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Hot Mantle May Prop Up the Seafloor
1 April 2010 4:57 pm
The seas may be rising due to climate change, but most of the seafloor is also dropping as part of the natural dynamics of Earth's crust. The question that has dogged scientists for decades, however, is why hasn't the ocean bottom sunk faster? An exhaustive analysis of the Pacific Ocean seabed may provide at least part of the answer—though experts think important questions remain.
Earth's crust is like an ice field floating atop the ocean. As tectonic plates move away from each other, the underlying mantle wells up between them. This process builds underwater mountain ranges, such as the central ridges of the Atlantic and Pacific oceans. The mantle also heats the plates near this upwelling, making them more buoyant—and thus raising the seafloor. As the new crust cools, it sinks under its own weight—a process called subsidence—so the seafloor should be deeper the farther it is from a mid-ocean ridge.
But that's not what scientists have found. In the oldest parts of the seafloor, which are also the parts farthest away from the mid-ocean ridges, the ocean bottom tends to be considerably shallower than expected—in some cases hundreds of meters shallower. What has been holding it up?
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.
Geophysicist Seth Stein of Northwestern University in Evanston, Illinois, predicts that the new model will catch on if it can reliably explain certain aspects of seafloor dynamics, such as whether the seafloor flattens in the Atlantic and Indian oceans in the same way as it does in the Pacific. Otherwise, he says, the standard models will probably persist because "they still do a reasonable job in predicting ocean depth, including the flattening."