Peering Beneath a Source of El Capitan’s Deadly Rockfalls


El Capitan rises over 3,000 feet above the floor of Yosemite National Park in California. Scaling this granite edifice is considered a rite of passage among elite climbers, who come from around the world to test themselves on its sheer face.

But this towering behemoth is the site of frequent rockfalls. Over 20 have occurred in the last decade, including one in 2017 that killed a climber. The majority of these falls have been linked to rock formations known as flakes, sheets of rock that are peeling off El Capitan like layers of onion skin.

With infrared imaging, scientists have now essentially peered behind two of the largest flakes, Boot Flake and Texas Flake, to determine how well they’re connected to El Capitan. The results, presented at a meeting of the European Geosciences Union in Vienna in April, suggest that the underlying structures linking each flake to the 100-million-year-old granite are surprisingly small. By visualizing these attachment points, scientists can monitor them to keep climbers safe.

“This is a beautiful study,” said Allen Glazner, a geologist at The University of North Carolina at Chapel Hill not involved in the research. It shows how much “glue” is holding these rocks up, he said.

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Hundreds of flakes dot the face of El Capitan. They’re unnerving to look at because they appear so precarious. “You can’t tell what holds those things up there,” Dr. Glazner said.

Boot Flake and Texas Flake are among El Capitan’s largest flakes: Boot Flake is about a third the size of a tennis court, and Texas Flake is about two times larger. They’re both located roughly halfway up a popular climbing route called The Nose. Climbers ascending The Nose move around these flakes, sometimes even shimmying between Texas Flake and the underlying rock face using a technique called “chimneying.”

In October 2015, researchers set up a camera capable of thermal imaging in El Capitan Meadow and photographed Boot Flake and Texas Flake. In all of the frames, the flakes stood out — they were a few degrees colder than the surrounding rock. That was consistent with cool air circulating around the backsides of the detached parts of the flakes, the scientists reasoned.

But a closer inspection of the images revealed something unexpected. Small sections of each flake — near the center of Boot Flake and in the middle and lower part of Texas Flake — were slightly warmer than the rest of the formation. These thermal anomalies revealed the rock bridges where Boot Flake and Texas Flake are connected to the face of El Capitan, the researchers realized.

“We know that there are points of attachment,” said Greg Stock, a Yosemite National Park geologist and member of the research team. “But we’ve never been able to see them.”

Rock conducts heat, so intact rock bridges will show up as slightly warmer, said Antoine Guerin, a doctoral candidate in geology at the University of Lausanne in Switzerland, who led the study. “The detached part of the flake is cooler.”

Using models of how heat flows through granite, Mr. Guerin and his colleagues calculated that Boot Flake’s rock bridge was roughly 55 square feet in area, or about 6.8 percent of the flake’s total area. Texas Flake’s rock bridge was much smaller, the team inferred — only roughly 16 square feet in area, or about 0.8 percent of the flake’s total area.

These rock bridges are “scarily small,” said Dr. Glazner. “I would have expected there would be more holding that on.”

Boot Flake is stable, at least for now, the researchers say. But Texas Flake’s tiny rock bridge likely isn’t sufficient to glue it in place. This formation is probably held up by another feature, an intact rock attachment that runs along its base.

Brian Collins, a geotechnical engineer at the United States Geological Survey in Menlo Park, Calif., and a member of the research team, climbed The Nose in 2001. He remembers Boot Flake and Texas Flake, and now looks at them in a different light.

“When you’re standing on them, it’s really quite amazing to think that this is just perched on the side of El Capitan and for some reason it’s still there,” he said.



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