Yosemite Valley has occupied a prominent place in environmentalism, tourism and the study of land forms.
Estimates of when it formed have ranged from 2 million to more than 50 million years ago.
But a lack of clear clues have made narrowing that gap impossible until a recent paper in Geological Society of America Bulletin showed how it could be done.
“The Yosemite Valley landscape doesn't have any of the features that would traditionally let a geologist infer the development of topography,” said lead author Kurt Cuffey of the UC Berkeley Department of Geography.
Cuffey explained that, while other areas might hold long-accumulating sediments or volcanic material to probe for clues, Yosemite Valley consists mainly of unvarying granite bedrock.
“Without this very sophisticated and relatively new technique, there's really no way to assess how or when the topography of Yosemite developed,” he said.
The technique is a twist on a tried-and-true trick: Using known rates of radioactive decay to treat certain minerals as a kind of geological clock. Here, the authors used helium released as a byproduct of uranium and thorium decay in phosphate minerals called apatites, small grains of which pepper the Yosemite Valley granite.
Helium only lingers in those minerals at temperatures of 120 degrees Fahrenheit or cooler. So young, deep rocks hold onto no radiogenic helium, whereas cooler rocks uncovered by erosion contain an amount proportional to temperature.
“The radiometric clock does not turn on until a certain temperature has been crossed, and the retention of the helium varies a lot with the temperature,” he said.
The deeper in the Earth a rock is buried, the hotter the conditions. As erosion removes overlying material, the rock cools until it is ultimately uncovered and exposed to surface conditions.
So, just as the products of radioactive decay provide a proxy for time, one product — helium — acts as a substitute thermometer — which, in turn, reveals when the rock was exposed and when the valley formed.
“Not only the total amount of helium, but its pattern within an individual crystal, will depend on how fast and when the rock cooled,” said Cuffey. “So those things together make this not only a clock for time, but also a measure the temperature history.”
Comparisons of those clocks with samples taken further west in Yosemite Valley, as well as from the uplands to the north in areas like Tuolumne Meadows and Tioga Road, show the rocks from the bottom of the canyon underwent rapid cooling recently, whereas those from up on the surrounding lands have cooled very little for a very long time.
“The upland surface has not eroded very much at all in the last 60 million years, whereas the canyon eroded quite recently,” said Cuffey.
Calculations suggest the deep canyon likely formed in the last 5 million years, during the great deformation that created the Basin and Range Province the covers much of the inland Western United States and northwestern Mexico.
As pressure from the Pacific tectonic plate weakened and the land surrounding Yosemite stretched, a fault created an escarpment on the east side of the Sierra Nevada, forming a deep river canyon.
“We also know that the cooling of the climate and the onset of glaciation definitely played a role to in deepening the valley and making it look the way it does today,” said Cuffey.
