The face of El Capitan after a rockslide on 27 September 2017 killed one person. Photo: Reuters

By Rachel Gutman 
2 October 2017

(The Atlantic) – Last Wednesday and Thursday, there were two major rockfall events at Yosemite’s El Capitan, a rock formation extremely popular with climbers. Wednesday’s rockslide killed one person, the first rockfall-related fatality in the park since 1999. Thursday’s released a volume of rock larger than 10,000 cubic meters, about four Olympic swimming pools’ worth of rock.

According to Roger Putnam, a climber and geologist at Moorpark College, these rockslides were not unusual from a geologic standpoint. In fact, he said, the quick succession of rockfalls “perfectly shows how a small rockfall happens and then—boom—that causes another rockfall, and another.”

Rockfalls are par for the course at Yosemite, where the National Park Service estimates 80 events happen every year. But despite their frequency, there is a possibility that warming temperatures and an unstable climate could cause even more rockfalls at Yosemite and worldwide.

Rockslides happen in rock with existing weaknesses, like large cracks, after they become unstable as a result of a trigger. Roy Sidle, a professor of geography at the University of the Sunshine Coast, said in an email that those triggers fall into four major categories: “freeze-thaw action, wetting and drying, temperature changes, and (of course) human disturbances (even rock climbing).”

Putnam said that, until recently, most geologists thought freeze-thaw cycles, also called frost wedging, caused the majority of rockfalls. The idea was that water seeped into cracks during winter rainstorms, then expanded when it froze—the same process that forms potholes. Then, in 2016, Yosemite’s park geologist published a paper in Nature Geoscience that found most rockslides in Yosemite happen on hot, sunny days, not in freezing temperatures. [more]

Will Climate Change Make Rockslides Worse?

ABSTRACT: Exfoliation of rock deteriorates cliffs through the formation and subsequent opening of fractures, which in turn can lead to potentially hazardous rockfalls. Although a number of mechanisms are known to trigger rockfalls, many rockfalls occur during periods when likely triggers such as precipitation, seismic activity and freezing conditions are absent. It has been suggested that these enigmatic rockfalls may occur due to solar heating of rock surfaces, which can cause outward expansion. Here we use data from 3.5 years of field monitoring of an exfoliating granite cliff in Yosemite National Park in California, USA, to assess the magnitude and temporal pattern of thermally induced rock deformation. From a thermodynamic analysis, we find that daily, seasonal and annual temperature variations are sufficient to drive cyclic and cumulative opening of fractures. Application of fracture theory suggests that these changes can lead to further fracture propagation and the consequent detachment of rock. Our data indicate that the warmest times of the day and year are particularly conducive to triggering rockfalls, and that cyclic thermal forcing may enhance the efficacy of other, more typical rockfall triggers.

Rockfall triggering by cyclic thermal stressing of exfoliation fractures



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