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How Do Glaciers Carve Valleys? Mystery Solved
Yosemite Valley.
Credit: National Park Service

Since the dawn of time, humans have wondered how mountains and valleys came to be. Now, a new research technique has uncovered the specifics of the million-year process by which glaciers carve valleys in the landscape.

Glacial valleys, such as Yosemite Valley in California and the Fiordland in the Southern Alps of New Zealand, are always U-shaped, and are surrounded on all sides by high mountain ridges. It was known previously that these low-relief areas were drudged out by enormous, slow-moving glaciers . But for the first time, geologists at the University of California at Berkeley led by David Shuster have discovered why the formations look the way they do.

Shuster developed a method (called helium-4/helium-3 thermochronometry) that reveals how rocks in the Fiordland gradually worked their way up to the surface. The method pinpoints the temperature of minerals in the rock at different times in their geologic history, and because temperature increases with a rock's depth, the temperature history of the mineral is tied to how deeply it was buried over the past few million years.

"The technique allows us to collect samples from the present surface and, based on observations, infer how they cooled ... over the last few million years, and thus, how deep they were when they cooled," Shuster said in a university press release. In the Fiordland, the rock currently on the surface was 1.5 miles underground when the Earth cooled and glaciers began forming 2.5 million years ago. The chronology of the rock's ascent helped Shuster and his team construct a picture of the Fiordland's formation, which they described in the April 1 issue of the journal Science.

As tectonic plates collided in the region to build up the region's mountains, a glacier slowly flowed downhill, gouging out a valley in the landscape. For the first million years, most of the land carving happened at the mouth of the glacier the end furthest downhill. Then, for the next million years, erosion occurring uphill took over as the primary rock mover and shaker. Erosion ate into the glacier's headwalls, creating the razorback ridges that surround a glacial valley at its mountainous end.

"Apparently, the heads of glaciers would be directly opposite one another on either side of a high ridge, and faster erosion at the headwalls caused the glaciers to eat their way inward to the spine of the mountain range, farther from the glacier's outlet," Berkeley glaciologist Kurt Cuffey said.

The Fiordland arrived at its present topography about half a million years ago. Earth entered an interglacial, warm period about 12,000 years ago, causing glaciers to melt and recede, and leaving the Fiordland, and other glacial valleys, ice-free.

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