by Julia Rosen Monday, April 20, 2015
This past winter, two athletes grabbed the world’s attention as they climbed up the sheer walls of Yosemite National Park’s El Capitan. But some geologists are more concerned with what falls down these precipitous faces — namely, rocks. Park geologist Greg Stock has developed a history of rockfalls in Yosemite valley dating back 15,000 years, which reveals how its cliffs have crumbled since glaciers disappeared at the end of the last ice age.
Stock focused on the great piles of talus that lie at the feet of Yosemite’s cliffs. Using a digital elevation model to estimate the shapes of cliff bases — which often lie buried beneath rubble — he calculated the total volume of fallen rock in the valley: a staggering 180 million cubic meters of gritty granite.
This means Yosemite’s 53 square kilometers of vertical faces have retreated 3 to 4 meters since deglaciation, Stock reported at the annual meeting of the American Geophysical Union last December in San Francisco. Annually, the valley’s walls shed about 12,000 cubic meters of rock, causing them to recede about 0.2 millimeters on average, which Stock suggests may reflect the rate at which granite layers flake off in a process known as exfoliation. This long-term geologic rate agrees with the historic trend witnessed since 1857.
Stock also investigated the size and frequency of individual rockfall events occurring in the last 15,000 years. He found that small events happen frequently but don’t contribute much to the total talus volume, whereas large events, possibly triggered by earthquakes, occur rarely but account for 20 percent of the debris.
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