Standing tall.
Erosion by Tibet's Yarlung Tsangpo River deformed Earth's crust enough to cause 7100-meter Gyala Peri to rise among the Himalayas.

Noah Finnegan

The River That Raised a Mountain

If you think erosion always wears down mountains, think again. Researchers now report a case in which a river created a new mountain, a dramatic example of how climate, plate tectonics, and erosion can affect each other.

To say that the Yarlung Tsangpo River flows through the Himalaya Mountains of Tibet is a little like saying Tyrannosaurus rex tiptoed through the world of the Cretaceous. The Yarlung Tsangpo is the highest and one of the roughest rivers in the world, and its total drop of about 3000 meters--more than twice that of the Mississippi River in less than half the length--makes it an extraordinary excavator. In one section, the river pushes past a mountain named the Namche Barwa-Gyala Peri massif and has cut a gorge nearly 5 kilometers deep. Here, researchers think they have made a startling discovery.

Although most of the Himalayas along the Yarlung Tsangpo's length have been rising at a uniform rate during the past 50 million years, the massif has shot up tens of times faster. Parts of it have reached more than 7700 meters in less than 2 million years, an international team concludes in the current issue of the Geological Society of America's GSA Bulletin. The event, which geologists call a tectonic aneurysm, occurred because the river removed so much material during that time frame from a corner of the Indian crustal plate. This lightened the plate enough to allow a small part of it to lift more rapidly than the rest, thrusting Namche Barwa-Gyala Peri above the surrounding landscape. "There's very compelling evidence" that erosion has allowed the mountain to grow, says geologist and lead author Noah Finnegan of Cornell University.

A feedback loop is also involved in this phenomenon, Finnegan says. The rise of the Himalayas has trapped much of the seasonal moisture blowing to the northeast off the Indian Ocean. That captured moisture provides India with its annual monsoon rains, but it also deposits a lot of snow on the mountains, which when melted boosts the Yarlung Tsangpo's flow. The process shows that "you can make weather by uplifting mountains," he says. And that weather, sometimes, can generate more uplift. The paper makes a convincing case that erosion and mountain building can be tightly coupled, says environmental scientist Cameron Wobus of the University of Colorado, Boulder.

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