Those visiting Yosemite Valley look at the EL Captain’s granite wall with amazement. One will wonder how the half-dome’s sliced face came to be. Many might be tempted to think that it took glaciers and rain a long time to create these wonderful scenes. But not many people have an idea of how long it took to have that appearance.
Did it start when the granite cutting through the valley got exposed 50 million years ago? Did it take place when the canyons began to form 30 million years ago? Or did the valley start to form six million years when the Sierra tilted towards the west?
But there’s light at the end of the tunnel. A new rock analysis technique has been used by geologists based in one of the universities in California to provide a more definite answer. The technology used revealed that a big part of the Valley’s depth was formed 10 million years ago.
The geologists discovered that rivers originally created a shallow valley that was already there. Although the scientists can’t give a precise answer, the new estimation is the first one based on a study on granite rocks close and inside Yosemite.
According to glaciologist, Kurt Cuffey, Yosemite Valley is among the most iconic topographic features on earth. “It is true that you will see signage indicating the number of years it took for it to form a deep canyon when you go to Yosemite. But that was only true until the recent study was conducted”, said Cuffey.
Greg Stock, a geologist at Yosemite national park admits that all the stories previously said regarding the origin of the iconic topography of the park have been unclear. This is because geologists are yet to agree concerning what happened to Sierra’s distinct granite that was formed below the mountain approximately 90 million years ago.
“We understand that Sierra was a great mountain even before the granite formed underneath. It was a series of volcanoes that probably resembled the Andes mountains found in South America”, said Stock. “The question in many people’s minds is whether the height has been happening due to erosion or it has been happening since that time”, said Stock.
Rock Cooldown
A planetary science professor known as Shuster developed a technique that might reveal the origins of the valley. The technique that was developed 15 years ago has baffled many scientists and proved to be a game changer in determining the age of Yosemite.
The technique known as helium-4/helium-3 thermochronometry works by reconstructing the history temperature of a rock as per the longitudinal distribution of helium-4 in minerals. This is measured by comparing it to an artificially-generated helium-3. Since temperatures rise based on the depth below, the temperature history can reveal when a rock was exposed due to the landscape erosion.
“The rock temperature is caused by the lowering down of the surface material into it”, said Shuster. “It is a process that takes a lot of time. The progression as time goes by coupled with the rock cooling leads to what is obtained from thermochronometry and geochemistry”.
The experiments carried out at the Berkeley Geochronology Centre showed that although the upland rock has been around for approximately 50 million years, the bedrock found at the Tenaya Canyon got exposed just recently.
Bedrock Studies
Geologists obtained granite samples from the surrounding highlands as well as at the bottom of the mountain. But no samples were taken from Yosemite Valley. However, since the two were around the same period, one can guess the time when Yosemite Valley was formed.
While the project doesn’t reveal the exact age of the valley, it does give close to an accurate answer to what has been puzzling geologists for many years.