Geoscientists Claudia Adam of the University of Évora in Portugal and Valérie Vidal of the University of Lyon in France, argue that the answer can be found in the behavior of the mantle. They reviewed measurements and topography of the seafloor at nearly 800 locations across the entire Pacific. That’s the largest stretch of ocean bottom on the planet, and the direction and rate of motion of the twin Pacific plates has remained reasonably constant for about 50 million years. The researchers compared those measurements with depths predicted by models and then analyzed the results using a new hypothesis about the flow of heat within the mantle.In tomorrow’s issue of Science, the duo reports that the discrepancies between the real depths of the sea bottom and the depths predicted by standard models can be accounted for by spreading out the heat from the mantle farther away from the mid-ocean ridges. That extra thermal energy gives the ocean plates more buoyancy as they move farther away from the ridges. Adam and Vidal found that if they applied this hypothesis to their models, the results closely coincided with their observations—enough to account for the shallower sea bottom.It’s an “intriguing” conclusion, says geoscientist Kevin Furlong of Pennsylvania State University, University Park. But the problem, as he sees it, is that the researchers “as yet have not provided the physical mechanism” to explain how the convection of heat proceeds through the mantle.

On the other hand, geophysicist Dave Stegman of the Scripps Institution of Oceanography in San Diego, California, calls the results of the paper “exciting and profound.” If confirmed, he says, the findings mean that we now understand “a very fundamental aspect of tectonics and the evolution of the Earth.” The findings might even explain the puzzling curve in the migration of the Hawaiian Islands chain over millions of years, he says, because their motion might have been governed by mantle processes instead of tectonics.