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El Capitan sits at over 3,000 feet above ground in Yosemite National Park, California. Scaling this granite building is considered a rite of passage by elite mountaineers from all over the world to test themselves.
But this imposing mastodon is the site of frequent landslides. More than 20 have occurred over the last ten years, including one in 2017 that killed a mountaineer. The majority of these falls were related to rock formations known as flakes, rock plates that stand out from El Capitan like layers of onion peel.
With infrared imaging, scientists have essentially focused behind two of the larger flakes, Boot Flake and Texas Flake, to determine how connected they are to El Capitan. The findings, presented at a meeting of the European Geoscience Union in Vienna in April, suggest that the underlying structures connecting each flake to 100 million year old granite are surprisingly small. By visualizing these attachment points, scientists can monitor them to ensure the safety of climbers.
"It's a great study," said Allen Glazner, a geologist at the University of North Carolina at Chapel Hill not involved in the research. This shows how much "glue" holds these rocks, he said.
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Hundreds of flakes dot the face of El Capitan. They are confusing to watch because they seem so precarious. "You can not know what holds these things back there," Dr. Glazner said.
Boot Flake and Texas Flake are among the biggest flakes of El Capitan: Boot Flake is about one-third the size of a tennis court and Texas Flake is about twice as big. They are both located roughly halfway up a popular climbing route called The Nose. Climbing climbers, The Nose, move around these flakes, sometimes even scintillating between Texas Flake and the underlying rock wall using a technique called "chimney".
In October 2015, researchers installed a camera capable of thermal imaging at El Capitan Meadow and photographed Boot Flake and Texas Flake. In all the frames, the flakes came out – they were a few degrees colder than the surrounding rock. This was consistent with the cold air flowing behind the loose parts of the flakes, according to the scientists.
But a closer inspection of the images revealed something unexpected. Small sections of each flake – near the center of Boot Flake and in the central and lower parts of Texas Flake – were slightly warmer than the rest of the training. The researchers realized that these thermal anomalies revealed the rock bridges where Boot Flake and Texas Flake are connected to El Capitan's face.
"We know that there are points of attachment," said Greg Stock, geologist of Yosemite National Park and member of the research team. "But we have never been able to see them."
As the rock conducts heat, the intact rock bridges will therefore appear slightly warmer, said Antoine Guérin, a PhD candidate in geology at the University of Lausanne in Switzerland, who had led the study. "The detached part of the snowflake is cooler."
Using models of heat flow in granite, Guerin and his colleagues calculated that Boot Flake's rock bridge had an area of about 55 square feet, or about 6.8 percent of the total area. total area of the flake. The team deduced that the Texas Flake Rock Bridge was much smaller: its area was only 16 square feet, or about 0.8% of the total flake area.
These stone bridges are "scary," Dr. Glazner said. "I was expecting it to be over."
Boot Flake is stable, at least for now, according to the researchers. But the little Texas Flake stone bridge is probably not enough to stick it. This formation is probably maintained by another characteristic, an intact rock fixation 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 looks at them now in a different light. .
"When you stand on them, it's really amazing to think that this is just perched on the side of El Capitan and for whatever reason he's still there," he said.
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