By Steve Gorman

LOS ANGELES (Reuters) - A 2,400-ton chunk of granite broke loose last week from the towering Half Dome formation in Yosemite National Park, altering one of North America's most popular rock-climbing routes, but no one was hurt and casual visitors will probably never notice.

The fallen slab, estimated to have measured about 800 cubic meters in volume, is believed to have crumbled from the middle the northwest face of Half Dome some time very late on July 2 or early July 3, park spokeswoman Jodi Bailey said on Wednesday.

No one saw the giant hunk of rock plunge to the valley floor in the heart of the park, located in California's Sierra Nevada range about 200 miles (320 km) east of San Francisco, and no climbers were affected when it fell.

Word has it a gap from the missing slab was discovered a day or two later by a group of mountaineers making their way up the northwest face of the iconic 4,700-foot-tall (1,432-meter) granite dome.

"This is probably one of the 50 classic climbs in North America," Bailey said, adding that the northwest route remains open with signs warning climbers they must navigate around the altered section due to loose remaining rock.

Park officials lack firm numbers for how many people scale Half Dome each year because no permits are required for technical rock climbing there. But one local mountaineering expert estimated that some 300 to 400 climbers ascend the northwest route annually, according to Bailey.

Hiking trails around Half Dome and the famed cable route up the back side have not been affected, and the general public is unlikely to detect any visual difference due to the immense scale of the granite formation as seen from the ground.

"There's already a boulder field at the base of it. You couldn't pick out the pieces that came from this slab," she said.

The uneven-shaped granite chunk that came down was about 2- to 3-feet (60 to 90 cm) thick and measured about 100 feet (30 meters) in length on one side and 200 feet (60 meters) along the other.

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Rock falls due to constant weathering are common in Yosemite Valley year-round, with events of this size occurring about once a year. But this one stood out because it sloughed off the face of Half Dome, Bailey said.

Heavy rains that swept Yosemite Valley on Thursday and Friday may have contributed to the latest fall, she said.

(Editing by Eric Walsh)