Why some Alaskan rock glaciers race downhill while their neighbors barely move

This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Two massive rock glaciers are creeping down mountain valleys in Alaska's Wrangell–St. They sit only a few miles apart and formed under the same sky.

One moves more than 5 feet (1.5 meters) a year. The other moves at less than half that pace. A team led by SMU researchers has figured out why, and the answer points to a factor that varies sharply from one slope to the next, even within a single park: which way a slope faces.

The study, published in Journal of Geophysical Research: Earth Surface, used five years of European Space Agency satellite radar data to map seven active rock glaciers in the park between 2018 and 2022. Rock glaciers are slow-moving piles of rocky debris bound together by ice, and scientists treat them as long-term indicators of permafrost health. "Air temperature peaks once a year, but the rock glaciers we studied sped up twice," said Qingyu Sui, lead author and a Ph.D. candidate in Dedman College of Humanities and Sciences.

"That told us something else was at work. We found that snowmelt and late-summer rain drive the seasonal motion, and solar radiation controls whether the subsurface is warm enough for that water to reach the layers where movement happens." The radar measurements revealed a striking pattern at the fastest-moving landform, called Sourdough Peak. The first speed-up occurred 72–84 days after spring snowmelt began, and the second occurred after late-summer rains.

Air temperature peaks only once a year, so temperature alone could not explain the double signal. Instead, the researchers found that water plays a critical role. As snowmelt and rain seep into the rocky surface, they raise pressure deep inside the landform and lubricate a buried sliding layer.

The nearby McCarthy Creek rock glacier showed almost no seasonal speed-up at all. Modeling work by the team traced the difference to sunlight exposure. Sourdough Peak faces south and sits in the open, while McCarthy Creek lies in the partial shadow of the valley wall.