Berkeley analysis reveals age of Yosemite Valley

Berkeley analysis reveals age of Yosemite Valley

In 2010, two of the research’s authors – UC Berkeley’s David Shuster (left) and Yosemite Park geologist Greg Inventory (proper) – collected rock samples from the sting of Yosemite Valley, the place granite has eroded solely slowly over the previous 50 million years. . Within the distance is the Higher Merced River Valley. Former UC Berkeley graduate pupil Johnny Webb is within the heart. Credit score: Kurt Cuffey, UC Berkeley

How outdated is Yosemite Valley in California?

New guests to Yosemite Valley gaze in amazement on the granite wall of El Capitan and the neatly carved face of Half Dome, understanding, maybe dimly, that it took rain and glaciers a very long time to chop and form this scene. However how lengthy did it take?

Did all of it begin 50 million years in the past, when the granite that runs by the valley was first uncovered to the weather? Was it 30 million years in the past that the canyons of the southern Sierra Nevada started to kind? Did the valley solely start to kind when the Sierra tilted west about 5 million years in the past or the glaciers that shaped in a cooled local weather 2 to Did 3 million years signify the vast majority of its formation?

The geologists of University of California, Berkeley, used a brand new rock evaluation method to get a extra exact reply, concluding that the majority of Yosemite Valley’s astonishing depth was dug 10 million years in the past, and sure much more lately. – during the last 5 million years. This reduces the oldest estimates by round 40 million years.

They discovered that the rivers initially carved out a shallow valley that already existed, and that lately rivers and ice have contributed to it.

Tenaya Canyon and Yosemite Valley

Tenaya Canyon (heart) and a part of Yosemite Valley (foreground) as seen from Glacier Level in Yosemite Nationwide Park. Tenaya Creek in all probability started carving out this granite canyon beneath 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 carve the basic contours of the glacial valley. Credit score: Greg Inventory/Nationwide Park Service

Though scientists are unable to be extra particular, the brand new estimate is the primary to be based mostly on an experimental research of granitic rocks in and close to Yosemite, fairly than inferences based mostly on what was occurring elsewhere within the Sierra Nevadas.

“Yosemite Valley is likely one of the most well-known topographical options on the planet,” mentioned glaciologist Kurt Cuffey, professor of geography and earth and planetary sciences at UC Berkeley. “And naturally in case you go to Yosemite Park and skim the signage, they gives you numbers for when it grew to become a deep canyon. However till this challenge, each declare about how outdated this valley was when it shaped a deep canyon was based mostly on assumptions and hypothesis.

Yosemite Nationwide Park geologist Greg Inventory admits the story being instructed in regards to the origin of the park’s iconic granite topography has been a bit obscure, as geologists nonetheless disagree on what occurred. handed for the reason that Sierra’s signature granite shaped underground between about 80 and 100 million years in the past. as much as 10 kilometers (6 miles) beneath a mountain vary that seemed very totally different than it does at present.

“We all know that the Sierra was a excessive mountain vary 100 million years in the past when the granite was forming at depth. It was a sequence of volcanoes that may have seemed a bit just like the Andes in South America,” Inventory mentioned. “The query actually is whether or not the elevation has simply eroded down since that point or whether or not it is gone down a bit after which been uplifted once more extra lately. At this level, based mostly on the research I have been doing for many of my profession and supported by this research, I see a variety of proof of latest uplift during the last 5 million years.

This uplift, which occurred concurrently an earthquake within the jap Sierra Nevada created a mile-high escarpment, steepened western slopes and rivers, inflicting them to incise quicker the valleys.

Cuffey, UC Berkeley geochemist David Shuster and their colleagues, together with Inventory, lately printed the leads to the journal Bulletin of the Geological Society of America.

Rock Cooldown

Shuster, a professor of earth and planetary sciences, developed a method 15 years in the past which he believed might on the time make clear the origins of the valley, which has fascinated him and Cuffey ever since they first noticed Yosemite after they had been children. Shuster, initially from California, has been visiting him since he was a toddler. Cuffey, a local of central Pennsylvania, made his first journey to the park when he was 15 years outdated.

