El Capitan in Yosemite National Park
Sierra Sun Times file photo
January 11, 2021 - 2020 proved to be a relatively mild year for rockfalls in Yosemite. Thirty-four rockfalls were documented in 2020, with a cumulative volume of about 3,120 cubic meters (9,285 tons). Both metrics are well below recent averages. The lower numbers may be due in part to below-average precipitation in 2020 (rainfall is a common rockfall trigger) but they more likely result from under-reporting. COVID-19 related access restrictions throughout most of 2020 meant that there were fewer people present to witness and report rockfalls.
The two largest rockfalls of 2020 happened in the summer. The first occurred at 7:14 pm on June 20, when a large exfoliation slab fell from the “Porcelain Wall” just west of Half Dome. Hundreds of park visitors around Mirror Lake watched as the slab, about 1,040 cubic meters in volume (nearly 3,100 tons), toppled from the wall and impacted a ledge below, exploding into thousands of fragments and generating a large dust cloud that filled Tenaya Canyon. Although spectacular, the rockfall was fortunately not consequential, as rock debris did not make it as far down as the Mirror Lake loop trail.
Another large rockfall occurred just two weeks later, when about 940 cubic meters (2,80 tons) of rock fell from low on Middle Brother at 8:42 am on July 4. Again, the rockfall was witnessed by hundreds of visitors enjoying the holiday. The rockfall created a dust cloud but did not damage infrastructure in the area. Four smaller rockfalls occurred from the same location later that night. This location has been intermittently active since 2016, demonstrating a progressive pattern that is relatively common in Yosemite. The Porcelain Wall and Middle Brother rockfalls both occurred on hot days, suggesting that heat may have triggered the rockfalls by thermal expansion of partially detached exfoliation slabs.
Other substantial rockfalls in 2020 occurred from El Capitan, Yosemite Falls, Glacier Point, and the Cascade Cliffs above Little Yosemite Valley.
If you witness a rockfall of any size, encounter fresh rock debris, or hear cracking or popping sounds emanating from the cliffs, please contact park geologist Greg Stock at 209/379-1420 or by email at greg_stock@nps.gov, or contact Park Dispatch by dialing 911 within the park. Documented rockfalls are added to the park database (http://pubs.usgs.gov/ds/746/), enabling long-term evaluation of rockfall activity to improve public safety.
Source: G. Stock/NPS
