“Usually glaciers are considered a powerful agent of erosion that carve mountains out, build deep valleys, and help destroy mountains,” said geologist Stuart Thomson of the University of Arizona, lead author of the new study September 16 issue of Nature. “But what we’re finding in Patagonia is that the mountains haven’t eroded at all.”

Mountains are formed when two continental plates smash together, thrusting the crust upward. This process is ongoing in several of Earth’s mountain ranges, including the Andes.

But when mountain glaciers appeared around 3 million to 5 million years ago, they started grinding away at the rocky peaks as they grew and flowed slowly downhill, carving them into their present jagged shapes. A mountain’s ultimate height and width is a balancing act between uplift from the Earth and wear-down from the ice.

The Andes were considered the textbook example of this effect, dubbed the “glacial buzzsaw.” But some of Thomson’s colleagues built computer models that suggested mountains could continue to grow under a glacier’s protective cover.

“If you shut down erosion, stop the mountain from eroding away, the mountains all grow,” Thomson said. “There were some papers a few years ago that postulated this idea. But no one had actually seen it in the real world.”

To test the models, Thomson and colleagues journeyed through the fjords of Patagonia on small boats rented from local fishers. They sailed through the same channels that Charles Darwin traveled on the Beagle; several landforms there are named for him and his crew.

Thomson and colleagues used hammers to break off football-size slabs of granite. When the geologists got back to the lab, they ground the rocks up and picked out small crystals of a mineral called apatite, which is made of stuff similar to tooth enamel.

The researchers hand-picked 146 apatite crystals, each less than a tenth of a millimeter long, and analyzed them for evidence of radioactive elements decaying within them. When uranium decays to lead, a natural process that happens at a steady rate, the split leaves a tiny track in the apatite that is visible under a microscope.

But these tracks are erased when the crystal is heated above 212 degrees Fahrenheit, the temperature of rock at 2.5 miles deep in the Earth. Counting the number of tracks lets geologists back-calculate the last time that the rock was that hot, and therefore how long ago it emerged from the Earth.

“If the rocks are very old, erosion is very slow. It’s taken a long time to come from four kilometers to the surface,” Thomson said. “But a young age, around one million years, shows the erosion rate is very fast.”

To check their calculations, the team used a similar dating technique that involved measuring the amount of helium in the crystals, a record of the last time the rocks were 158 degrees Fahrenheit.

Both methods gave nearly the same results: North of about 45 degrees latitude, the rocks showed that erosion started accelerating between 5 million and 7 million years ago, around the time when the Patagonian glaciers formed. This means the buzzsaw was active in the north.

But south of 45 degrees, where the mountains are about 3,000 feet higher, all the rocks were older than 10 million years. The buzzsaw somehow turned off.

“It was quite a surprising thing,” Thomson said. “Our original motivation was to look at how the glaciers destroy the mountain. We were expecting to see lots of erosion in the south.”

Whether glaciers cut or coddle the mountains below depends on climate, Thomson says. In the southern part of Patagonia, the climate is much colder and the glaciers move much more slowly.

“We call this glacial armoring because it’s actually protecting the mountains from erosion and allowing them to grow much higher than they would have normally,” Thomson said.

Some models suggest that the glaciers actually stick to the rocks and don’t move at all. Large ice sheets like those that covered North America and northern Europe during the last ice age were known to be frozen to the ground below, says geologist Jean Braun of the Joseph Fourier University in Grenoble, France, who was not involved in the new work.

“However, that mountain glaciers (i.e., those usually fast flowing along and eroding mountain sides) would be frozen to the bedrock and thus protect mountain topography had never been demonstrated,” he told Wired.com in an email. The new study is “very important,” he said, because it is “one of the best proof to date that the link between climate and mountain belts dynamics is real and quantifiable.”

Images: 1) The south flank of Cordillera Darwin (elevation 2488 m), the highest point on Tierra del Fuego, Chile (photo taken from the Beagle Channel). Credit: Stuart Thomson. 2) Nature/Jean Braun. 3) Geologists transferring from research vessel “The Foam” to shore in a Zodiac in front of the north side of Cordillera Darwin. Credit: Stuart Thomson.

See Also:

• Earth’s Most Stunning Natural Fractal Patterns
• Enormous Ice Block Breaks Off Greenland Glacier
• Earth From Space: Greenland Glacier Shrinks Overnight
• Stunning Views of Glaciers Seen From Space
• NASA Deploys Rubber Ducks to Track Glacier Water

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Authors: Lisa Grossman

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