by First-time visitors to Yosemite Valley gasp at the sheer granite wall of El Capitan and the carefully hewn face of Half Dome, aware, perhaps dimly, that rain and glaciers must have taken a long time to cut and sculpt that landscape. But how long?
Did it all start 50 million years ago, when the granite that runs through the valley was first exposed to the elements? Was it 30 million years ago, when the data suggests that canyons began to form in the southern Sierra Nevada? Did the valley only begin to form after the Sierra tilted west about 5 million years ago, or was it mainly due to glaciers forming in a cooler climate 2-3 million years ago?
Geologists at the University of California, Berkeley used a novel rock analysis technique to get a more precise answer and concluded that much of Yosemite Valley’s impressive depth was excavated 10 million years ago, and probably even more recently. during the last 5 million years. This cuts about 40 million years from the oldest estimates.
Rivers did the initial carving in a pre-existing shallow valley, they determined, and then both rivers and ice recently contributed.
While scientists can’t be more precise, the new estimate is the first to be based on an experimental study of granite rocks in and near Yosemite, rather than inferences based on what was happening elsewhere in the Sierra Nevada. .
“Yosemite Valley is one of the most famous topographical features on the planet,” said glaciologist Kurt Cuffey, a UC Berkeley professor of geography and earth and planetary sciences. “And of course if you go to Yosemite Park and read the signage, they’ll give you the numbers for when it became a deep canyon. But until this project, all the claims about how old this valley was, when it formed a deep canyon , it was only based on assumptions and speculations”.
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 vague, because geologists still don’t agree on what happened since the granite characteristic of the Sierra was formed underground between 80 and 100 million years ago. up to 10 kilometers (6 miles) ago under a mountain range that looked very different than it does today.
“We know that the Sierra was a high mountain range 100 million years ago, when granite was forming deep down. It was a chain of volcanoes that might have looked a bit like the Andes in South America,” Stock said. . “The question really is has the elevation just been going down due to erosion since that time or has it gone down a bit and then up again more recently. At this point, based on studies I’ve done for most of my career and I have supported. Based on this study, I see a lot of evidence for a recent uplift that occurred sometime in the last 5 million years.”
That uplift, which occurred at the same time as earthquake faulting in the eastern Sierra Nevada created an escarpment several kilometers high, steepened western slopes and rivers, causing them to cut through valleys more quickly.
Cuffey, UC Berkeley geochemist David Shuster, and their colleagues, including Stock, published the findings this week in the journal Bulletin of the Geological Society of America.
rock cooling
Shuster, a professor of earth and planetary sciences, developed a technique 15 years ago that he thought at the time might shed light on the valley’s origins, something that has fascinated both him and Cuffey since they first saw Yosemite when they were kids. kids. Shuster, a native of California, has visited him since childhood. Cuffey, from central Pennsylvania, made his first trip to the park at the age of 15.
Much of what they remember learning is that the valley was carved by glaciers, giving little thought to what happened before Ice Age glaciers reached the Pleistocene about 2.5 million years ago.
“What I learned from signage in the valley as a child was not quite right, given what the scientific literature was saying at the time. However, the topography has been interpreted to be significantly modified by ice,” Shuster said. “How to quantify that with geochronological tools, rather than just making up a story 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 against a uniform distribution of artificially produced helium-4. 3. Because temperature increases with depth underground, temperature history can indicate when a rock was discovered as the landscape eroded.
“The temperature of the rock is a function of the surface going down to it,” Shuster said. “It’s very similar to taking off a down comforter: the rock below it progressively cools. This progression in time with the cooling of the rock is what we get from geochemistry and thermochronometry.”
The expectation is that exposed granite bedrock in the broad Sierra highlands will show a long history of cool surface temperatures, having been exposed for tens of millions of years longer than more recently exposed bedrock on the floor of the Tenaya Canyon, which feeds into Yosemite Valley from the northeast.
The experiments, conducted at the Berkeley Geochronology Center, indicated that while upland rock has been near the surface for about 50 million years, bedrock at the bottom of Tenaya Canyon has been exposed much more recently. . The rock temperature history obtained 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 ago, and most likely only 5 million. of years. behind. This means that a kilometer of rock has since been eroded away.
“This upland area that people are familiar with in parts of Tioga Road and Tuolumne Meadows, is a very old landscape,” said Cuffey, who is the Martin Distinguished Chair in Ocean, Earth and Climate Sciences. “The question is: What about the deep canyon? Is it also very old or is it relatively young? And what we found in our study, our big contribution, is that it is quite young. The best guess for the moment is in the last 3 to 4 million years, but perhaps as far back as 10 million years to the start of the rapid incision.”
Bedrock studies
Geologists collected samples of granite bedrock from nearby highlands and the bottom of Tenaya Canyon, but not Yosemite Valley bedrock, which lies buried under about 1/3 mile (500 meters) of sediment that today forms the valley floor. But since the two formed at the same time, one can infer the time of Yosemite Valley’s formation from the time of Tenaya Canyon’s erosion.
“The brief history of Yosemite Valley would be that some kind of valley existed for tens of millions of years, a canyon carved out by a river associated with the ancient Sierra Nevada. And then in the last 5 million years or so, El renewed uplift of the range through the westward tilt caused the rivers to bend 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 in the last 2 to 3 million years, as the climate cooled and glaciers descended through Tenaya Canyon and into Yosemite Valley, they further carved out the rock, deepening those valleys. And in the case of Yosemite Valley, widening it considerably. So, there is some component of an ancient Yosemite Valley. But I think this recent work shows that a lot more of that topography is younger, rather than older.”
Stock, who served as the park’s geologist for 17 years and is the park’s first geologist, said the new study will review how the park tells Yosemite Valley’s geological story.
“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 include these new results on those displays.” he said. “It’s a perfect place to tell that story, because there’s a direct view of the Tenaya Canyon.”
