Why are tons of rocks falling in Yosemite?

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Rockfalls are not uncommon.

But this week, the death of an English hiker and the injuries of two others at Yosemite National Park because of massive rockfalls from El Capitan, have reignited the question: Why are rocks falling in Yosemite National Park -- the quintessential summer outdoors spot for tourists living in California and beyond.

The answer in many cases: Hot weather, preceded by water that has seeped into cracks and frozen, causing the cracks to grow.

Heat not only moves the rock, it deforms it, the geologists described. Using lasers, the geologists observed a phenomenon known as thermal bowing, in which the center of the slab bulges outward as the exterior heats up while the interior remains cool. As the cracks get longer, the stress on the points where the slab is still attached grows larger. This further separates the slab from the cliff. El Capitan is a granite monolith on the north side of the park, which 4 million people visit each year. It's among the favorite climbing spots in the world.

This winter has been especially rainy, and this summer has experienced many triple-digit days.
Although many conditions can trigger rockfalls,  some are more likely to happen in the hottest part of the day, during the hottest part of the year, according to a joint study by Brian Collins of the US Geological Survey and Greg Stock of the National Park Service.

“Our research provides clear evidence that thermal effects can move large slabs of rock and that these movements, over time, can lead to rock falls,” he said in a study that was published in 2016 in Nature Geoscience.

However, it wasn’t immediately clear if hot weather alone contributed to the death of a man this week. Sometimes, rocks fall and no one knows why.

Stock told KTVU on Friday that in this case, scientists will have to study if in fact it was heat and water that caused the two days of rockfalls.

The temperature in Yosemite Valley was 81 degrees on Wednesday when 32-year-old Andrew Foster of Wales, England and his wife were hiking at the bottom of El Capitan when a chunk of granite about 12 stories tall broke free and plunged down. Foster died, and his wife remains hospitalized. Foster is the 17th person to die by a rock fall in Yosemite, according to USGS data, which dates back to 1857.

And on Thursday, the mercury was 80 degrees when another rockfall occurred, this one 10 times the size of the 1,3000 tons that fell the day prior. One person was injured in that case.

Geologists have been documenting rock falls in Yosemite since 1857. Deaths occur, but they are not common. Including Foster’s death, 17 people have died, and 87 have been injured, from rock falls in that area.

The last time a climber was killed by a rock falling at Yosemite was in 2013, when a Montana climber fell after a rock dislodged and sliced his climbing rope. It was preceded by a 1999 rock fall that crushed a climber from Colorado.

The park noted that rockfall activity in 2016 was slightly lower than in previous years, with 58 documented rockfalls and slides. One of the more major rockslides was on Jan. 9, when 270 tons of rock slide onto the El Portal Road. In February, about 3,000 tons of rock fell at Middle Brother.

And yet for all this science, Stock noted that for any given rockfall, there is always a large degree of uncertainty about what exactly triggered it. Historical records indicate that more than half of all documented rockfalls in Yosemite were not associated with any recognizable trigger.

As of Thursday, Northside Drive was closed from Camp 4 to El Capitan Crossover, with a detour available on Southside Drive, which was set up for two-way traffic.