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The secret history of glaciers

Do glaciers really erode mountains faster than rivers? How fast helium-4 dissipates out of rocks may hold the answer.

Two scientists have come up with a way to better determine how glaciers have carved mountains and when rocks first emerged from Earth's depths.

The technique involves determining how rapidly helium-4 has leaked out of rocks found on the planet's surface. Rock gets steadily warmer as depth increases. In the parts of British Columbia where the field research took place, the underground temperature climbs about 25 degrees Celsius with each kilometer.

Heated rocks dissipate helium-4 at a faster rate than cooler ones. Thus, by determining how fast the helium-4 dissipated, one can figure out how quickly the rock cooled, how deeply it was buried and when it came to the surface.

In practice, determining these numbers is quite difficult. David Shuster, of the Berkeley Geochronology Center, and Ken Farley, chairman of the geological and planetary sciences division at the California Institute of Technology, along with colleagues came up with a way of bombarding rock samples with helium-3, a fairly rare isotope of helium, in a cyclotron from a medical lab.

Once the sample is saturated with helium-3, the scientists study the ratios between the two forms of helium.

So far, the technique has allowed the researchers to determine that two kilometers of the overlying rock in the Klinaklini Valley in British Columbia eroded six times faster when glaciers arrived than before. This bolstered the arguments that glaciers erode mountains far quicker than rivers.

The rock in the valley began to cool rapidly about 1.8 million years ago. The valley took about 300,000 years to carve.

"Rather than taking evidence from a single instant, we can, for the first time, see an integral of hundreds of thousands of years. So this is a new way to get at the rate at which glaciers do their work," Farley said in a statement.

Shuster also hypothesized that the technique could be used to study the effect of climate change on geographic features.