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2010 Rock Fall Year in Review

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avatar 2010 Rock Fall Year in Review
January 10, 2011 01:34PM
Although not as newsworthy as those of recent years, many significant rock falls occurred in Yosemite in 2010. The largest occurred on October 11 from the southeast face of El Capitan, midway up the cliff and along the path of Horsetail Falls. This was actually the largest in a series of rock falls from that area over several days. There were no associated injuries despite the fact that these rock falls occurred during the peak of the fall El Capitan climbing season. The cumulative volume of these failures was approximately 1,700 cubic meters (5,000 tons).

Another area of notable rock fall in 2010 was the Rhombus Wall immediately north of the Ahwahnee Hotel. This area was first active in August of 2009, with subsequent failures in September 2009. Quiet for the early part of 2010, the cliff experienced renewed activity beginning in August, with at least five rock falls radiating outward from the initial failure point between August and November 2010. The cumulative rock fall volume over this interval was 187 cubic meters (550 tons). This area of the Rhombus Wall is an impressive example of a progressive failure due to stress redistribution and crack propagation, and presents a unique opportunity to learn more about this complex process.

Ironically, the most serious rock fall of 2010 was also one of the smallest. On October 5, 2010, following several days of intense rain, local children Serra Weber, Carmen Ortiz, and Angel Ortiz were playing on the talus slope near the Church Bowl Picnic Area when a 1.7 cubic meter (5 ton) rock fell from low on the cliff. The rock fell directly onto Serra, pinning her and causing life-threatening injuries. Carmen and Angel ran to the nearby Yosemite Medical Clinic and notified the medical staff, who responded immediately. Serra was flown to a local hospital where she remained for several weeks. Serra recently returned to school in Yosemite Valley, where she, Carmen, and Angel were honored for their bravery during this event.

Other areas in Yosemite experiencing rock falls in 2010 include the Porcelain Wall (on the western shoulder of Half Dome), Glacier Point, Middle Brother, Middle Cathedral, Indian Canyon, and the Merced River Gorge. In all, there were 59 documented rock falls in 2010, with an approximate cumulative volume of 2,900 cubic meters (8,500 tons), more than half of which derives from the El Capitan rock falls. For comparison, the cumulative volume for 2009 was roughly 17 times greater, at about 48,120 cubic meters (142,000 tons); in that year the large volume was dominated by the March 2009 Ahwiyah Point rock fall. Small rock falls are much more likely to occur than large rock falls, but large rock falls represent a far more important source of rock fall debris. The database of rock falls and other geologic events in Yosemite, begun in 1857, now documents over 740 events, making it one of the longest and most detailed landslide databases in the world. (G.Stock-01-10-11)

It is very likely that there were additional rockfalls in 2010, but these events either were not witnessed or went unreported. 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 at greg_stock@nps.gov, or contact Park Dispatch. Predicting rock falls is not yet possible, but understanding the events that do happen is an important step toward this goal. For more information on rock falls and rock fall research in Yosemite, please see the Park’s web page: http://www.nps.gov/yose/naturescience/rockfall.htm
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