SAN FRANCISCO--A huge slab of granite that fell from a cliff at Yosemite National Park last year created a supersonic blast of air when it hit the ground, according to a new model presented here Friday at the fall meeting of the American Geophysical Union. The dramatic event, which knocked down more than 1000 trees, killing a hiker, has given scientists their most detailed look at the physics of air blasts from rockfalls.
Geologists knew the 10 July 1996 rockfall was unusual when it registered with the force of a magnitude-2.15 earthquake at seismic stations up to 200 kilometers away. Four separate chunks broke loose from one of Yosemite Valley's glacier-carved walls, a natural process called "exfoliation." The 80,000-ton main slab launched from a slope, then dropped 550 meters and landed near the Happy Isles Nature Center. Because there had been no scientific instruments nearby, geophysicist Meghan Morrissey of the Colorado School of Mines and her colleagues did some sleuthing to reconstruct the impact.
Morrissey used tornado studies to correlate the observed tree
damage with wind speeds. Then she devised a numerical model of the
rockfall, which suggested that thick clouds of dust had filled the
air from a smaller chunk that fell 14 seconds earlier. Close to the
impact, she found, the air blast exceeded 430 kilometers per
hour--faster than the speed of sound in such dusty conditions. Dust
from the blast helped strip bark and knock down trees, but it could
have been worse: If there had been no dust to absorb the main
blast's energy, the shock wave would traveled farther, causing more
widespread damage. She notes that a small ridge shielded the nature
center by deflecting most of the wind upward.
Yosemite-lovers need not give up their hiking. "Air blasts of this magnitude from free-falling rocks are extremely rare," says civil engineer Gerald Wieczorek of the U.S. Geological Survey in Reston, Virginia. He found records of only a half-dozen such events around the world in the last 2 centuries, including just one other at Yosemite--following an earthquake in 1872.
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