First-time visitors to Yosemite Valley look in awe at El Capitan’s sheer granite wall and the neatly carved face of Half Dome, perhaps vaguely aware that rain and glaciers must have taken a long time to carve and shape that landscape. to sculpt. But how long?
Did it all start 50 million years ago, when the granite that cuts the valley was first exposed to the elements? Was it 30 million years ago when data suggests canyons in the southern Sierra Nevada started forming? Did the valley only begin to form after the Sierra tilted west about 5 million years ago, or was it mainly due to glaciers that formed in a cool climate 2 to 3 million years ago?
Geologists at the University of California, Berkeley, used a new technique of rock analysis to get a more accurate answer, concluding that much of the impressive depth of Yosemite Valley has been carved out since 10 million years ago, and most likely even more recently — in the past 5 million years. This shaves off about 40 million years from the oldest estimates.
Rivers performed the first carving in a pre-existing shallow valley, they determined, and then both rivers and ice contributed recently.
While the scientists couldn’t be more accurate, the new estimate is the first to rely on an experimental examination of the granitic rocks in and near Yosemite, rather than inferences based on what was going on elsewhere in the Sierra Nevada.
“Yosemite Valley is one of the most famous topographical features on Earth,” said glaciologist Kurt Cuffey, a professor of geography and Earth and planetary sciences at UC Berkeley. “And of course, if you go to Yosemite Park and read the signage, they’ll give you numbers for when it became a deep canyon. But until this project, any claim about how old this valley is, when it formed a deep canyon , was just based on assumptions and speculation.”
Yosemite National Park geologist Greg Stock admits that the story told about the origins of the park’s iconic granite topography has been a bit vague, as geologists still disagree on what has happened since the Sierra’s signature granite. formed underground between about 80 and 100 million years ago, up to 10 kilometers (6 miles) below a mountain range that looked very different from today.
“We know that the Sierra was a high mountain range 100 million years ago, when the granite formed at depth. It was a chain of volcanoes that may have looked a bit like the Andes in South America,” Stock said. “The real question is whether the elevation has just come down since then due to erosion or if it’s come down a bit and then lifted back up again. Right now, based on studies I’ve done for most of my career and supported by this study, I see a lot of evidence for recent revival sometime in the past 5 million years.”
That elevation, which occurred at the same time the eastern Sierra Nevada earthquake created a slope several miles high, steepened the western slopes and rivers, pushing them into valleys more quickly.
Cuffey, UC Berkeley geochemist David Shuster and their colleagues, including Stock, published the findings this week in Bulletin of the Geological Society of America.
rock cool down
Shuster, a professor of Earth and planetary sciences, developed a technique 15 years ago that he believed could shed light on the valley’s origins at the time, something that has fascinated both him and Cuffey since they first saw Yosemite as children. saws. Shuster, a California native, has frequented it since early childhood. Cuffey, from central Pennsylvania, made his first trip to the park at age 15.
Much of what they remember is that the valley was carved out by glaciers, which made short work of what happened before Ice Age glaciers arrived in the Pleistocene some 2.5 million years ago.
“What I learned from the signage in the valley when I was a kid didn’t quite add up, given what the scientific literature said at the time. Nevertheless, the topography has been interpreted as significantly altered by ice,” Shuster said. “How you can quantify that with geochronological tools, rather than just making up a story about it based on geomorphology, is something we tried to do here.”
Shuster’s technique, called helium-4/helium-3 thermochronometry, reconstructs the temperature history of a rock sample based on the spatial distribution of natural helium-4 in minerals, which is measured by comparison with an artificially produced uniform distribution of helium-3. Because temperatures rise with depth underground, temperature history can show when a rock was uncovered as the landscape eroded.
“The temperature of the rock is a function of the surface that sinks into it,” Shuster said. “It’s similar to removing a down comforter — the rock underneath gets progressively colder. This progression over time with the cooling of the rocks is what we get from geochemistry and thermochronometry.”
Granite bedrock exposed on the broad highlands of the Sierra is expected to show a long history of cool surface temperatures, as they have been exposed for tens of millions of years longer than bedrock recently uncovered on the floor of Tenaya Canyon, which feeds Yosemite Valley. from the northeast.
The experiments, conducted at the Berkeley Geochronology Center, indicated that while rock from the highlands has been close to the surface for about 50 million years, rock at the bottom of Tenaya Canyon has been uncovered much more recently. The temperature history of the rock obtained from the bottom of Tenaya Canyon — from an exposed area of bedrock at the base of Half Dome — indicates it was more than a kilometer underground 10 million years ago, and most likely only 5 million years ago. This means that a kilometer of rock has been eroded since that time.
“This high-altitude surface that people know from parts of the Tioga Road and Tuolumne Meadows — that’s a very ancient landscape,” said Cuffey, the Martin Distinguished Chair in Ocean, Earth and Climate Science. “The question is, what about the deep chasm? Is it also very old, or relatively young? And what we found in our research, our major contribution, is that it’s quite young. The best guess for the timing is in the last 3 to 4 million years, but maybe 10 million years back before the onset of the rapid incision.”
The geologists collected samples of granite bottom from nearby highlands and the bottom of Tenaya Canyon, but not from the bottom of Yosemite Valley itself, which is buried under about 500 meters (1/3 mile) of sediment that makes up the valley today. But since the two formed at the same time, one can deduce the timing of Yosemite Valley’s formation from the time of scouring Tenaya Canyon.
“The short history of Yosemite Valley would be that for tens of millions of years there was a kind of valley – a river-carved canyon associated with the ancient Sierra Nevada. And then, in the last 5 million years or so, renewed range elevation westward tilt caused rivers to steepen and deepen the canyons they were in,” Stock said.
“So that probably carved out more of Yosemite Valley and may have started to form Tenaya Canyon. And then over the last 2 to 3 million years, as the climate cooled and glaciers came through Tenaya Canyon into Yosemite Valley, they further formed the rock, deepening those valleys. And in the case of Yosemite Valley, widening it considerably. So there’s a part of an old Yosemite Valley. But I think this recent work shows that a lot more of that topography is younger than older.”
Stock, who has held the position of park geologist for 17 years and is the park’s first geologist, said the new study will review how the park tells the geologic history of Yosemite Valley.
“The timing of this new study is perfect in that in the coming years we hope to completely redo the Geology Hut displays at Glacier Point. I’m very excited to incorporate these new results into those displays,” he said. “It’s a perfect place to tell that story because there’s a view right out to Tenaya Canyon.”
Hot days can cause Yosemite rockfalls
Quote: How old is Yosemite Valley in California? (2022, October 20) retrieved October 21, 2022 from https://phys.org/news/2022-10-california-yosemite-valley.html
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