Park officials have confirmed that a recent rockfall at Yosemite National Park was in fact the largest such documented event in the park in more than two decades. As a result of the rock and other debris, the southern portion of the Mirror Lake loop trail is closed to hikers indefinitely.
According to a recent park report,
On March 8th, a large rockfall occurred from Ahwiyah Point near Half Dome. Rocks fell roughly 1,800 feet, knocking down hundreds of trees and burying hundreds of feet of trail on the southern portion of the Mirror Lake loop trail. The impact generated ground shaking equivalent to a magnitude 2.4 earthquake. Numerous smaller rockfalls have occurred from Ahwiyah Point since then. There were no injuries or structures affected.
The approximate volume of the initial rockfall was 43,000 cubic meters, or 115,000 tons. This is considerably larger than the 1996 Happy Isles rockfall, which was estimated at 30,000 cubic meters in volume. Therefore, this rockfall is the largest one in Yosemite National Park since the 1987 Middle Brother event.
That Middle Brother event was the largest recorded rock fall in historical times in Yosemite. It deposited an estimated 1.5 million tons of debris at the base of Three Brothers, and closed Northside Drive for several months.
Rockfalls are nothing new in Yosemite, and these natural events shouldn't be a surprise, given the dramatic geology of the park.
Historical records indicate that at least 600 rock falls have occurred in the park over the last 150 years. Massive piles of “talus” or rock debris at the base of Yosemite Valley’s cliffs are reminders of these dramatic events.
Does last month's significant event signal the start of a more active cycle of falling rocks in the park? The park staff says
Because of the most recent rockfall activity around Yosemite Valley, there has been speculation that rockfall has become more frequent. Based on historical databases and recent events, park geologists are unable to discern a geologically significant increase in rockfall activity in Yosemite Valley
Natural processes like rockfalls help to create the beautiful and changing scenery in Yosemite National Park, but they also present potential hazards. The park website includes additional information about rock falls, including safety tips for visitors in this geologically active park.
A map of 150 years of Yosemite Valley rockfalls (from 1857-2007) is available on the park's website.