A lot of what they keep in mind studying is that the valley was carved out by glaciers, disregarding what occurred earlier than the ice age glaciers arrived within the Pleistocene there. about 2.5 million years outdated.

“What I discovered about signage within the valley as a toddler was not totally correct, given what the scientific literature mentioned on the time. Nonetheless, the topography was interpreted to be considerably altered by ice,” Shuster mentioned. “The right way to quantify this with geochronological instruments, fairly than simply making up a narrative about it based mostly on geomorphology, is one thing we had been attempting to do right here.”

Shuster’s method, referred to as helium-4/helium-3 thermochronometry, reconstructs the temperature historical past of a rock pattern based mostly on the spatial distribution of pure helium-4 within the minerals, which is measured by comparability to an artificially produced uniform distribution of helium-3. As a result of temperature will increase with depth underground, temperature historical past can point out when a rock was found because the panorama eroded.

“The temperature of the rock is a perform of the floor taking place to it,” Shuster mentioned. “It is similar to eradicating a down comforter — the rock beneath will get progressively colder. This development in time with the cooling of the rock is what we get from geochemistry and thermochronometry.

The uncovered granite bedrock on the huge Sierra Uplands is anticipated to have a protracted historical past of cool floor temperatures, as they’ve been uncovered for tens of tens of millions of years longer than the bedrock most lately uncovered on the ground of Tenaya Canyon, which feeds into Yosemite Valley from the northeast.

The experiments, performed on the Berkeley Heart for Geochronology, indicated that though upland rock has been close to the floor for round 50 million years, the bedrock on the backside of Tenaya Canyon has been uncovered rather more lately. . Rock temperature historical past obtained from the underside of Tenaya Canyon—from an space of ​​uncovered bedrock on the base of Half Dome—signifies that it was greater than a mile underground 2015 in the past. 10 million years in the past, and almost certainly solely 5 million years in the past. Because of this one kilometer of rock has been eroded since that point.

“This upland floor that individuals know from elements of the Tioga Freeway and the Tuolumne Meadows – it is a very outdated panorama,” mentioned Cuffey, Martin Distinguished Chair in Ocean Science, Earth and local weather. “The query is, what in regards to the deep canyon? Is it additionally very outdated, or is it comparatively younger? And what we present in our research, our huge contribution, is that it is fairly younger. The very best estimate presently is throughout the final 3-4 million years, however presumably way back to 10 million years for the onset of speedy incision.

Bedrock research

Geologists have collected samples of granite bedrock from close by highlands and the underside of Tenaya Canyon, however not from the bedrock of Yosemite Valley itself, which is buried about 500 meters (1/3 of mile) of sediment that at present varieties the underside of the valley. However for the reason that two shaped on the similar time, the timing of Yosemite Valley’s formation will be inferred from the timing of the stripping of Tenaya Canyon.

“The transient historical past of Yosemite Valley could be that there was a form of valley in place for tens of tens of millions of years – a river-cut canyon related to the traditional Sierra Nevada. After which, over the previous 5 million years or so, additional west-dipping uplift of the vary triggered steepening and deepening of the canyons by which they had been positioned,” Inventory mentioned. “In order that in all probability carved out extra of Yosemite Valley and possibly began to kind Tenaya Canyon. After which, during the last 2-3 million years, because the local weather cooled and glaciers descended into Tenaya Canyon and Yosemite Valley, they additional carved the rock, deepening these valleys. And within the case of Yosemite Valley, widen it significantly. So there is a component of an historic Yosemite Valley. However I believe this latest work exhibits that a lot of this topography is youthful than older.

Inventory, who served because the park’s geologist for 17 years and is the park’s first geologist, mentioned the brand new research will revise how the park tells the geological historical past of Yosemite Valley.

“The timing of this new research is ideal within the sense that over the following few years we hope to utterly redo the Geology Hut shows at Glacier Level. I’m very joyful to incorporate these new leads to these screens,” he mentioned. “It is an ideal place to inform that story as a result of there is a direct view of Tenaya Canyon.”

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


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