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Another Look Beneath Hawaii Knocks Islands Off Their Riser Pipe
26 May 2011 5:20 pm
Hawaii may be a vacation destination and a surfers’ paradise, but for some earth scientists it is also an exemplar of a long-sought geological structure. The Big Island of Hawaii sits atop a vast “plume,” or upwelling, of hot rock from Earth’s deep mantle—at least, so some scientists argue. However, new seismic imaging reported in tomorrow’s issue of Science reveals no plume under Hawaii. Instead, it draws attention to a new sort of feature well west of Hawaii, one that may or may not be part of a new kind of plume system. It seems the prize for bagging the first bona fide plume will remain out of reach for a while longer.
The most recent sighting of a standard Hawaiian plume came when seismologists imaged the mantle by monitoring how quickly seismic waves from distant earthquakes passed upward beneath and around the islands (Science, 4 December 2009, p. 1330). The waves appeared to slow when passing through hotter-than-normal rock in a column extending at least 1500 kilometers beneath the islands. That could be the upper half of a hot, vertical plume of rock rising like smoke from a chimney. It might extend from near where the rocky mantle meets the molten iron of the core 2900 kilometers down. At Earth’s surface, such a plume would deliver molten rock to fuel the Hawaiian volcanic hot spot. A few dozen other hot spots around the world might be stoked by other mantle-spanning plumes.
Now, the latest look beneath Hawaii using seismic waves differently reveals a different mantle. Seismologists Qin Cao of the Massachusetts Institute of Technology (MIT) in Cambridge and colleagues use seismic waves that rise through the mantle and reflect from the underside of mantle layers, where, thanks to changing pressure, different minerals form. The depth of a given mineral transformation reflects the temperature there. Using this reflected wave technique, Cao and colleagues found elevated temperatures a few hundred kilometers beneath Hawaii, much as the earlier study found. However, deeper down they saw no sign of a rising hot column. There is no deep plume beneath Hawaii in their observations.
More than 1000 kilometers to the west, however, they find what they take to be unusually hot rock almost 700 kilometers down, where the upper mantle and lower mantle meet at a prominent boundary. Although rock can rise or sink through the upper-lower mantle boundary, passing through it isn’t easy. Cao and colleagues speculate that the thousand-kilometer-wide flattened blob of hot rock they see may be trapped at the boundary with only a bit at its eastern edge managing to leak upward and feed Hawaii. And if the hot blob is old enough to have created the 60-million-year-old chain of Hawaiian Islands, they say, then the blob would likely be replenished by hot rock rising from below.
So the new view beneath the Pacific could be revealing a sort of offset plume. “It’s very difficult to make a simple, undistorted plume that extends all the way to Hawaii,” says seismologist Rob van der Hilst of MIT, a co-author on the paper. The reflection method could be revealing the upper part of a rising mantle plume, as if a smokestack’s plume rose to hit a glass ceiling—the 700-kilometer boundary—only to flow to an open window at the ceiling’s edge and rise again in another plume.
As is common in the field of mantle imaging, the new view gets a mixed reception from others in the field. “It might be right; it’s hard to tell,” says Donald Forsyth of Brown University. He and others point to inconsistencies between the new view and other studies using much the same kind of seismic waves analyzed using different techniques. “Very careful studies using different techniques don’t agree,” says seismologist Edward Garnero of Arizona State University, Tempe. The possible offset plume “is a prediction about the Earth. Let’s see how it holds up.”