We've all enjoyed the spectacular results when glaciers carved their way through the landscape and it seems intuitive that glaciers inhibit mountain growth due to erosion.

Not necessarily, say geologists, they can actually encourage mountain growth.

A new study in Nature found that glaciers in the southern reaches of the Patagonian Andes have acted as a kind of protective shield throughout that mountain range's 25-million-year history.    As most people realize, glaciers erode mountains and slow their growth once the mountain peak reaches above the snowline. Above this elevation, where glaciers remain permanently frozen, scientists believed that the masses of ice carve away at the mountain face as they slide down its surface—an idea known as the 'buzzsaw' theory.

patagonian andes
 
At the relatively low elevations found in the southern Andes, they expected the buzzsaw effect would have had a major impact on the mountains throughout the 25 million years they've been building.

Instead, the team found just the opposite. They measured the ages of rock samples from a vast track of the Patagonian Andes and discovered that at higher southern latitudes—where the mountains are at lower elevation and so the glacial buzzsaw should have a bigger impact—the rock was older than expected, meaning erosion has been taking place at a much slower rate than previously thought.

Rather than slicing away at the mountain peaks, the scientists found that the glaciers instead seem to have helped the mountains grow. "The glaciers act like armor to protect the uplifting mountains from erosion, allowing them to reach heights well above those predicted by the glacial buzzsaw theory," said Mark Brandon, a Yale geologist and co-author of the new study.

Despite the lower elevations found in the Patagonian Andes, the glaciers remain cold enough that their bases are frozen and stuck to the mountain surface, Brandon said. Whereas warmer glaciers melt at their base and slide down the mountain, these colder glaciers appear to have provided an icy shield.

The new finding presents the first evidence to contradict the glacial buzzsaw theory, Brandon said—a theory that has proven difficult to test on large scales and over geological time scales.

Next, the team will try to use the results to calibrate a global erosion model to understand how climate affects the mountain building process.

Citation: Stuart N. Thomson, Mark T. Brandon, Jonathan H. Tomkin, Peter W. Reiners, Cristián Vásquez, Nathaniel J. Wilson, 'Glaciation as a destructive and constructive control on mountain building', Nature 467, 313-317 (16 September 2010) doi:10.1038/nature09365