SU Researcher's International Team Discovers Unsuspected Molten Layer in
Himalayan Crust, This Week's Issue of Science Reports
"We Should Think Of How Molasses Looks When It's Pushed by a Squeegee," Says Earth Sciences Professor K. Douglas Nelson
SYRACUSE--An article principally authored by a Syracuse University earth sciences professor in this week's issue of Science pulls together research by a multinational team to describe a partially molten crust beneath the Himalaya and the Tibetan Plateau.
At the premier place in the world where two continents are colliding head on, says professor K. Douglas Nelson, researchers came to trace the Indian continent's downward plunge under Tibet but additionally discovered a molten layer within the crust, which may change the way earth scientists view the evolution of earth's continents.
"We tend to think about the earth's plates moving along discrete faults," says Nelson. "But here we should be thinking about flow, we should think of how molasses looks when it's pushed by a squeegee."
Nelson's research involved 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. Led by Nelson, researchers from more than half a dozen other U.S. universities, the Geological Survey of Canada, Germany's Potsdam University, and the Chinese Academy of Geological Science worked together under the umbrella of Project INDEPTH (International Deep Profiling of Tibet and the Himalaya).
Funded in part by the National Science Foundation, INDEPTH has been under way since 1992.
Other methods of exploration included seismic refraction (measuring return waves that have refracted rather than reflected), broadband earthquake (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). Two Syracuse University graduate students worked with Nelson in seismic reflection and in field work, mapping strata and geological structures and returning to the University to analyze the gathered data.
Plate tectonics, a view of the earth's crust as an array of shifting pieces, is only three decades old. The current generation of geologists, says Nelson, is working to clarify this picture. Since the 1980s, Nelson, interested in the creation of mountain belts, has been involved with seismic reflection as a method for delineating large-scale structures underground.
"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 in the world where a classic continental collision is happening on a large scale," Nelson says.
Attempts to arrange for a Himalayan study began in 1984, says Nelson, but only when the Chinese political situation allowed greater access to foreigners did the project take shape.
"For 50 million years, India has been driving north into Asia like a bulldozer pushing a pile of debris, moving at about 5 centimeters a year," he adds. "It's pushed up the Himalayas and the Tibetan Plateau, and pieces of central Asia are pushing out like melon seeds into the Pacific."
The plates of the earth's lithosphere move because the underlying mantle is hot and convecting. The traditional understanding of these plates is that they are rigid. The continental crust forms the upper part of the plates in continental regions. The upper crust deforms by faulting, which produces earthquakes, while the lower crust deforms more plastically, as would heated metal. Nelson says that scientists have puzzled over the presence and role of magma, superheated liquid rock, in the crust in collision zones.
INDEPTH's research revealed a molten layer within the 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.
The discovery may also go a long way toward explaining how certain chemicals and minerals find their way to particular levels in the earth's crust, a problem, says Nelson, believed to involve magmatism.
"It may be that collisions like this act like big moonshine stills," he says. "Continental crust moves around and, in the 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."