacific-Suiko and Detroit-and found that they had formed more than 1000 kilometers north of the present location of the Hawaiian hot spot. But the limited data didn't convince most researchers that the hot spot had moved. Paleomagnetist John Tarduno of the University of Rochester, New York, wanted to convince his colleagues that hot spots can move, so he arranged to do the job right. He and colleagues persuaded the Ocean Drilling Program to send its drill ship JOIDES Resolution to the Emperor chain of seamounts northwest of Hawaii for a full 2 months of drilling. Last summer, he, co-chief scientist Robert Duncan of Oregon State University in Corvallis, and the shipboard scientific party of Leg 197 drilled four seamounts, penetrating a total of 1200 meters of volcanic rock beneath the bottom sediments. That was enough to average out short-term variations in the orientation of the magnetic field, something earlier, less ambitious drilling had not done convincingly. With these higher quality data from more sites, preliminary shipboard analyses suggest that the Hawaiian hot spot moved southward at rates as high as 30 and 50 millimeters per year-faster than North America is moving between 81 million and 43 million years ago. 'It's a very nice result,' says Tarduno, who presented his findings on his poster. 'It's pretty hard to argue against hot-spot mobility now.' Tectonophysicist Seth Stein of Northwestern University in Evanston, Illinois, agrees. 'His results raise a very major question about the long-term fixity of hot spots,' he says. 'You're talking about hot-spot motion at the speed of plates. A lot of questions about mantle dynamics and plate motions will need rethinking.''

Of ocean weather and volcanoes

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