For almost 40 years, scientists have recognized that Earth's ocean floors jostle and slide past one another like enormous rigid plates. But whether continents always do the same has been less clear. Now, satellite measurements of the Tibetan Plateau suggest that when continents go head-to-head, they can behave more like unbaked pizzas.
The latest evidence for squishy continents comes from a satellite-borne technique called interferometric synthetic aperture radar (InSAR). By bouncing radar waves off Earth's surface and letting the recorded waves from a satellite's different orbital passes interfere, researchers can calculate the number of radar wavelengths by which the distance between points on the surface has changed. The technique can measure the motion of spots a few tens of meters across with a precision of a few millimeters.
Tim Wright of the University of Oxford, U.K., and colleagues used InSAR to investigate a 500-kilometer-long swath cutting across two great faults in the Tibetan Plateau. Tectonic forces are driving the Indian subcontinent into the Asian underbelly 4.5 centimeters every year, and the Tibetan Plateau bears much of the impact. If the plateau sandwiched between the two faults (the Altyn Tagh and the Karakoram) were a rigid plate, it's been calculated, northward compression would be squeezing it between the faults like a watermelon seed between two fingers at a rate of 20 to 30 millimeters eastward per year.
Examining InSAR data that the European Remote Sensing satellites 1 and 2 had gathered between 1992 and 1999, Wright and colleagues found that the Karakoram fault was slipping at most 7 millimeters per year--and might not be moving at all. "That's quite a robust measurement," says Wright's Oxford colleague Barry Parsons. "I think you can rule out" fast slip on the Karakoram. On the Altyn Tagh, less certain results yielded a fault motion of only 5 ± 5 millimeters per year. The plateau, the researchers report in the 9 July issue of Science, is yielding and deforming like so much putty.
The InSAR observations "look reasonably persuasive to me," says tectonophysicist Wayne Thatcher of the U.S. Geological Survey in Menlo Park, California. In addition, Global Positioning System data back up the slow movement, notes geophysicist Peter Molnar of the University of Colorado in Boulder. The plateau's thicker crust, he says, must leave underlying mantle rock so deep and therefore so hot that it can flow under India's squeeze. "In my view, there's no question," says Molnar. "Continental tectonics is not plate tectonics."