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On the Rocks

Betsy Mason

New Scientist, 174 (2341), p 13. 4th May 2002

To learn some geology, try pouring a cool cocktail.

Plate tectonics may be about to be turned on its head. Instead of being heated from below like water on a stove, the Earth's interior could have more in common with a neat gin on the rocks.

While geologists agree that the plates of the Earth's crust are constantly moving, nobody is quite sure why. Some believe that heat from within the Earth causes convection currents in the molten interior that drive the plates. But geophysicist Don Anderson of the California Institute of Technology in Pasadena thinks the convection could be triggered by the plates themselves. Efforts to model mantle convection based on heat from below have failed.

"Those convection calculations have never come up with plate tectonics or anything resembling the present situation on Earth," Anderson says.

Thinking about the problem from the top down may be the key. The new view is that where the hot mantle comes into contact with the relatively chilly crust above, it cools and sinks. The same thing happens when you order a drink on the rocks, says Anderson.

"If you put two ice cubes in a glass of gin, the gin next to the ice cubes will get cold and sink."

By studying the pattern of the plates today and in the past, Anderson concludes that the Earth is in a transitional state between so-called "ideal configurations" – low-energy, stable states seen in other spherical systems such as buckyballs and viruses.

"If things ever settle down to an equilibrium state, then the ideal number of plates seems to be somewhere around 12," he adds.

Although Earth does have roughly 12 plates, they have widely different shapes and sizes. It's an unstable configuration but may eventually even out into patterns where three similar plates meet at triple junctions. This is typical of systems in nature that are controlled by surface processes, and Anderson speculates that the Earth's plates are controlling the interior, not vice versa.

"It shows that plate tectonics is much more powerful than people thought," he says.

If Anderson is right, geologists may have to reconsider why some volcanoes appear where they do. Volcanic activity where two plates collide can easily be blamed on plates melting as one is forced under the other. But when volcanoes emerge in the middle of a plate, geologists usually invoke a narrow plume of hot mantle rising up from deep within the Earth and bursting through the crust.

Anderson, however, believes that weak spots in the plates rather than plumes control where the hot magma erupts onto the surface.