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Plate tectonics
New dates defy fixed hotspots

How the Hawaiian Island-Emperor Seamount chain got its characteristic dogleg bend is a classic example of the fixed hotspot theory: The bend in the chain occurred when the Pacific plate, which had been steadily moving north over a fixed plume of magma from the mantle, suddenly changed direction 47 million years ago and started heading northwest.

Proponents of this idea have long pointed to the geometry of similar bends in other Pacific volcanic chains as evidence of a plate uniformly changing direction over fixed hotspots. All the bends should be the same age, but until now, they had not been dated. A new study, however, has found that the bends formed up to 20 million years apart, challenging the fixed hotspot theory.

Geochemists Anthony Koppers and Hubert Staudigel of the Scripps Institution of Oceanography in San Diego, Calif., investigated two Pacific seamount chains with bends similar to the Hawaii-Emperor chain — Gilbert Ridge and Tokelau Seamounts. Argon isotopic dating of basalt samples, dredged from submerged volcanoes at the bends, revealed that the Gilbert bend occurred 67 million years ago, or 20 million years before the Hawaii-Emperor bend, and the Tokelau bend occurred 57 million years ago.

Koppers and Staudigel say that the findings, published in the Feb. 11 Science, show that such “asynchronous bends cannot be reconciled with the stationary hotspot paradigm.” Instead, they suggest that the varying ages indicate hotspot motion or that local stretching of the Pacific plate caused cracks that allowed magma to well up. This process, they say, is unrelated to any large-scale directional shift but is due instead to local stresses in the plate that result from plate tectonics in general.

“The major contribution here is that they’ve been able to show that you can’t just use the simple geometry of the seamount tracks to reconstruct plate motion or to make arguments in favor of fixed hotspots,” which has been done in the past, says John Tarduno, a geophysicist at the University of Rochester. In 2003, he and colleagues published paleomagnetic data indicating that the seamounts and islands of the Hawaiian-Emperor chain did not all form at the same latitude, contrary to what would be expected with a fixed hotspot (see Geotimes, March 2003). Such evidence, indicative of hotspot movement, has weakened the popular theory in recent years.

Geophysicist Pål Wessel at the University of Hawaii, Manoa, welcomes the new results but cautions that the bends are not as unambiguous as Hawaii’s. “Perhaps the geometric classification of those kinks as ‘bends’ was simply not correct,” he says, which would make their dates irrelevant to the fixed hotspot theory.

Loren Kroenke, a colleague of Wessel’s at Hawaii, is also not convinced that the fixed hotspot theory is dead. “When the plate changes direction, it places stress on the plate,” he says. And because Koppers and Staudigel say that local stress could be the culprit, the new findings do not necessarily rule out large-scale directional changes or “affect the fixed hotspot theory very much,” he says.

Kroenke also questions the age of the samples. “For example, there are ridges coming off the Hawaiian chain,” he says, “and if you looked at some of those, you’d be off by 60 million years in the timing of the plume activity.”

Tarduno counters that the discrepancies among the bend ages are so large, up to 20 million years, that sampling error is unlikely. “The fixed hotspot model has been so appealing because it provides an elegant, simple mechanism to explain volcanic chains,” Tarduno says, “but when you look at the details, you realize that the chains must also reflect more complex processes.”

Sara Pratt
Geotimes contributing writer

Links:

"Bending thoughts about Hawaiian chain," Geotimes, March 2003
"The Hawaiian hotspot debate: an update," Geotimes Web Extra, Aug. 11, 2003

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