AIR BLAST CREATED BY FALLING BOULDER A FIRST.

Byline: Stuart Gordon Modesto Bee

Rock slides in Yosemite Valley are about as common as bears, but few have triggered a devastating explosive effect like that from the 300-foot slab of solid granite that slammed to earth below Glacier Point on Wednesday.

``What makes this different is that it was such a large slab falling so far that it created an air blast which we haven't seen before,'' said National Parks Service spokesman James Snyder, who co-authored a report on Yosemite Valley rock slides for the U.S. Geological Survey.

Hundreds of trees were knocked over like bowling pins by a combination of an explosive air blast created ahead of the falling slab and the shock wave it caused when it smacked into the ground, scientists and engineers said.

Snyder estimates the chunk of granite weighed several thousand tons and fell nearly 2,000 feet from the almost sheer cliff between Washburn and Glacier points.

The face of the granite walls of Yosemite Valley are constantly ``peeling,'' geologists said. ``They are very unstable and unpredictable. About the only thing you can predict is that slides are going to occur,'' Snyder said.

Rarely, though, do they result in death. On Nov. 16, 1980, a rock slide killed three people near Yosemite Falls. During the 1920s, Snyder said a rock slide killed one person. Two others may have lost their lives in the early years after Yosemite became a National Park, Snyder added.

But for the most part, he said, slides go unnoticed by tourists and only attract the attention of curious geologists and park rangers.

Usually, the slabs of rock hit the cliff's face and break up as they fall to earth. Wednesday's slide was unusual because it appears the huge slab remained intact until it was pulverized when it hit the ground.

University of the Pacific geology professor Curtis Kramer said visitors to Yosemite Valley can see ample evidence of past rock slides at the base of the granite walls, the result of a process scientists call exfoliation.

``The base of all the slopes have a large amount of talus,'' he said. Talus is the term used to describe the fractured granite debris, remnants of larger slabs that plunged from the cliffs above.

``Sometimes the slides are triggered by earthquakes,'' Kramer said. ``The freezing and thawing of water in fractured joints of the exposed granite, which is constantly expanding, causes large slabs of rock to peel off and slide down the slopes at velocities of well over 100 miles per hour. In the natural history of the area, it's not an unusual event.''

Much of Yosemite Valley's natural beauty is a result of the process of exfoliation, he noted.

David Fletcher, chairman of UOP's division of Civil Engineering, likened the air blast created by Wednesday's rock slide to a bomb explosion.

``Pick up a book and keep one hand on the desk. When you drop the book, the hand on the desk can feel the air rushing out from beneath the falling book,'' he said. ``Now imagine the rush of air being squeezed out from underneath that massive falling rock in Yosemite. It must have sent out a high pressure air wave of tremendous force.''

Just as the blast from a sonic boom can rattle windows, he said, the momentum of highly pressurized air can uproot or snap mature trees like they were toothpicks.