The mountains of Sonora Pass may seem immovable, but don’t let your eyes fool you.
A geologist studying the Sierra Nevada says the entire region may be slowly drifting west toward the ocean as its own mini-continent.
The strange-sounding concept was explained to geology teachers at Baker Station last weekend by Cathy Busby, a professor at the University of California, Santa Barbara. Her recent research points to a continental rift along the eastern Sierra, contrary to previous emphasis on the San Andreas Fault.
The rift system in eastern California, called the Walker Lane Belt, is an extension of the same rift that separated Baja California from Mexico about 6 million years ago. It covers a wide area that includes Reno, Carson City and Lake Tahoe.
Given millions of years more, the same rift could cause the Sierra and Central Valley to split from North America in the style of Baja California.
It’s not likely that the movement west will pose a danger to Sonora anytime soon, according to Busby.
“It’s going to take another 5 or 6 million years before that happens, so it’s not something that property owners have to worry about,” she said.
Scientists previously thought the San Andreas Fault was the only site of movement between the North American and Pacific tectonic plates, which are giant slabs of rock that float on the earth’s mantle.
Data collected from GPS units since the 1990s led a group of geologists to conclude that the Sierra Nevada mountain range is actually part of a mini-continent, or “microplate” in scientific terms.
“The Pacific plate is dragging the Sierra Nevada microplate with it and trying to capture it,” Busby summarized.
In Tuolumne County, the most visible evidence of the rifting process is Table Mountain — which was created when lava erupted from the rift at Sonora Pass 10 million years ago.
Busby’s recent research has found evidence of massive volcanic eruptions at around the same time, one of which released the lava that flowed along the ancient Stanislaus River and later became Table Mountain.
That’s when the movement of the Sierra Nevada mini-continent began, according to Busby and others who have done research in the area.
“In geologic time, if it keeps doing what it’s doing, the continent will rift and the ocean may make it closer,” said Columbia College geology professor Jeff Tolhurst. “Ten million years from now, you’ll have beachfront property.”
The Walker Lane Belt, the site of the rifting, includes a series of smaller faults or fractures in the earth’s crust.
The portion of the Walker Lane Belt at Sonora Pass runs from Kennedy Creek to Highway 395, Busby said. It doesn’t represent an active fault and hasn’t caused earthquakes in the recent past.
But some Walker Lane faults have. One, the Owens Valley Fault in eastern California, generated one of the three worst earthquakes in California history in 1872.
A recently discovered active fault courses through Truckee, a California town near Carson City and Reno, Nev.
“Because it’s such a long fault, that means there could be a big earthquake,” Busby said. “The longer the fault, the bigger the earthquake.”
During the past few years, Busby has researched evidence of the mini-continent theory around Sonora Pass. She found it in what she describes as the “voluminous volcanic rocks” in the area.
Science teachers and geology professors heard about Busby’s research this weekend, and got to see some evidence for themselves, at the Baker Station conference.
“Her talk was really good,” said Tolhurst. “She just found out all this amazing new stuff. It’s really kind of startling, in a way. It’s totally changed the thinking about the whole area.”
The Sierra Nevada are still rising about a millimeter a year, Tolhurst said. Movement of the plates means that the Sea of Cortez, which separates Baja California from Mexico, could work its way along the eastern side of the mountains.
Far from worrying about the effects of the Walker Lane Belt, Tolhurst wishes the ancient volcanoes around Sonora Pass were still active.
“What I want to have happen is one of these volcanoes wake up again so we can have hot springs around here,” he said.
In the meantime, Sonora Pass continues to be a wonderland for geologists and graduate students working on research.
“Everyone loves the Sierra for geology,” Busby said.
This weekend’s conference at Baker Station’s High Sierra Institute was hosted by the National Association of Geoscience Teachers and Modesto Junior College. Its participants included students, teachers at the secondary school level, and college instructors such as Tolhurst.
Sonora High earth science and physics teacher Jason Westfall also attended. Learning about new theories makes for better teaching, he said.
“Most of the people (who) go increase their content knowledge, and it just gives them a deeper understanding of their subject,” Westfall said. “The textbook says one thing, and the recent research actually suggests something different.”
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