International Research Team Discovers Unsuspected Molten Layer in Himalayan Crust
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At the premier place in the world where two continents are colliding head on, researchers have traced the Indian continent's downward plunge under Tibet and discovered a molten layer within the crust, changing the way scientists view the evolution of earth's continents. The scientists conducted their research as part of project INDEPTH (International Deep Profiling of Tibet and the Himalaya), sponsored by the National Science Foundation.
"This discovery may go a long way toward explaining how large volumes of the earth's crust are redistributed via magmatism," says Leonard Johnson, director of NSF's continental dynamics program, which funded the research. Magmatism is the development and movement of magma within the earth.
"We tend to think of earth's plates as moving along discrete faults," says Doug Nelson, an earth scientist at Syracuse University and author of a paper on the subject in this week's issue of the journal Science. "But we should be thinking about flow, about how molasses looks when it's pushed by a squeegee."
Project INDEPTH's research revealed a molten layer within earth's crust under the Tibetan Plateau, but Nelson says this is not magma that has been injected into the crust from beneath. Rather, this soft area is the result of an extremely thick crust. The thick crust's lower regions, since they run so deep, are exposed to higher than usual temperatures, melting the crustal materials. "If you thicken a crust enough and give it tens of millions of years," says Nelson, "it will melt."
This molten crust may explain the relative flatness of the Tibetan Plateau, researchers surmise. "It may be that collisions like this act like big moonshine stills," Nelson adds. "Continental crust moves around and, in major collisions, this heating occurs, and the crust is chemically refined. That's not a new idea, but now we have the suggestion that collision-induced magmatism may be the big elephant in the process."
Nelson's research used seismic reflection, a procedure that involves detonating explosions on the earth's surface and recording the waves reflected back by sub-surface structures. "No one method alone would have presented compelling evidence for what we found," says Nelson, so multiple techniques were employed in searching the earth's hidden layers.
Other methods of exploration included seismic refraction (measuring return waves that have refracted rather than reflected), broadband earthquake recording (in which stations measure waves generated by worldwide earthquake activity), and magnetotelluric (determining how conductive geological structures are by measuring induced electrical and magnetic fields).
Plate tectonics, a view of the earth's crust as an array of shifting pieces, is only a few decades old. The current generation of geologists, says Nelson, is working to clarify this picture. "Those of us involved with seismic reflection have long thought that if we could pick one place in the world to do this work, it would be the Himalayan-Tibetan Plateau, because this is the one place where a classic continental collision is happening on a large scale." For 50 million years, India has been driving north into Asia like a bulldozer pushing a pile of debris, moving at about five centimeters a year. Says Nelson, "It's pushed up the Himalayas and the Tibetan Plateau, and pieces of central Asia are pushing out like melon seeds into the Pacific."
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