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| Afternoon sun on the flanks of Half Dome in Yosemite National Park. (C. Fredrickson Photography/Moment/Getty Images) |
Monoliths with names like El Capitan and Sentinel Rock, which rise thousands of feet above the meandering trails of the Yosemite Valley floor, keep watch like mythical titans of old.
It is unclear how long they have dominated the landscape; previous estimates range from tens of millions of years ago to just 15 million years ago.
The stunning granite skyline of Yosemite Valley is thought to have emerged over the course of the past ten million years, according to a new attempt to determine when it developed.
Additionally, the majority of the sculpting may have occurred as recently as the past 5 million years, according to a team of researchers led by geologists from the University of California, Berkeley.
According to UC Berkeley glaciologist Kurt Cuffey, "this upland surface that people are familiar with from parts of the Tioga Road and Tuolumne Meadows – that's a very old landscape."
"The inquiry is: What about the extensive gorge? Is that as well very old, or is it rather recent? Additionally, our major finding in our research was that it is relatively new. The beginning of the rapid incision may have occurred as far back as 10 million years, but the best guess is between 3 and 4 million years ago.
The Sierra Nevada, a mountain range formed by a chain of volcanoes caused by the collision of continental plates around 100 million years ago, includes the jagged peaks of Yosemite National Park.
The forces beneath the surface have continued to push the bedrock upward, while the elements have continued to polish the granite and remove the limestone and shale to create natural features renowned for their raw beauty around the world.
Yosemite Valley itself is home to a few monoliths that stand out because of their impressive facades. These near-vertical rock faces draw tourists, photographers, naturalists, and rock climbers who are willing to risk life and limb to reach them.
The sculpting of the rocks' flanks is a different matter entirely, whereas the rocks themselves are straightforward to examine for evidence of aging.
"When the granite was forming at depth, we know that the Sierra was a high mountain range 100 million years ago. According to Yosemite National Park geologist Greg Stock, "it was a chain of volcanoes that may have looked a little like the Andes Mountains in South America."
"Really, the question is whether the elevation has just been coming down through erosion since that time or whether it came down a little and then was uplifted again more recently," says the author.
Around the Sierra Nevada, there are numerous signs that both mechanisms are working, steepening slopes and making parts of the range more susceptible to weathering and erosion.
Geologists believe that Yosemite could have experienced similar phenomena tens of millions of years ago based on these events.
The senior author and UC Berkeley geochemist David Schuster developed a method that the researchers used to challenge preconceived notions; one that is based on evidence embedded within the minerals that make up the prominent rock formations rather than on models of a changing landscape.
The technique, which is known as helium-4/helium-3 thermochronometry, involves evaluating the helium-4 isotope, which slowly transforms into helium-3 over time.
This change sharply increases above 30 °C (approximately 86 °F), making it possible to approximate the rate of cooling of a rock sample distribution.
These results can give researchers an idea of how long a large rock has been above the warmer parts of the crust, similar to how a thermometer can tell how long a cake has been cooling out of the oven.
The researchers mapped a history of cooling across the vast expanse of igneous rock by thermochronometrying mineral samples from 16 locations within and around Yosemite Valley.
Now, we might imagine that over the course of tens of millions of years, a river carved a path for what would eventually become a valley in the Sierra Nevada geology.
Recurrent activity beneath the crust tilted the range about 5 million years ago, accelerating the flow of water and deepening the ravine.
Glaciers carved the valley floor and its rock walls with their icy chisels during the cooling of the climate over the past two to three million years, pushing the boundaries and shaping the familiar cliffs that dominate us today.
Every year, millions of people come to Yosemite and are awestruck by these massive monoliths. Now that they can see formations as old as the dinosaurs and as recent as humanity's own emergence in a single glance, those feelings of wonder may even intensify.
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