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Syracuse University researcher Doug Nelson on team exploring what lies beneath the Tibetan Plateau
Syracuse University researcher Doug Nelson on team exploring what lies beneath the Tibetan PlateauMay 05, 2001Judy Holmesjlholmes@syr.edu
Each summer, dozens of intrepid explorers embark on a precarious journey to climb to the top of Mt. Everest, located in the heart of the Himalayan Mountains. This summer, an international group of researchers from Syracuse University, the University of Alberta, the Geological Survey of Canada, the Chinese Academy of Geological Sciences and the China University of Geosciences will embark on a mission to discover what lies 70 to 80 kilometers beneath the Tibetan Plateau, the world’s largest plateau, located at the northern foot of the Himalayas. The experiments they will conduct build on the work and discoveries the group has made during the past 10 years–the latest of which was published in the April 27 issue of Science. That study confirmed the researchers’ earlier suspicions that, while the Tibetan Plateau, with an elevation of 5,000 meters above sea level, contains some of the world’s thickest crust, the vast majority of that crust is molten or in a semi-liquid state. The research is being funded by the National Science Foundation. “The plateau is basically like a gigantic waterbed,” says SU’s Doug Nelson, acting chair of the Department of Earth Sciences. “There is no other place in the world like it. The plateau is so flat because the crust is fundamentally weaker than what you would normally expect to find.” Through the new experiments, Nelson and his colleagues Martyn Unsworth, associate professor at the University of Alberta’s Institute for Geophysical Research, and Alan Jones, SU adjunct professor and a researcher at the Geological Survey of Canada, will use the latest magnetotelluric surveying technology to determine the thickness of the molten layer and what lies beneath. Magnetotelluric surveying is a technique that uses naturally occurring electromagnetic waves to measure the electrical conductivity of Earth’s crust. These waves are mainly produced by variations in the solar wind hitting Earth’s magnetosphere and by lightning. Normally, rock is fairly resistive to the naturally occurring electric and magnetic fields that cut through the planet, Nelson says. But on the Tibetan Plateau, the researchers found large areas of highly conductive material at depths of 10 to 20 kilometers below the surface.
The researchers first discovered pockets of conductive material about six years ago, when they conducted some of the first geophysical studies–using seismic reflection profiling (sonar) and magnetotelluric surveying–on the southern edge of the plateau. The studies were designed to image the part of the Indian continent that is colliding with and pushing underneath the southern edge of Asia, a process that has been ongoing for some 50 million years and which has produced the tallest mountains in the world and some of the most devastating earthquakes. Their findings were documented in Science in 1996 in a series of five articles. “We found the part of the Indian continent that lies beneath Asia using seismic techniques,” Nelson says. “But by using magnetotelluric surveying, we also found a pocket of extremely conductive material below a layer of resistive rock, which was striking. For crust to be this conductive, extreme circumstances would have to occur.” The Tibetan Plateau crust averages 70 to 80 kilometers thick, whereas the average crust in Syracuse is 40 kilometers, Nelson says. “Crust that is of normal thickness never gets hot enough to melt, he says, but if you double the thickness of the crust, you will reach the melting point in rock before reaching the base of the crust.” The 1996 findings raised questions about the extent of the conductive layer and its composition. The researchers returned to Tibet in 1998 to conduct a magnetotelluric survey across the entire Tibetan Plateau, which is about two-thirds the size of the continental United States. That study, published in the April 27 issue of Science, indicates that the entire Tibetan Plateau is probably underlain by molten rock. According to the study, the molten rock in southern Tibet lies just 15 to 20 kilometers beneath the surface and is most likely due to partial melting and aqueous fluids in the crust. In northern Tibet, the top of the molten layer is 30 to 40 kilometers below the surface. This summer the team will try to determine how deep the molten layer extends beneath Tibet. Is it confined to the crust or does it extend down into the upper mantle? The results could help answer long-standing questions about how the crust is formed and modified through geologic time. The research team, which will include SU first-year graduate student Jessica Spratt, will position the sensors at specially selected sites on the Tibetan Plateau and leave them there for longer periods of time than done in previous studies. This summer is an especially good time to conduct the experiments because the sun is currently at the peak of its 11-year sunspot cycle. During the sunspot peak, it spews out large and frequent solar flares, producing a big electromagnetic signal for the experiment. “If you want to look deeply into the crust, this year is the time to do it,” Nelson says. “If we find that the mantle beneath Tibet is molten, it would imply that magma is working its way up and contributing to the melting of the crust,” Nelson says. “That could change a lot of theories about how crust is modified and perhaps formed in continent-continent collisions.”