Expert: Yosemite Rockfalls Due To Location In Sierra Nevada

Published: Wednesday, October 4, 2017 - 3:37pm
Updated: Friday, October 6, 2017 - 2:17pm

In a check of regional news, the El Capitan monolith in Yosemite National Park experienced two massive rockfalls last week, one of which claimed the life of a British tourist at the base of the cliff.  One of the pieces of rock that sheared off was described as “as big as an apartment building."

Ryan Porter, associate professor of geophysics at Northern Arizona University, says this is due to Yosemite being located within the Sierra Nevada, which are large blocks of granitic rock formed deep within the Earth. Over time these rocks rise to the surface because the pressure on them reduces.

“As the temperature heats up and cools down throughout the day it actually causes the rocks to contract and expand, which can actually further make those cracks bigger," Porter said.

The rock that breaks off as a result of the cracks expanding is known as exfoliation.

Ken Yager, president of the Yosemite Climbing Association has made more than 50 climbs of El Capitan. He’s seen a number of rockfalls especially at this time of year.

“The massive ones happen in August, September into October and I can compare it to an earthquake. You get a big earthquake and then you’re going to have a lot of little aftershocks afterward," Yager said.

Despite the rockfall, it’s peak climbing season in Yosemite and park officials say there's no more danger than usual. As for El Capitan and the possibility of it going away, Porter said, “probably not in our lifetime but if we were to fast forward hundreds of thousands to millions of years it’s likely to look very different than it does now.”

Over the last century, according to news and park reports, rockfalls at Yosemite have resulted in 17 deaths, 85 injuries and damage to buildings, roads and trails.  

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