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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