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

In brief:

• Two rock glaciers a few miles apart in Alaska's Wrangell–St. Elias National Park move at sharply different speeds, and the difference traces to which direction each slope faces.

• Using five years of European Space Agency satellite radar, researchers found that snowmelt and late-summer rain, not air temperature, drive the seasonal motion, producing two speed-ups a year rather than one.
  
  
• Solar radiation controls whether the subsurface is warm enough for that water to reach the layers where movement happens: the south-facing, open Sourdough Peak sped up markedly while the shaded McCarthy Creek showed almost none.
  
  
• The findings suggest satellite radar paired with hydrologic and thermal modeling can monitor rock glaciers and other permafrost landforms across remote mountain regions without setting foot on the ice.

Two massive rock glaciers are creeping down mountain valleys in Alaska's Wrangell–St. Elias National Park. They sit only a few miles apart and formed under the same sky. One moves more than 5 feet 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.

Snowmelt and rainfall drive seasonal rock glacier motion

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 sped up took place 72 to 84 days after spring snowmelt began and the second 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.

Why slope direction and solar radiation set the pace

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. The extra solar energy at Sourdough Peak warms the subsurface enough to let meltwater reach the layers where motion happens. At the colder, shaded site, the upper frozen layer acts as a barrier.

"Satellite radar lets us measure very small surface movements across mountain areas that are difficult to reach on foot," said Zhong Lu, professor in SMU's Roy M. Huffington Department of Earth Sciences and corresponding author on the study. "When we pair those measurements with hydrologic and thermal modeling, we can see how climate forcing interacts with local conditions like slope, aspect and solar radiation. That combination is what makes it possible to monitor rock glaciers and other permafrost landforms across Alaska year after year."

Monitoring permafrost across remote mountain regions

The findings matter beyond Alaska. Rock glaciers store water that mountain communities depend on, particularly in dry years, and most have never been measured directly. The SMU team's approach offers a way to monitor thousands of them across the world's high mountain regions without setting foot on the ice.

Sui and Lu co-authored the study with Jinwoo Kim, Vamshi Karanam and Kang Liang of SMU; Tyler M. Meng of Washington University in St. Louis; Bretwood Higman of Ground Truth Alaska; Chunli Dai and Emmanuel Junior Budukumah of the University of Florida; Sam McColl of Earth Sciences New Zealand; Ian Howat of The Ohio State University's Byrd Polar and Climate Research Center; and Chad Hults of the National Park Service's Alaska Regional Office.

The research was supported by NASA's Earth Surface and Interior Program and the U.S. Geological Survey Landslide Hazards Program. Computing was provided by SMU's high-performance computing cluster (ManeFrame III) through the O'Donnell Data Science and Research Computing Institute.