Scientists discover reason for "sudden" tectonic movements

Eric Hopton for redOrbit.com – Your Universe Online

Movement of tectonic plates beneath the surface of the earth is, thankfully, super-slow. Any significant movements are normally measured in tens or even hundreds of millions of years.

But sometimes tectonic plates get a little too “hasty” and make “sudden” movements over a million years or so. These speedy shifts have puzzled geologists for decades, but now, scientists from Yale University believe they’ve found the answer–and it’s all about crust.

The rigid outer shell of the earth, made up of the crust and upper mantle, is broken up into tectonic plates. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries. This latest research helps us understand how those processes work.

The results of the Yale-led research will be published the journal Proceedings of the National Academy of Sciences (PNAS). The paper’s authors believe the secret to understanding abrupt tectonic plate shifts, which have been identified in many parts of the world, lies in the way both thick “crustal plugs” and weakened mineral grains interact. That interaction could explain a range of “relatively speedy moves among tectonic plates around the world, from Hawaii to East Timor.”

“Our planet is probably most distinctly marked by the fact that it has plate tectonics,” said Yale geophysicist David Bercovici, lead author of the research. “Our work here looks at the evolution of plate tectonics. How and why do plates change directions over time?”

Normal explanations of tectonic plate movements involve the process of subduction. This is what occurs at the convergent boundaries between plates. One tectonic plate moves under another tectonic plate and sinks into the Earth’s mantle as the plates converge. Regions where this process occurs are known assubduction zones. Subduction speeds are normally measured in inches per year. The average rate of convergence is between one and three inches per year.

“Subducting” slabs are the result of the cold top layer of the Earth’s rocky surface becoming colder and heavier. This causes the slabs to sink into the deeper mantle below. But, for sudden plate shifts to happen, the slabs must detach from their plates. How this happens as always been a mystery as scientists believed that the slabs should be too cold and too stiff to break away.

This latest research from Yale, however, has found there are other factors at play. First, they discovered, “thick crust from continents or oceanic plateaux is swept into the subduction zone, plugging it up and prompting the slab to break off”. Once this detachment process starts, movement “is accelerated when mineral grains in the necking slab start to shrink, causing the slab to weaken rapidly”.

When this combination of movements in the crust and shrinking mineral grains reaches a critical point we get those much faster horizontal tectonic plate shifts. The end result, say the Yale team, can even be “continents suddenly bobbing up”.

“Understanding this helps us understand how the tectonic plates change through the Earth’s history,” Bercovici said. “It adds to our knowledge of the evolution of our planet, including its climate and biosphere.”

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