VOLCANO MADNESS: [Geology2] Stop-and-go plate tectonics

Thursday, April 19, 2012

[Geology2] Stop-and-go plate tectonics

Stop-and-go plate tectonics

Ancient crustal plates may have dived deep, time and again

By Alexandra Witze

Web edition : Monday, April 9th, 2012

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HOT EARLY MANTLEA simulation of plate tectonics today (left) shows a crustal slab (green) plunging more than 400 kilometers into Earth's mantle. Billions of years ago (right), when the mantle was 200 degrees Celsius hotter, the slab would have barely penetrated the depths before breaking off and foundering. Geologists propose this difference led to intermittent plate tectonic activity early in Earth's history.J.F. Moyen et al/Geology 2012

Plate tectonics might have gotten a fitful start on the early Earth.

Today, the process of Earth's crustal movement called plate tectonics dictates nearly everything about the planet's appearance, from the sites and heights of the mightiest mountain ranges to the depths of the oceans' trenches. But geologists have long argued about when thin, rigid crustal plates first formed on the young planet and began jostling against one another.

Now, two researchers propose that plate tectonics started and stopped over and over, billions of years ago, before running continuously. The work, published online March 26 in Geology, could explain how a sort of proto-plate tectonics on the hot early Earth evolved into the style geologists see today.

"As far as I'm aware we are some of the first people to come up with this scenario to explain how plate tectonics started," says Jeroen van Hunen, a geophysicist at Durham University in England who did the work with Jean-François Moyen, of Jean Monnet University in Saint-Etienne, France.

A defining feature of plate tectonics is how one crustal plate sometimes dives beneath another, a process known as subduction. When that diving plate gets deep enough, the high pressures and temperatures inside Earth chemically change its rocks. If those rocks are later uplifted to the surface, geologists can recognize the chemical alteration and show that plate tectonics has occurred.

Many scientists have spotted this signature in rocks from the Archean eon, which stretched from about 3.8 billion to 2.5 billion years ago, and so have argued that plate tectonics must have happened then. But the Earth's mantle, or layer beneath the crust, was also several hundred degrees Celsius hotter thanks to residual heat from the planet's birth. A hotter mantle makes subduction tough, because the diving edge of the crustal plate weakens and breaks off before the plate can get too deep.

So Moyen and van Hunen decided to look for evidence of short-term plate tectonics. Some ancient outcrops, as in western Australia and in Zimbabwe, show altered rocks interleaved with more pristine rocks in layers formed every couple of million years. These repetitions may represent subduction turning on and off and on again over time, the scientists say.

Next they simulated how plates might subduct under various mantle temperatures. At temperatures 200 degrees hotter than today, the calculations showed how crustal plates would make it only partway down before breaking off and foundering.

"Each time a slab breaks off, you temporarily stop the whole process," says van Hunen. "It's not so easy to get a subduction system going. You get a kind of start-and-stop scenario."

Plates would then have to cool at the surface and become dense enough to again start sinking into the mantle, starting the process over. Only when the mantle cooled sufficiently — perhaps by around 2.7 billion years ago — could permanent, modern-style subduction take hold.

Of course, "a sporadic record of subduction is not the same as sporadic subduction," says Hugh Rollinson, a geologist at the University of Derby in England who has studied Zimbabwe's Archean rocks. But he says the basic concept, that plate tectonics emerged intermittently, is "a good idea, and one to test."