Berkeley Analysis Reveals the Age of Yosemite Valley

In 2010, two of the research’s authors — UC Berkeley’s David Shuster (left) and Yosemite Park geologist Greg Stock (proper) — collected rock samples from the rim of Yosemite Valley, the place the granite has eroded solely slowly over 50 million years. final. . In the distance is the higher Merced River valley. Former UC Berkeley graduate pupil Johnny Webb is at heart. Credit: Kurt Cuffey, UC Berkeley

How previous is California’s Yosemite Valley?

First-time guests to Yosemite Valley gaze in marvel at the sheer granite wall of El Capitan and the clean-cut face of Half Dome, realizing, maybe vaguely, that it will need to have taken a very long time for rain and glaciers to carve and formed that scene. But how lengthy did it take?

Did all of it start 50 million years in the past, when the granite that runs by the valley was first uncovered to the components? Was it 30 million years in the past when canyons started to type in the southern Sierra Nevada? Did the valley start to type solely when the Sierra tilted west about 5 million years in the past, or did glaciers that fashioned in a cooling local weather 2 to three million years in the past account for many of its formation?

Geologists from the University of California, Berkeley, used a brand new rock evaluation method to get a extra exact reply, concluding that a lot of Yosemite Valley’s beautiful depth was reduce 10 million years in the past. and most certainly even later – inside the final 5 million years. . This dwarfs the oldest estimates by about 40 million years.

They discovered that rivers first carved a shallow valley that already existed, and later each rivers and ice contributed.

Tenaya Canyon (heart) and half of Yosemite Valley (foreground) as seen from Glacier Point in Yosemite National Park. Tenaya Creek seemingly started scouring this granite canyon under Half Dome (heart proper) about 5 to 10 million years in the past, with glaciers arriving about 2.5 million years in the past to sculpt the traditional glacial valley outlines. Credit: Greg Stock/National Park Service

While scientists are unable to be extra exact, the new estimate is the first to be based mostly on an experimental research of granite rocks in and close to Yosemite, reasonably than conclusions based mostly on what was occurring elsewhere in the Sierra Nevada.

“Yosemite Valley is one of the most famous topographic features on the planet,” stated glaciologist Kurt Cuffey, UC Berkeley professor of geography and earth and planetary science. “And of course, if you go to Yosemite Park and read the signage, they’ll give you the numbers of when a deep canyon was made. But until this project, any claim about how old this valley is when it formed a deep canyon was based on guesswork and speculation.”

Yosemite National Park geologist Greg Stock admits that the story told about the origin of the park’s iconic granite topography has been a bit fuzzy because geologists still don’t agree on what happened since the Sierra granite formed underground between ca. 80 and 100 million years. ago, up to 10 kilometers (6 miles) below a mountain range that looked much different than it does today.

“We know that the Sierra was a high mountain range 100 million years ago when granite was forming at depth. It was a chain of volcanoes that might look a bit like the Andes mountains in South America,” Stock said. “The question is really whether the elevation has only gone down by erosion since then or if it went down a bit and then rose again recently. At this point, based on the studies I’ve done for most of my career and supported by this study, I see a lot of evidence for the last uplift to have occurred sometime in the last 5 million years.”

That uplift, which occurred at the same time as the earthquake struck in the eastern Sierra Nevada, created an escarpment several kilometers high, steepening the western slopes and rivers, causing them to cut valleys faster.

Cuffey, UC Berkeley geochemist David Shuster and colleagues, including Stock, recently published the findings in the journal Bulletin of the Geological Society of America.

Rock cooling

Shuster, a professor of earth and planetary science, developed a technique 15 years ago that he thought at the time could shed light on the valley’s origins, something that has fascinated him and Cuffey since they first saw Yosemite. as a child. Shuster, a California native, has been visiting since early childhood. Cuffey, from central Pennsylvania, made his first trip to the park at age 15.

Most of what they remember learning is that the valley was carved by glaciers, giving a short shrift to what happened before the Ice Age glaciers reached the Pleistocene about 2.5 million years ago.

“What I learned from the signage in the valley as a child was not at all correct, given what the scientific literature was saying at the time. However, the topography has been interpreted to have been significantly modified by the ice,” Shuster said. “How to quantify this with geochronological tools, rather than just creating a history based on geomorphology, is something we were trying 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 temperature increases with depth underground, the temperature history can tell when a rock was exposed as the landscape eroded.

“The temperature of the rock is a function of the surface going down on it,” Shuster said. “It’s very analogous to removing a comforter – the rock underneath gradually gets colder. This progression over time as the rocks cool is what we get from geochemistry and thermochronometry.”

It is anticipated that the granite rocks uncovered in the huge Sierra Mountains ought to present an extended historical past of chilly floor temperatures since they’ve been uncovered for tens of thousands and thousands of years longer than the just lately uncovered rocks on the ground of Tenaya Canyon, i which feeds into Yosemite Valley from the northeast.

The experiments, conducted at the Berkeley Geochronology Center, showed that, while rock from the mountains has been near the surface for about 50 million years, the rocks at the bottom of Tenaya Canyon have been exposed much more recently. The temperature history of rock taken from the bottom of Tenaya Canyon—from an exposed area of ​​rock at the base of Half Dome—indicates that it was more than a kilometer underground 10 million years ago, and most likely only 5 million years ago. This means that one kilometer of rock has eroded since then.

“This upland that persons are acquainted with from elements of Tioga Road and Tuolumne Meadows — it is a very previous panorama,” said Cuffey, who is the Martin Distinguished Chair in Ocean, Earth and Climate Science. “The question is: What about the deep canyon? Is he too old, or is he relatively young? And what we found in our study, our big contribution, is that it’s quite new. The best guess for the time is in the last 3 to 4 million years, but possibly as long as 10 million years for the onset of rapid shearing.”

Bed studies

Geologists collected samples of granite bedrock from the nearby highlands and from the bottom of Tenaya Canyon, but not from the bedrock of Yosemite Valley itself, which lies buried under about 500 meters (1/3 mile) of the sediment that forms the floor today. of the valley. But since both were formed at the same time, the time of formation of Yosemite Valley can be deduced from the time of the clearing of Tenaya Canyon.

“The quick historical past of Yosemite Valley could be that there was a form of valley in place for tens of thousands and thousands of years—a river-carved canyon linked to the historical Sierra Nevada. And then, in the final 5 million years or so, the uplift “Renewal of the range through westward tilting caused the rivers to steepen and deepen the canyons they were in,” Stock stated. “So that probably carved out more of Yosemite Valley and may have started to form Tenaya Canyon. And then, in the last 2 to 3 million years, as the climate cooled and the glaciers came down through Tenaya Canyon and into Yosemite Valley , they sculpted the rock further, deepening those valleys. And in the case of Yosemite Valley, widening it considerably. So there are some components of an old Yosemite Valley. But I think this recent work shows that much more of that topography is younger than older.”

Stock, who has held the place of park geologist for 17 years and is the park’s first geologist, stated the new research will reexamine how the park tells the geologic historical past of Yosemite Valley.

“The timing of this new study is perfect in that, over the next several years, we hope to completely redo the Geology Hut displays at Glacier Point. I’m very excited to incorporate these new results into those shows,” he stated. “It’s a perfect place to tell that story because it has a view toward Tenaya Canyon.”

Reference: “Late Cenozoic Deepening of Yosemite Valley, USA” by Kurt M. Cuffey, Alka Tripathy-Lang, Matthew Fox, Greg M. Stock and David L. Shuster, 19 October 2022, Bulletin of the Geological Society of America.
DOI: 10.1130/B36497.1