Glacial Blankets Can Protect Mountains From Erosion

15 September 2010 2:26 pm

Glaciers have a reputation as geological buzz saws. They grind down mountains and carve deep valleys. But a new study reveals that glaciers can sometimes protect mountains as well. Over the past 5 million years, recurring glaciers near what is today Tierra del Fuego, at the tip of South America, have wrapped themselves around the southern Patagonian Andes, preventing the mountains from being eroded like their sister peaks to the north.

Periodic glaciation—a process that continues to this day—has wreaked havoc on parts of the Patagonian Andes. Ice, up to thousands of meters thick accumulated from millennia of precipitation, sits atop rock that is being pushed upward by the Pacific Ocean tectonic plate slipping under South America. In the more temperate part of the range, from 38˚ to 49˚ south latitude, the glacial grinder has shaved off as much as 1000 meters from the mountains' peaks, flattened their slopes, and smoothed their surfaces. But farther south, between 49˚ and 56˚ latitude, the mountains have been spared: The peaks are higher—some nearly 4000 meters—and the ridges are much more rugged.

To find out why, a team led by geologist Stuart Thomson of the University of Arizona in Tucson spent several months collecting rock samples from various locations and elevations along the Patagonian Andes. The researchers tested 146 of those samples using a technique called thermochronology, whereby they fired a laser at grains of the mineral apatite in the rocks. The laser heated the grains, releasing isotopes of helium trapped in the apatite. By calibrating the ratio of those isotopes, the researchers can determine when the rock in question cooled below 70°C and trapped the helium in the first place. The younger the rock, the more deeply it must have been imbedded in the mountain, and thus the more the mountain must have eroded.

Based on the analyses, Thomson and colleagues conclude that the rocks from the southern portion of the Patagonian Andes are uniformly older than the samples collected in the north. What happened, they report in the 16 September issue of Nature, is that along the northern portion of the mountain range, temperatures remained moderate enough to create thin layers of water under the ice, which provided enough lubrication for the glaciers to slide and grind down the mountain peaks and slopes. But farther south, temperatures were too low for the water layers to form. The ice stayed frozen solid, bonding to the rock and protecting it from wind, water, and the wearing effects of the ice itself when it slides down the slopes. It's a compelling sign, Thomson says, that "climate has a strong influence on mountain building."

It's a "very good paper" that demonstrates a concept that "had been very difficult to demonstrate in the field," says geologist Jean Braun of Université Joseph Fourier in Grenoble, France. Erosion, or the lack of it, can change the dynamics of mountain building, he explains, so the key is to quantify the relationship. The researchers have provided "one of the best proofs to date [that] the link is real and quantifiable."

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