Geologists: Yosemite’s Unpredictable Rock Slides are Rarely Deadly

YOSEMITE -- After two days of rock slides on El Capitan in Yosemite, officials are keeping visitors clear of the area.

The first in a series of rock slides starting Wednesday killed a climber from England and severely injured his wife.

There are around 80 rockfalls a year in the park, and many others that go unreported.

Despite the imposing rock formations above the valley, a pattern of erosion has been going on for tens of thousands of years. As formations are being pushed up, the weathering process wears them down.

Mike Oskin, a University of California, Davis geologist, said it's no surprise that another rock slide came a day after the first one.

Yosemite is known for its waterfalls, but water seeping and freezing into vertical cracks help break up the toughest granite over time. A before and after picture on El Capitan shows how slabs weighing thousands of tons can let loose without warning.

A few decades of climbing, or a millisecond in geological time, probably has had no effect on erosion.

The park has actually done detailed surveys on where rockfalls are likely to occur using sophisticated laser and electronic scanning. Yet, predicting when they will occur is like predicting an earthquakes -- you can’t.

Since the park opened in 1857 there have been 16 deaths and 100 injuries caused by falling rocks. That’s not a lot, considering Yosemite gets 4 million visitors a year. Despite this week's tragedy, its a risk frequent visitors like Kathy McBride are willing to live with.

“It’s that kind of random kind of stuff that happens here in the park... doesn’t seem to have dissuaded people from visiting," she said. "No, it’s beautiful out here. You can’t let that stop you."

Just as many Californians live with the possibility of earthquakes, Yosemite visitors will have to live with the occasional rock slide.

"Rockfall in Yosemite is something that’s always going to be there," said Chris Wills, a geologist with the California Geological Survey. "You can minimize it by being aware of your surroundings, but you can’t eliminate it completely.

The park is still open to visitors, even though some roads in the valley are still closed.