The Captain rises more than 3,000 feet above the floor of Yosemite National Park in California. Climbing this granite building is considered a rite of passage among elite climbers, who come from all over the world to prove themselves on its steep face.
But this imposing giant is the site of frequent rock jumps. More than 20 occurred in the last decade, including one in 2017 that killed a climber. Most of these falls have been related to rock formations known as scales, sheets of rock that are falling off El Capitan as layers of onion skin.
With infrared imaging, scientists have now essentially looked behind two of the larger flakes, Boot Flake and Texas Flake, to determine how well they are connected to El Capitan. The results, presented at a meeting of the European Geosciences Union in Vienna in April, suggest that the underlying structures that link each scale to the 100-million-year-old granite are surprisingly small. By visualizing these junctions, 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 who was not involved in the research. It shows how much "glue" holds these rocks, he said.
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Hundreds of snowflakes dot the face of El Capitán. They are disconcerting to look at because they seem very precarious. "You can not say what holds those things up there," said Dr. Glazner.
Boot Flake and Texas Flake are among El Capitan's largest flakes: Boot Flake is about one-third the size of a tennis court, and Texas Flake is about twice as large. Both are located approximately halfway along a popular climbing route called The Nose. Climbers who ascend The nose moves around these scales, sometimes it even moves between Texas Flake and the underlying rock using a technique called "chimneys".
In October 2015, the researchers installed a camera capable of taking thermal images in El Capitan Meadow and photographed Boot Flake and Texas Flake. In all the frames, the flakes stood out: they were degrees colder than the surrounding rock. That was consistent with the fresh air that circulated in the back of the flaked parts of the flakes, the scientists reasoned.
But a closer inspection of the images revealed something unexpected. The small sections of each scale, 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 investigations revealed that the thermal anomalies revealed the rock bridges where Boot Flake and Texas Flake are connected to the face of El Capitán.
"We know there are attachment points," he said. Greg Stock, geologist of Yosemite National Park and member of the research team. "But we've never been able to see them."
The rock conducts the heat, so the intact rock bridges will be slightly warmer, said Antoine Guerin, a Ph.D. in geology at the University of Lausanne in Switzerland, who led the study. "The detached part of the scale is colder."
Using models of how heat flows through the granite, Mr. Guerin and his colleagues calculated that the Boot Flake rock bridge had an area of approximately 55 square feet, or approximately 6.8 percent of the total area of the scale. The Texas Flake rock bridge was much smaller, the team inferred: it only had approximately 16 square feet of area, or about 0.8 percent of the total area of the scales.
These rock bridges are "terribly small," said Dr. Glazner. "I would have expected there to be more enduring that."
According to the researchers, Boot Flake is stable, at least for now. But the small Texas Flake rock bridge is probably not enough to stick it in place. This formation is probably sustained by another feature, an intact rock fixture that runs along its base.
Brian Collins, a geotechnical engineer from the United States Geological Survey in Menlo Park, California, and a member of the research team, joined The Nose in 2001. He remembers Boot Flake and Texas Flake, and now looks at them from a different perspective. .
"When you're standing over them, it's really amazing to think that this is only next to El Capitán and, for some reason, it's still there," he said.