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Glacial pace of erosion was not so slow, new technique shows

December 12, 2005

ANN ARBOR, Mich.-Glaciers, rivers and shifting tectonic plates have shaped mountains over millions of years, but earth scientists have struggled to understand the relative roles of these forces and the rates at which they work.

Now, using a new technique, researchers at the University of Michigan, California Institute of Technology and Occidental College have documented how fast glaciers eroded the spectacular mountain topography of the Coast Mountains of British Columbia.

Their work is described in the Dec.9 issue of the journal Science.

U-M assistant professor of geological sciences Todd Ehlers has been working in a remote region of the Coast Mountains for the past three years, studying rates of glacial erosion and topographic change. Using a new geochemical tool developed by the Caltech researchers, he and his collaborators were able to quantify the rates and magnitude of glacial erosion across a major valley. They found that glaciers radically altered the landscape around 1.8 million years ago, about the time that Earth began to experience a number of ice ages.

The erosion rates documented in the study suggest that glaciers eroded the mountains six times faster than rivers and landslides had before glaciation began. The researchers also found that glaciers scraped at least 2 kilometers (about 1.2 miles) of rock from the mountains.

"These results are exciting," Ehlers said, "because they clearly document that glaciers are the most efficient method for sculpting the topography of the range. They also demonstrate the utility of a new geochemical tool that can be applied to study erosion in other mountain ranges."

The study relied on a technique called helium-helium thermochronometry, developed by Caltech's Ken Farley and his former student David Shuster, now at Berkeley Geochronology Center in Berkeley, California. "It's an unwieldy name, but it gives us a new way to study the rate at which rocks approached Earth's surface in the past," Shuster said.

The new technique rests on three facts: one, that rocks on the surface have often come from beneath the surface; two, that the ground gets steadily warmer as depth increases; and three, that helium leaks out of a warm rock faster than a cold one. By determining how fast the helium leaked out of a rock, it's also possible to determine how fast the rock cooled and, ultimately, how deeply it was buried, as well as when and how fast it got uncovered.

The team showed that the cooling of the rock happened very quickly and that the entire valley was carved out in about 300,000 years.

"We can say that the glacier was ripping out a huge amount of material and dumping it into the ocean," Farley said. "And rather than taking evidence from a single instant, we can for the first time see an integral of hundreds of thousands of years. So this is a new way to get at the rate at which glaciers do their work."

Why the intense erosion occurred 1.8 million years ago is not `well understood, Shuster said, "but it seems to coincide with some very interesting changes that took place in Earth's climate system at that time."

In addition to Ehlers, Farley and Shuster, Margaret Rusmore, a geology professor at Occidental College in Los Angeles, was a coauthor on the paper. The research was supported by grants from the National Science Foundation.

The Regents of the University of Michigan