They found that rivers initially sculpted a shallow valley that already existed, and recently both rivers and ice contributed.
Tenaya Canyon (center) and part of Yosemite Valley (foreground) as seen from Glacier Point in Yosemite National Park. Tenaya Creek most likely began to search this granite canyon listed below Half Dome (center right) about 5 to 10 million years back, with glaciers getting here about 2.5 million years ago to sculpt the classic glacial valley lays out. Credit: Greg Stock/National Park Service
While the researchers are not able to be more exact, the new price quote is the first to be based upon an experimental study of the granite rocks in and near Yosemite, rather than on inferences based upon what was going on elsewhere in the Sierra Nevada.
” Yosemite Valley is among the most well-known topographic features on the planet,” stated glaciologist Kurt Cuffey, UC Berkeley teacher of location and of earth and planetary science. “And obviously, if you go to Yosemite Park and read the signage, they will offer you numbers for when it ended up being a deep canyon. Up till this job, every single claim about how old this valley is when it formed a deep canyon, was just based on presumptions and speculation.”
Yosemite National Park geologist Greg Stock admits that the story outlined the origin of the parks renowned granite topography has actually been a little unclear, since geologists still do not concur about what has happened because the Sierras signature granite formed underground in between about 80 and 100 million years ago, as much as 10 kilometers (6 miles) under a range of mountains that looked a lot different than it does today.
” We know that the Sierra was a high range of mountains 100 million years back when the granite was forming at depth. It was a chain of volcanoes that may have looked a bit like the Andes Mountains in South America,” Stock stated. “The question actually is whether the elevation has just been coming down through erosion since that time or whether it came down some and after that was boosted again more recently. At this point, based on research studies Ive provided for many of my profession and supported by this study, I see a great deal of proof for current uplift taking place at some point in the last 5 million years.”
That uplift, which happened at the same time that earthquake faulting in the eastern Sierra Nevada developed a cliff a number of kilometers high, steepened the western slopes and rivers, causing them to incise valleys faster.
Cuffey, UC Berkeley geochemist David Shuster and their colleagues, including Stock, just recently released the findings in the journal Geological Society of America Bulletin.
Rock cooldown
Shuster, a teacher of earth and planetary science, established a strategy 15 years ago that he believed at the time may clarify the origins of the valley, something that has actually amazed both him and Cuffey given that they first saw Yosemite as kids. Shuster, a California local, has visited it since early youth. Cuffey, from central Pennsylvania, made his very first journey to the park at the age of 15.
Much of what they keep in mind discovering is that the valley was carved by glaciers, giving brief shrift to what occurred before Ice Age glaciers arrived in the Pleistocene some 2.5 million years ago.
” What I discovered from the signage in the valley when I was a kid wasnt quite right, provided what the clinical literature said at the time. Nonetheless, the topography has been translated to be considerably modified by ice,” Shuster stated. “How to quantify that with geochronological tools, rather than just comprise a story about it based on geomorphology, is one thing we were attempting to do here.”
Shusters method, called helium-4/ helium-3 thermochronometry, reconstructs the temperature level history of a sample of rock based upon the spatial circulation of natural helium-4 in minerals, which is measured by comparison to an artificially-produced consistent distribution of helium-3. Because temperature level increases with depth underground, the temperature history can inform when a rock was discovered as the landscape deteriorated.
” The temperature of the rock is a function of the surface lowering down into it,” Shuster said. “Its extremely comparable to removing a down comforter– the rock below it gradually gets cooler. This progression through time with the rock cooling is what we receive from the geochemistry and thermochronometry.”
The expectation is that granite bedrock exposed on the broad uplands of the Sierra ought to reveal a long history of cool surface area temperature levels because theyve been exposed for tens of millions of years longer than bedrock more just recently exposed on the flooring of Tenaya Canyon, which feeds into Yosemite Valley from the northeast.
