Yale scientists are claiming to have solved one of geology's great puzzles: Why do tectonic plates sometimes make swift, dramatic shifts after unfathomable eons of microscopic progress? By “swift” geologists mean something that can take up to a million years, rather than the sort of event that wreaks havoc on those living near fault lines. Nevertheless, the work may provide insight to help predict those shifts rapid enough to pose a threat to life and limb.
"Our planet is probably most distinctly marked by the fact that it has plate tectonics," says Professor David Bercovici, lead author of the paper published in Proceedings of the National Academy of Sciences. "Our work here looks at the evolution of plate tectonics. How and why do plates change directions over time?"
The standard explanation of plate tectonics is that slabs near the Earth's surface become colder than the hot interior, and sink, or subduct, into the Earth. As they do so, they drag the surface plates behind them. Rapid changes in direction occur when slabs peel off the plates they are attached to, but geologists have been mystified by how this occurs. Cold stiff slabs should not be detachable, yet in Hawaii and Timor we see evidence of plates moving at speeds that models can't explain.
However, Bercovici says that crustal material is washed into the subduction zone and this affects the slab's behavior. Material plugging up the zone is enough to start the process of separating the slab from the plate, and this is accelerated when huge forces occurring at such depths shrink mineral grains, weakening the slab.
Freed from its slab, the tectonic plate may slide sideways, or in the case of a continent, rise relative to the oceanic plates around it.
Bercovici has been working on explaining plate tectonics for much of his career. Last year alone, he published nine papers related to this topic, including explanations of why Venus doesn't seem to have developed similar geology to Earth, despite being so alike in size and composition. Bercovici proposes that Venus was too hot at the surface for the formation of weakened slabs, preventing the spread of plate behavior.
When plate movements were first confirmed, it was widely assumed that they were driven by forces from below, and that what happened at the surface was irrelevant. However, in recent years, it has become increasingly clear that the whole planet is an interwoven system, and that events at the surface, including those driven by biology, can have an impact on even the mightiest processes below.