Steep mountain slopes have surprisingly long lifetimes

The critical threshold angle, also known as the angle of repose, is the point at which slopes should not be. In the Himalayas and in a national park, you can see these steep slopes. How long the slopes stay that way is one of the questions that some geologists ask.

The rock mass in these oversteepened parts are prone to fail. Gen Li is a geology professor at the University of Santa Barbara.

The residence times, or "lifetimes," of oversteepened slopes are explored in a new study. The study combines 20-year records of landslides, long-term uplift rates, and models of erosion to estimate the lifetime of slopes. They looked at the idea of "excess" rock, the volume of rock that extends beyond that critical threshold angle, which varies between 29 and 39 in their study region. The researchers found something unexpected when they calculated the lifetimes of slopes with different amounts of excess rock.

The slopes are steep for a long time. The average life span for all oversteepened slopes in the study area was 61,000–157,000 years, but very steep slopes have a life span that is much longer.

It's an interesting observation that the slopes can stay longer.

The result is incomprehensible at the surface. Competition between rock mass growth and removal resulted in the longer lifetime. As the rock mass grows and the failure increases, the chunks of rock and soil are removed. The result is a long-lived steep slope because rock removal can't keep up with the amount of rock mass that accumulates.

Scientists who study cliff faces will be helped by the new findings. "We're not yet to the point where we can say when a specific hillslope will fail," said Seulgi Moon, a geomorphologist at the University of California, Los Angeles, and a study coauthor.

More information: Gen K. Li et al, Residence Time of Over‐Steepened Rock Masses in an Active Mountain Range, Geophysical Research Letters (2022). DOI: 10.1029/2021GL097319 Journal information: Geophysical Research Letters

The story was re-posted by the Americangeoscience Union. The original story can be read here.