Pouring water on the theory of hot spots.

Pouring water on the theory of hot spots 

The term "wet spot" lacks the flair of "hot spot," but it may enter the geophysical lexicon if one oceanographer's argument proves correct.

According to the hot spot theory, many of Earth's volcanoes are fed by thin columns of unusually warm rock rising from deep within the mantle. But an analysis of seafloor rocks now suggests that volcanic activity at certain presumed hot spots arises from water, not warmth, says Enrico Bonatti of Columbia University's Lamont-Doherty Geological Observatory in Palisades, N.Y.

"This should cause at least some rethinking on the concept of hot spots, which has been so important in the last 20 years," Bonatti asserts.

He reached his controversial conclusion after examining rocks from the ocean bottom near the Azores in the North Atlantic. Geoscientists recognize these volcanic islands as the current location of a hot spot that once lay beneath Newfoundland. The hot spot's apparent southeastward migration is an illusion created by plate tectonics; the spot itself has remained stationary while Newfoundland and the Atlantic Ocean have migrated to the northwest over it.

Bonatti studied peridotite rocks collected by dredges, drill ships and deep submersibles. Peridotites originate in the mantle layer, offering scientists a window into the conditions beneath the tectonic plates. Using the chemistry of key peridotite minerals as geothermometers, Bonatti estimated the rocks' temperatures as they rose through the mantle to the surface. Chemical variations indicate how quickly the peridotite cooled.

Bonatti compared peridotites from the Azores with those from other volcanic sites in the Atlantic unassociated with hot spots. He surmised that the Azores samples would record a more rapid cooling rate because these hotter, more buoyant mantle rocks should rise faster. Instead, he found they recorded similar or slower cooling rates.

His research report in the Oct. 5 SCIENCE challenges the idea that a hot spot feeds volcanic activity in the Azores by melting mantle rock. Bonatti says his findings suggest that the mantle under the Azores is not hotter than normal, but instead contains unusually high concentrations of water and other volatile ingredients, such as carbon dioxide and chlorine. By lowering the melting temperature of rock, volatiles would encourage the uppermost mantle to melt, providing an alternative explanation for the area's volcanic activity.

Basaltic rocks previously collected from the ocean floor near the Azores support the wet spot theory, Bonatti says. These volcanic rocks hold two to three times as much water as normal basalts and also contain extra chlorine, bromine and fluorine.

Bonatti says his findings don't raise questions about most hot spots. But some areas, such as the Azores, may not fit the traditional interpretation, he says. In particular, he suggests that other so-called hot spots in the equatorial Atlantic may be wet spots.

Many geophysicists believe hot spots provide a pipeline for transporting heat from the planet's core to its surface. But Bonatti's work suggests that some hot spots may instead signify sites where volatile-rich material escapes upward.

Petrologist Henry J.B. Dick says he has also used peridotites as Atlantic geothermometers and has obtained results similar to Bonatti's. But he thinks the wet spot hypothesis doesn't hold water.

Dick, of the Woods Hole (Mass.) Oceanographic Institution, disputes Bonatti's assumption that peridotite at hot spots must rise quickly. In the Azores region, Dick says, hot mantle material might well rise slowly because volcanic activity there has created an extra-thick crust that could inhibit the upwelling rock.