The experiments, conducted at the Berkeley Geochronology Center, showed that, while rock from the uplands has been close to the surface for about 50 million years, bedrock at the bottom of Tenaya Canyon has actually been exposed much more just recently. The temperature level history of the rock acquired from the bottom of Tenaya Canyon– from an exposed area of bedrock at the base of Half Dome– indicates that it was more than a kilometer underground 10 million years earlier, and most likely just 5 million years ago. This means that a kilometer of rock was eroded away since that time.
“The question is: What about the deep canyon? The best guess for the timing is in the last 3 to 4 million years, but possibly as far back as 10 million years for the start of the quick incision.”
Bedrock research studies
The geologists collected samples of granite bedrock from neighboring highlands and the bottom of Tenaya Canyon, but not from the bedrock bottom of Yosemite Valley itself, which lies buried under about 500 meters (1/3 mile) of sediment that today forms the valley flooring. Since the 2 formed at the same time, one can presume the timing of the formation of Yosemite Valley from the time of the searching of Tenaya Canyon.
” The quick history of Yosemite Valley would be that there was some kind of valley in location for 10s of millions of years– a river-carved canyon associated with the ancient Sierra Nevada. “So, that most likely carved out more of Yosemite Valley and might have begun forming Tenaya Canyon. And then in the last 2 to 3 million years, as the environment cooled and glaciers came down through Tenaya Canyon and into Yosemite Valley, they further shaped the rock, deepening those valleys.
Stock, who has held the position of park geologist for 17 years and is the parks first geologist, stated the new research study will revise how the park informs the geological history of Yosemite Valley.
” The timing of this brand-new research study is perfect in the sense that, over the next numerous years, were wishing to totally renovate the Geology Hut displays at Glacier Point. Im very thrilled to consist of these new lead to those screens,” he said. “Its a best location to tell that story due to the fact that theres a view directly up Tenaya Canyon.”
Recommendation: “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, Geological Society of America Bulletin.DOI: 10.1130/ B36497.1.
In 2010, two of the studys authors– David Shuster of UC Berkeley (left) and Yosemite Park geologist Greg Stock (right)– gathered rock samples from the rim of Yosemite Valley, where the granite has actually eroded only gradually 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. Credit: Kurt Cuffey, UC Berkeley
How old is Californias Yosemite Valley?
First-time visitors to Yosemite Valley stare in awe at El Capitans sheer granite wall and Half Domes cleanly sliced face, understanding, perhaps slightly, that it must have taken rain and glaciers a long period of time to cut and form that scene. However how long did it take?
Did it all begin 50 million years back, when the granite that the valley cuts through was very first exposed to the components? Was it 30 million years earlier when canyons in the southern Sierra Nevada began to form? Did the valley only begin to form when the Sierra slanted toward the west around 5 million years ago or did glaciers that formed in a cooling environment 2 to 3 million years ago account for most of its development?
Geologists from the University of California, Berkeley, used a brand-new rock analysis method to get a more accurate response, concluding that most of Yosemite Valleys astonishing depth was cut 10 million years earlier, and more than likely even more recently– within the last 5 million years. This decreases the oldest quotes by about 40 million years.
In 2010, two of the studys authors– David Shuster of UC Berkeley (left) and Yosemite Park geologist Greg Stock (ideal)– collected rock samples from the rim of Yosemite Valley, where the granite has actually worn down just gradually over the last 50 million years. Did the valley only start to form when the Sierra slanted towards the west around 5 million years ago or did glaciers that formed in a cooling climate 2 to 3 million years ago account for the majority of its formation?
Tenaya Creek most likely started to scour this granite canyon listed below Half Dome (center right) about 5 to 10 million years back, with glaciers getting here about 2.5 million years ago to sculpt the traditional glacial valley lays out.” The quick history of Yosemite Valley would be that there was some kind of valley in place for tens of millions of years– a river-carved canyon associated with the ancient Sierra Nevada. And then in the last 2 to 3 million years, as the environment cooled and glaciers came down through Tenaya Canyon and into Yosemite Valley, they further shaped the rock, deepening those valleys.
