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  Geotimes - March 2008 - A break between plate breakups?

A break between plate breakups?

Driven by heat from its interior, the face of Earth is being slowly but dramatically rearranged through plate tectonics, as continents collide, oceanic plates subduct and entire ocean basins open and close. If current plate motions continue, 350 million years from now, Earth will look very different. New crust forming at the mid-ocean ridges in the Atlantic Ocean will push its coasts farther apart, and the Pacific Ocean — which currently contains most of the world’s subduction zones — will essentially be closed.

Scientists have assumed that this reorganizing of tectonic plates, called the Wilson Cycle, is a constant, albeit slow, process. But some evidence suggests that plate tectonics isn’t continual, but actually occurs in fits and starts, according to geophysicists Paul Silver, of the Carnegie Institution of Washington in Washington, D.C., and Mark Behn, of the Woods Hole Oceanographic Institute in Woods Hole, Mass., who published their ideas Jan. 4 in Science. That concept of “intermittent” plate tectonics has implications for the thermal history of Earth, as well as for determining when plate tectonics first began.

Subduction zones are plate boundaries where two oceanic plates, or an oceanic plate and a continental plate, collide. The denser plate slides underneath the other, ultimately descending into the mantle. Conventional wisdom holds that, when one subduction zone closes, another opens up elsewhere, swallowing old oceanic crust and thus balancing the spreading of the seafloor at mid-ocean ridges in an ongoing cycle. Thus, when an ocean filled with active subduction zones — such as today’s Pacific Ocean — closes, other subduction zones would form to keep the cycle going.

Silver and Behn propose an alternate hypothesis, however: that instead of new subduction zones forming, plate motions will simply come to a temporary halt. “There’s no inherent reason that if you just shut [a subduction zone] off, that should initiate one elsewhere,” Behn says. “But people have implicitly assumed that.”

The Pacific Ocean is currently shrinking as the Pacific Plate subducts along multiple boundaries: It is slipping under the Nazca Plate to the east, the North American Plate to the north and the Australian Plate to the west. Figuring out what will happen when the Pacific Ocean fully closes — whether new subduction zones will form or not — is key to understanding the history of plate tectonics, Silver and Behn suggest. But it’s not an easy problem to solve. How new subduction zones form is still not well understood, and the tectonic recycling of the seafloor means that traces of subduction older than about 200 million years don’t exist.

Other lines of evidence may help scientists estimate rates of ancient subduction, however. For example, radioisotopic data from crustal rocks suggest that the growth rate of continents has been episodic, not continuous. Although scientists have sought to explain this pattern through dramatic mantle events, such as superplumes, another possible explanation could be intermittent plate tectonics, Behn and Silver say.

Another proxy may be volcanic activity related to subduction. Changes to the mantle’s geochemical makeup from that volcanic activity suggest that about a billion years ago, there was a period of almost no volcanism. That lack of volcanism could be tied to the Grenville orogeny, a mountain-building episode of plate collisions that occurred 1.3 billion to 1 billion years ago. Those collisions formed the supercontinent Rodinia and also may have closed an ocean containing most of the world’s subduction zones — and therefore shutting off related volcanic activity, the authors say.

Intermittent plate tectonics also has implications for Earth’s thermal history. One long-standing puzzle has been why Earth is cooling much more slowly over time than current models predict. Those models are based on how much heat is generated in the planet’s interior and on how much is currently lost at Earth’s surface — but intermittent plate tectonics would periodically cap the mantle like a lid on a boiling pot, slowing down its rate of cooling, Silver and Behn suggest.

The thermal implications are perhaps most compelling, says Bob Stern, a geoscientist at the University of Texas at Dallas. Of the terrestrial bodies in the solar system, Earth is the only planet with plate tectonics, he says; Venus, Mars and even the moon have a hot mantle beneath a lithosphere that doesn’t move, called a “stagnant lid.” If Earth, too, has had a stagnant lid at times, it falls along that continuum of planetary behaviors, he says.

Indeed, many geophysicists have already been entertaining the idea that plate tectonics is episodic, says Don Anderson, a geophysicist at Caltech in Pasadena, Calif. “Standard models of whole mantle convection and plumes, and steady state plate tectonics, cannot be correct,” he says. “Earth is a finite sphere, and continent collisions and ridge trench annihilations must occur and reorganize everything.”

Although the concept of intermittent plate tectonics may be just one way to rethink standard ideas in the field, this paper “is going to get people thinking,” Stern says. “What excited me about this paper was that the geophysical community is starting to take seriously the idea that plate tectonics may have had a more complicated history than we have heretofore considered.”

Carolyn Gramling

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