FRESNO, Calif. — A massive sheet of rock fell from the vertical face of Half Dome in Yosemite National Park, making one of the most popular routes attempted by climbers in North America even more challenging, park officials said Tuesday.

The granite sheet that peeled off measured roughly 100 by 200 feet. It was discovered by climbers who were attempting the route and had to turn back.

Half Dome draws climbers who insert cams into rock fissures for leverage. But it’s not likely climbers caused a piece of granite to fall off, the park geologist says. 2011 Associated Press file

“What used to be relatively easy climbing has gotten much more difficult,” park geologist and climber Greg Stock said.

Half Dome – an icon of the park that is visible throughout Yosemite Valley – is scaled by hundreds of experienced climbers each year using ropes and cams inserted into rock fissures for leverage.

However, climbers likely did not trigger the recent rock fall, Stock said.

He believes the culprit was the weather that heats and cools the brittle granite that can crack. In addition, he said, rain fell in Yosemite about the same time as the rock fell.

Yosemite is an active geologic zone. A rock equal in size fell this spring in Hetch Hetchy Valley and temporarily blocked a trail.

A deluge of boulders in 2008 fell on historic Curry Village, a camp of canvas and wooden cabins. The equivalent of 570 dump trucks of boulders hit 17 cabins, flattening one and sending schoolchildren scrambling for their lives.

The most recent rock fell from halfway up the sheer face of Half Dome, which rises more than 1,000 feet. A path for hikers using cable handrails was not changed.

There are several routes for climbers to scale Half Dome, but the one impacted when the rock fell ranks as one of the top 50 climbing destinations in North America, said Mike Gauthier, Yosemite’s chief of staff and an avid climber.

“Now is their chance to find a new work-around,” Gauthier said of Half Dome. “And they will.”

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