Early last Saturday morning, March 28, a huge rockfall broke lose near the summit of Ahwiyah Point near Half Dome in Yosemite National Park. The rockfall, occurring at 5:26AM, dropped 1,800 feet to the floor of Tenaya Canyon in seconds, knocked down hundreds of trees, and buried the south part of the popular Mirror Lake trail. The impact of this major geologic event registered on seismometers around California as a magnitude 2.5 earthquake.
Park geologist Greg Stock reported on Supertopo: ďThe volume of the Ahwiyah Point rock fall is still being determined, but it was clearly one of the largest rock falls in the past decade; for perspective, this rock fall was many times the size of the recent October 2008 rock falls behind Curry Village.Ē
Yosemite Valley is a major area of active rockfall, and, because of its accessibility, is one of the most studied rockfall areas in the world. Yosemite rockfalls are caused by a wide variety of factors and natural processes, including earthquakes, precipitation like heavy downpours, freeze and thaw cycles, thermal stresses, and gravity. In the last few years, Half Dome and the immediate surrounding area has been particularly active, with at least eight rockfalls between June 2006 and June 2007.
One of the largest rockfalls in the past couple decades occurred on July 10, 1996 when over 75,000 tons of granite fell 2,000 feet at 250 miles per hour from Glacier Point high above the southern edge of Yosemite Valley. This massive rockfall killed a hiker, injured six others, damaged the Happy Isles Nature Center, destroyed a snack bar, and filled the air with thick dust and grit, some of which settled in inch-deep layers on nearby picnic tables. Geologists later wrote in a report, "Dust from the cloud rose rapidly into the air and plunged the area near Happy Isles into darkness for some minutes." The air blast from the rockfall, equal to that of a tornado, toppled over a thousand trees, some a half-mile away from the impact zone.
Itís a fact of climbing and geology that rockfall happens. Rock, which we think of as a solid enduring medium, has a shelf life. When we climb, we always have to be aware of the potential for rockfall. To learn more about rockfall and loose rock for climbers, check out this section on Avoiding Loose Rock.
Photograph above: A massive rockfall from Glacier Point in June 1998. Photograph courtesy Marshall Minobe/SuperTopo.com