Southern Sierra rose as underlying rock sank away, scientists say

By: Associated Press | Saturday, July 31, 2004 10:06 PM PDT ∞

LOS ANGELES -- The southern Sierra Nevada, including California's highest peak, rose up when underlying material broke away and sank into Earth's mantle about 3.5 million years ago, a new study concludes.

Researchers, using high-tech seismometers to record distant earthquakes, said they created an underground image of the range and located the massive body of rock that fell away from the 22-mile-deep crust.

"Our measurements show how material at the bottom of the Sierran crust descended into the mantle, which seems to confirm that the mountain range popped up as the weight on the crust dropped off," said one of the study's authors, Craig Jones, an associate professor of geological science at the University of Colorado at Boulder.

The study, published in the July 30 issue of the journal Science, was also authored by CU-Boulder doctoral student Oliver Boyd and associate professor Anne Sheehan.

The towering peaks of the Sierra Nevada run some 400 miles along the eastern side of California.

"Our work builds on a shift in thinking about how these mountains formed," Jones said in a university press release. "For a long time, the Sierras were thought to be geologically 'boring,' so there's quite a huge shift under way in our understanding of these mountains."

The team from Colorado focused its research on the southern portion of the range -- north from Bakersfield to just south of Yosemite National Park near Mammoth Lakes, and east from Fresno to the western flank of Death Valley National Monument. The study area covered both the valley and mountain range and includes Kings Canyon and Sequoia National Parks and the state's highest peak, 14,494-foot Mount Whitney.

The researchers' seismometers recorded earthquakes in different wavelengths to create an underground image of the Sierra to a depth of 125 miles, in a process something like the way CAT scans and sonograms image the human body.

The method allowed the researchers to determine different rock types in the crust, which is Earth's upper layer, and the mantle, the layer of rock below the crust.

According to Sheehan the study area had never been imaged with such clarity.

The researchers concluded that rock known as eclogite broke away from the Sierran crust 3 million to 4 million years ago and was replaced by hot, young and mobile mantle rock called spinel peridotite, which allowed the Sierra to rise.

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