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Fast and slow plates

In the mid 1970s, geoscientists studying Earth’s plates made a key observation: oceanic plates move toward subduction zones roughly 3.5 times faster than continental plates. The reason for this difference remains cloudy, but a study in the Oct. 4 Science suggests an answer. Based on a simple model of lithosphere and mantle interactions, geophysicists at the University of Michigan have found that two major tectonic forces — slab pull and slab suction — can interact to generate the observed plate movements.

The researchers argue that those forces may in fact drive all plate dynamics. “We can explain the majority of plate movements through these two forces,” says geophysicist and lead author Clint Conrad.

Geoscientists first described slab pull and slab suction in the 1970s. What is new about this work is that the researchers integrated those forces into a compute model that made specific predictions, explains co-author Carolina Lithgow-Bertelloni. “This is the first model to test the relative contributions of pull and suction forces to plate movements.” The work was funded by a fellowship from the David and Lucile Packard Foundation.

Geophysicists have found that two major tectonic forces — slab suction and slab pull — can interact to explain most observed plate movements today. Image by Clinton Conrad.

Slabs are the leading edges of subducting plates, and they generate a tremendous amount of energy as they dive into the mantle. A slab can pull the rest of its plate behind it (the pull force). Or, the slab can drag against the viscous mantle and cause the mantle to flow in toward the subduction zone. That flow sucks in nearby plates, like pulling a plug from a bathtub (the suction force).

The researchers found that when either the pull force or the suction force acts alone, their model does not predict the observed difference in the rates at which oceanic and continental plates head toward subduction zones. Only when the two forces act in concert do the predictions move in line with reality.

Since oceanic plates are thinner and denser than continental plates, a collision between the two results in the ocean plate subducting, with its leading edge forming the downgoing slab. The study found that for oceanic plates, pull and suction forces combine to create a relatively quick march into subduction zones. The overriding continental plates lack the pull force, and so move more slowly. In addition, as oceanic plates drag against the mantle, they set up a current that offsets the suction force. This offset slows both the oceanic and continental plates.

The ability of this relatively simple model to explain the difference in plate speeds may represent a significant step in viewing slabs as the driving force behind all plate movements. Ever since plate tectonic theory took hold in the 1960s, geoscientists have argued over what drives plates: mantle upwelling at ridges that pushes plates apart, mantle circulation that drags plates along, or slab forces. According to Don Anderson, a geophysicist at Caltech Seismological Laboratory, “the paper confirms the idea that slabs drive tectonics. There is no need for other driving mechanisms such as plumes or mantle convection that are independent of plate tectonics.”

Greg Peterson

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