The blue waters of the Pacific hide a profusion of submerged mountains, the residue of more than 10,000 undersea volcanoes. Geophysicists have long thought that these are the products of a dozen or so "hot spots," fixed sites of deep-seated volcanism that left a trail of islands and seamounts as the oceanic plate moved over them. The locations of some of these hot spots--some still smoldering, others burned out long ago--have been notoriously difficult to pin down, however. But now, two marine geophysicists have come up with an elegant way to track them down, and their results are shaking up some assumptions about the Pacific's volcanic history.
Until now, geophysicists have tried to locate hot spots by a technique that requires knowing the ages of seamounts, and that information can be hard to come by. The new method, described by Paul Wessel and Loren Kroenke of the University of Hawaii in the current issue of Nature, relies on simple geometry.
Wessel and Kroenke used a computer to trace the path of a single seamount over millions of years, as the plate motions dragged it away from the hot spot first in one direction and then another. When they repeated this for several seamounts, they noticed that these "sea-floor flow lines" formed two trends, caused by the shift in the direction of the plate. The trends crossed to form an X marking the one position every seamount had passed across: the site of the hot spot. So Wessel could search for a hot spot behind any seamount whose position was mapped, not just those with known ages. And a recent burst of declassification of military-satellite data filled in the picture with information on 8800 Pacific seamounts. "It's a really elegant method, very clever," says marine geophysicist Seth Stein of Northwestern University.
When they applied their new technique, Wessel and Kroenke found X's marking hot spots near the traditional sites of three known Pacific hot spots--Hawaii, Rarotonga, and Louisville--one of which they pinned down even better than the old technique. In this preliminary analysis, however, no obvious X's appeared to mark the rest of the dozen or so other supposed Pacific hot spots. Wessel and Kroenke say that problems such as inaccurate assumed plate motions could explain the fuzzy patches. On the other hand, the problem may not be in the data but in traditional hot-spot theory, which holds that very deep, fixed plumes of hot rock feed hot spots. Shallow, rootless hot spots that drift about could create the fuzzy pattern, say Wessel and Kroenke.
The hot-spotting results are "pointing up the real problems we have with plume theory," says geophysicist Marcia McNutt of the Massachusetts Institute of Technology. Still, the answers aren't clear yet. Even Wessel agrees that "things aren't suddenly easy now that we have this technique."