How old is California’s Yosemite Valley? First-time visitors to Yosemite Valley stare in astonishment at El Capitan's sheer granite wall and Half Dome's cleanly sliced face, understanding, perhaps vaguely, that it must have taken rain and glaciers a long time to cut and shape that scene. But how lo
In 2010, two of the study’s authors — David Shuster of UC Berkeley and Yosemite Park geologist Greg Stock — collected rock samples from the rim of Yosemite Valley, where the granite has eroded only slowly over the last 50 million years. In the distance is the upper Merced River valley. Former UC Berkeley graduate student Johnny Webb is at the center.
While the scientists are unable to be more precise, the new estimate is the first to be based on an experimental study of the granite rocks in and near Yosemite, rather than on inferences based on what was going on elsewhere in the Sierra Nevada. That uplift, which happened at the same time that earthquake faulting in the eastern Sierra Nevada created an escarpment several kilometers high, steepened the western slopes and rivers, causing them to incise valleys more quickly.
Shuster’s technique, called helium-4/helium-3 thermochronometry, reconstructs the temperature history of a sample of rock based on the spatial distribution of natural helium-4 in minerals, which is measured by comparison to an artificially-produced uniform distribution of helium-3. Because temperature increases with depth underground, the temperature history can tell when a rock was uncovered as the landscape eroded.
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