Hotrod Continent
By Phil Berardelli
ScienceNOW Daily News
17 October 2007

You might not consider an object that takes a whole year to move the distance between your outstretched thumb and pinky to be a speedster, but in the field of plate tectonics, that's zipping along like a Formula One racer. The continental plate that comprises the Indian subcontinent once was such a mover, and now a team using a cutting-edge seismic method thinks it has figured out why.
Earth's rocky crust, or lithosphere, is made up of 14 massive plates that float atop the planet's semimolten mantle and make up the major landmasses and ocean basins. Five of these tectonic plates are the progeny of one--a giant plate known as Gondwanaland, which began to break up about 140 million years ago and eventually gave rise to Africa, Antarctica, India, Australia, and South America. Most of these fragments moved away from one another at about 5 centimeters per year, taking millions of years to arrive at their present locations. But the Indian plate raced along at 20 centimeters per year and eventually slammed into southern Asia with so much force that the collision gave rise to the Himalayas, the highest mountain range on dry land.

What enabled the Indian plate to move so fast? An Indian-German team may have discovered the answer: The plate is considerably thinner than its Gondwanaland siblings. Using a new technique called S-wave receiver function, which can distinguish how long seismic waves take to travel through the lithosphere versus through the underlying semimolten layer called the asthenosphere, the researchers were able to compute plate thicknesses by comparing the travel times of the waves. Readings obtained by 35 monitoring stations indicate that the Indian plate is 100 kilometers thick, only one-third as thick as other pieces of the former supercontinent, the researchers report tomorrow in Nature.

Plumbing the continent's depth was no easy task. "These are very weak signals in seismic records, and many high-quality seismic data must be available for a reliable observation," says geophysicist and co-author Rainer Kind of GeoForschungsZentrum, an earth science institute in Potsdam, Germany. The researchers think the Indian plate is so thin because, when it was part of Gondwanaland, it lay over a particularly hot zone of the asthenosphere that melted its deeper roots. The thin plate was then able to slide more easily across the mantle.

The paper is particularly interesting because it connects the thickness of a plate to the speed at which it can drift, says geophysicist Jason Phipps Morgan of Cornell University. "If true," he says, "this will be very important in how we think about the links between surface plate motions and deep mantle convection." Geophysicist Michael Steckler of the Lamont-Doherty Earth Observatory in Palisades, New York, says he's so impressed with the clarity of the S-wave data that he'd like his collaborators to apply the technique to their own studies of continental plates. Combined with other data, Steckler says, "the conclusion that India once had a root that is now gone is solid--all plates are not created equal."