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 ccu.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 park in California's Sierra Nevada mountains, and no climbers were affected when it fell.