A massive sheet of rock, outlined in red on this photo, recently peeled off Half Dome in Yosemite National Park/NPS, Greg Stock

A massive triangular slab of rock, roughly two-thirds the length of a football field in size, recently peeled off the face of Half Dome, adding a challenging new dimension to the classic Regular Northwest Face climbing route while also underscoring that Yosemite National Park's granite monoliths are not at all static.

Exactly when the slab fell away from the iconic dome isn't known, but Yosemite staff geologist Greg Stock guesses it most likely occurred sometime overnight July 2-3.

“It's somewhat surprising for a rock fall of this size and prominent location that we don’t actually know when it occurred," he said Wednesday during a phone call. "But there are several things suggesting that it occurred early in the morning on July 3. The first thing is that this is peak season for climbing the face of Half Dome. Normally there would be lots of climbers up there on this climbing route, but as far as we know there was nobody up there at this time."

The other factor was that the park was experiencing "intense thunderstorm activity" last Thursday into Friday. The storms were most intense "in the middle of night," said Stock. "Since nobody saw it, it probably occurred in the middle of the night."

The rockfall was revealed by a climbing party that attempted the popular route on July 5 and had to retreat.

"Just got a call from a buddy who is currently at the top of pitch 11 (pitch above the Robbins traverse) of the regular route on Half Dome and he says the ledge there is missing!" Dave Miller wrote in a post on Supertopo. "Anyone heard of recent rock fall on HD? He's done the route before so I kinda trust what he's saying, as outlandish as it sounds.

"Apparently they bailed cause they couldn't reach the anchor which is still there but with no ledge below it," the climber added. "He said there was a big dirt outline where the ledge was supposed to be. He also mentioned the area around the base of HD looked kinda gnarly but he just thought it was from the recent rain storms. Will be interested to find out what's going on up there, that's one of my favorite routes in the world."

This route up Half Dome is considered one of the top 50 classic climbs in North America, according to Mike Gauthier, Yosemite's chief of staff. Its first ascent was recorded in 1957, according to Climbing Magazine.

Yosemite Valley is well-known for its rockfalls. A massive fall in the Yosemite Valley in October 2008 convinced park officials to close 233 visitor accommodations, or roughly one-third of the available housing in Curry Village. Also closed were a shower house, restrooms, and 43 concessioner employee housing units.

That decision paid off in February 2014, when a rockfall rained boulders down onto areas where cabins once stood. A boulder with a volume of about 1 cubic meter impacted within the footprint of a former wooden cabin and then came to rest within the foundation of another.

"We’ve had about 10 rockfalls off the face of Half Dome in the last 10 years," Stock said. "So on average, one a year from somewhere on the face. This is probably the largest in the last 10 years, but there was one in 2006 that was also substantial. I really think in many ways that this is common. This is what happens in Yosemite.”

While freeze-thaw cycles are one cause of rockfalls, heavy rains also can lead to them, according to the geologist.

“Water getting into cracks in the granite tends to elevate pressures, it creates a hydraulic head" that can generate enough pressure to cause a break, he explained.

“It’s a pretty typical rockfall for the valley," said Stock. "It’s almost sort of textbook, it’s an example of exfoliation, the peeling away of a thin layer of rock from the face.”