The work, presented online May 27 in Nature Geoscience, shows that once the angle of a slope exceeds 30 degrees – whether from uplift, a rushing stream carving away the bottom of the slope or a combination of the two – landslide erosion increases significantly until the hillside stabilizes.
"I think the formation of these landscapes could apply to any steep mountain terrain in the world," said lead author Isaac Larsen, a University of Washington doctoral student in Earth and space sciences.
The study, co-authored by David Montgomery, a UW professor of Earth and space sciences and Larsen's doctoral adviser, focuses on landslide erosion along rivers in the eastern Himalaya region of southern Asia.
The scientists studied images of more than 15,000 landslides before 1974 and more than 550 more between 1974 and 2007. The data came from satellite imagery, including high-resolution spy satellite photography that was declassified in the 1990s.
They found that small increases in slope angle above about 30 degrees translated into large increases in landslide erosion as the stress of gravity exceeded the strength of the bedrock.
"Interestingly, 35 degrees is about the same angle that will form if sand or other coarse granular material is poured into a pile," Larsen said. "Sand is non-cohesive, whereas intact bedrock can have high cohesion and should support steeper slopes.