Glaciers boost mountain growth in Andes

Glaciers have carved some of the planet's most dramatic landscapes, from Yosemite National Park to the Himalayas. Now geologists have discovered that glaciers can do more than erode mountain peaks and shape valleys - they can actually encourage mountain growth.

A new study, which appears in the September 16 issue of Nature, found that glaciers in the southern reaches of the Patagonian Andes have acted as a kind of protective shield throughout the mountain range's 25-million-year history, providing the first evidence to contradict the widely held belief that glaciers inhibit mountain growth.

Until now, scientists thought that glaciers ubiquitously erode mountains, slowing 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.'

At the relatively low elevations found in the southern Andes, scientists expected the buzzsaw effect would have had a major impact on the mountains throughout the 25 million years they've been building, said Mark Brandon, a Yale geologist and an author of the new study.

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 armour to protect the uplifting mountains from erosion, allowing them to reach heights well above those predicted by the glacial buzzsaw theory,' Brandon said.

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.