Wednesday, August 26, 2009

Geologic Change in Yosemite: Remembering the Happy Isles Slide of 1996

I was looking for something else in the massive pile of old slides and photographs at the department today, and ran across a pair of photographs of taken a few weeks after the Happy Isles Rockfall of 1996, one of the more damaging historical mass wasting events in Yosemite Valley. The pictures recall a flurry of posts last spring about the Ahwiyah Rock slide near Half Dome. The 1996 event killed one person and severely injured another, destroyed a food stand, and damaged the Happy Isles Nature Center.

The rock involved in the event was a slab of exfoliated granitic rock roughly the size of a football field that broke off from the top of the cliff near Glacier Point, slid for several hundred feet and then fell 1,800 feet, hitting the floor of the valley at a speed of something over 200 mph. The rocks exploded on impact generating an airblast that toppled 1000+ trees. U.C. Berkeley has an excellent analysis of the slide here, and formal article from the GSA Bulletin can be found here.

Regrowth of trees and shrubs has obscured much of the damage, but the Happy Isles Nature Center has a nice exhibit and viewpoint; walk out the back door of the museum to find it. The slide was not the first in the area. Huge talus piles cling to slopes below the breakaway point, and a much larger prehistoric slide has been mapped beneath the thick forest. It forced the Merced River into a new channel, and contributed to the formation of the marshy area west of the campground.
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