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Slimmer Indian Continent Drifted Ten Times Faster

Anne Casselman
for National Geographic News
October 17, 2007
 
The thinness of the Indian continent allowed it to speed northward at ten times the rate of other tectonic plates, a new study suggests.

The oldest parts of India are about a half to a third as deep as similarly old portions of Australia, Africa, and Antarctica, geophysicists measured using a new technique.

This allowed India to far outstrip the speed of the other sluggish and thicker tectonic plates, also known as lithosphere, after the breakup of a massive supercontinent 150 million years ago.

"The Indian lithosphere is only 62 miles [100 kilometers] thick, whereas the African, Australia, and Antarctica lithospheres are 124 miles [200 kilometers] or more," said Rainer Kind, one of the study's authors.

"India was able to move faster and farther because it was much thinner. If it is thinner, it's easier to push away—the resistance is less," added Kind, who is a geophysicist at the Geoforschugsventrum in Potsdam, Germany, and the Freie Universität in Berlin.

The new study appears in this week's issue of the journal Nature.

Tracking the Continents

Earth is covered by several major continental plates, which have jostled around its surface for billions of years. Their movements shift the world's oceans and land masses around at a snail's pace—but one that adds up over enough time.

(Related: "Oldest Known Ocean Crust Found on Greenland" [March 22, 2007].)

"From mapping both the gravity and magnetic field of the ocean, we can then [reconstruct] the relative motions between plates," said Dietmar Müller, an earth scientist at the University of Sydney, Australia, who was not involved in the study.

"Paths of the individual tectonic plates are frozen into the ocean floor a little bit like railroad tracks, and we can see these tracks in the ocean's gravity field."

Geologists can also study rock samples to map the path of a continent relative to the magnetic poles through time.

Such geological forensics have long revealed that India moved extremely fast to the north after it broke away from Gondwanaland, the supercontinent that dominated the world map 150 million years ago.

Other continents that belonged to Gondwanaland, such as Africa, Antarctica, and Australia, broke apart at the steady pace of 0.8 to 1.6 inches (20 to 40 millimeters) a year.

India sped ahead at about 8 inches (200 millimeters) a year.

"This is the highest speed ever with which any of the continents was ever traveling," study author Kind said.

These four continents' motions created the Indian Ocean between them.

But what enabled India's fast movement was a mystery until now.

Breaking Up is Hard to Do

Kind and his colleagues used seismic waves generated by earthquakes to measure the continents' thickness.

Similarly to how the depth of lakes is studied with sound echoes, the characteristics of seismic waves change as they travel through different materials inside Earth.

The team was able to pinpoint the boundary where the continental lithosphere ends and the more plastic layer beneath it begins, which told them how thick the lithosphere was.

The researchers also devised a theory that Gondwanaland's break-up was triggered by a hot plume of mantle, which created a huge convection current. This slowly pushed the floating continents away from its hot center.

The team suggests that heat originating from this plume was probably responsible for melting off the bottom of India's continental plate, thereby thinning it.

But the University of Sydney's Müller isn't so convinced by that part of the theory.

"It could very well be that Gondwanaland broke up because the mantle got hotter and hotter underneath, but whether that was really driven by a giant mantle plume is a different question," he said.

Müller also isn't certain that this hot plume caused the Indian plate's thinness.

"That the loss of the root coincides with the breakup of Gondwanaland is pure speculation, not supported by observations," he said.

Regardless of what caused India to lose the bottom portion of its continental lithosphere, the result was the same.

"If it hadn't lost its root sometime in the past, it would have not yet collided with Eurasia, and Mount Everest and the entire Tibetan plateau would not exist at all today," Müller said.

Echo of Success

The new measurement technique may also help address many longstanding puzzles in plate tectonics.

That theory was widely adopted in the 1960s, but questions remain about what exactly drives the motion of continental plates, explained Michael Bostock, an earthquake seismologist at the University of British Columbia in Vancouver, Canada.

"These are the sorts of issues that are going to become increasingly clear now that we have methods that allow us to accurately measure the thickness of the tectonic plates," said Bostock, who also was not involved in the study.

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