After the first chunk fell Wednesday afternoon, officials said, smaller pieces of rock broke away over the next several hours from the same part of El Capitan. The block that fell on Thursday landed in the same area.
“What happened Wednesday, it was a flake,” Mr. Gediman said. “Today was a chunk, a slab of rock.”
Joseph Moring said he was taking photos along the base of El Capitan on Thursday afternoon when he heard a crashing sound high above. He immediately started recording video and captured the tumbling cloud of dust and rock as it crashed down the wall, slamming to the ground about 600 feet in front of him.
Mr. Moring, who was visiting Yosemite with his wife from their home outside London, said he ran back to the car to check on his wife, who stayed there while he took photos.
“She said she felt the car shake like it was an earthquake,” he said. “I’m literally shook up.”
Ken Yager, the president and founder of the Yosemite Climbing Association, said he was driving in the park when he spotted a “giant dust cloud” that had begun creeping around a rock formation like a slow-moving fog. Eventually, Mr. Yager said, the cloud even briefly blocked out the sunlight.
“I’ve probably only witnessed clouds that big maybe six times,” said Mr. Yager, who has lived in or near the park for decades and has climbed El Capitan many times.
Yosemite documents an average of more than 80 rockfalls a year, but injuries from such events are rare. But El Capitan, which is about 3,000 feet tall, is one of the top destinations within the park — especially for adventurous rock climbers — making a rockfall there more likely to strike someone.
“The fact they are happening now is not unusual,” Mr. Gediman said about the rockfalls. “It’s just in an area with a lot of climbers.”
He said park rangers placed signs near El Capitan after the rockfall on Wednesday letting visitors know about it.
A 2013 report prepared by the United States Geological Survey and the National Park Service presents an inventory of 925 rockfalls, rock slides and other so-called slope movements that occurred from 1857 to 2011. Those events killed 15 people and injured at least 85, the report said.
Nicholas Sitar, a professor of civil engineering at the University of California, Berkeley, said very steep rock faces like El Captain are cut into blocks by what he called “regular fractures” or “joints.”
Freezing and thawing during the seasons cause more and more fractures develop over time and rock slowly breaks down, Dr. Sitar said. “Eventually, a block or a series of blocks become unstable and they fall,” he said in an email late Thursday.
Sometimes, Dr. Sitar said, it is possible to hear the cracking before a fall. But most of the time, there is no warning.
Beth Christensen, a geologist and the director of environmental studies at Adelphi University in Garden City, N.Y., noted that rocks are not solid throughout and have cracks — or “planes of weakness” — that tend to run parallel to a rock’s face.
“It’s kind of serendipitous as to which rock will fall when, but they’re all going to fall eventually,” she said. “You’re never safe standing under a rock face, but then again, you’re never safe getting into a car on the highway.”
Dr. Christensen said that it was possible that Thursday’s rockfall was related to the same crack that caused a slab to fall the day before — a further “expression of weakness in a plane,” she said.
She called rockfalls “random acts of nature” and said that people should be aware of their surroundings.
“In the same way you wouldn’t stand next to a building the day after it burned — you would think there would be some structural weakness — you should probably exercise that same caution in the natural world.”
Twenty to 25 people were killed in the park last year in drownings, falls and other accidents.
Yosemite, which includes about 750,000 acres, is among the most visited national parks in the country. It is about 150 miles east of San Francisco.
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