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Long Valley Caldera is calm...for now |
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Friday, 13 June 2008 |
By Stacey Powells
Mammoth Times Staff Writer
“It’s highly unlikely that our children, grandchildren or even great grandchildren will see any kind of significant activity from the Long Valley Caldera,” said Dr. Dave Hill, the scientist in charge of the Long Valley Observatory.
Hill spoke to a full house at the Forest Service Auditorium in Mammoth Lakes on June 4. But those of us with our ears to the ground-in-motion still wonder. It's been approximately 760,000 years since the Long Valley Caldera blew its lid but in geologic time, that's practically a nano-second. “We geologists have a very warped sense of time,” Hill said.
Hill said that a series of “things” underground come together in the Eastern Sierra that cause movement. The Eastern Sierra comprises roughly 25 percent of the relative motion between the Pacific Plate and the North American Plate. Such strike slip faults as the San Andreas Fault are in our area. For example, the 7.2 earthquake that struck the Owens Valley in 1872 was the result of a strike-slip fault.
Strike-slip faults involve motion that is parallel to the strike of the fault – frequently described as a "side-by-side" motion. Strike-slip faults are further described as "right-lateral" (dextral) or "left-lateral" (sinistral) depending if the block opposite the viewer moved to the right or left, respectively.
There are other processes beneath our feet that have made our topography look like it does today.
“Fluids are also a key process driving the unrest in the Eastern Sierra,” Hill said. “Some of the fluid is magma, which produces quakes and the deformation we see.”
Another process that has been occurring for millions of years is the slow stretch of the Great Basin Range. “The entire area of the Great Basin Range from the Eastern Sierra across to the Wasatch Range in western Utah is being slowly stretched apart by about a half an inch a year. It’s the same as when you stretch taffy. When it thins it allows material from the mantle to come closer to the surface and that is part of the reason we see volcanic activity in the Eastern Sierra as well as along eastern side of the Great Basin in Utah.”
Geologically young
In geologic time our area is very young. According to Hill it was just before Columbus made his way to the Americas when the Obsidian Dome, South Deadman's Dome and the Glass Creek Flow erupted. During the past 1,000 years there have been at least 12 volcanic eruptions along the Mony-Inyo Crater chain. Geologists believe that during the last 5,000 years, an eruption has broken out somewhere along this chain every 250 to 700 years. Additionally, Negit Island in the middle of Mono Lake also erupted less than 700 years ago and Paoha Island in Mono Lake is said to have erupted no more than 250 years ago.
A USGS fact sheet states that as long as increased volcanic unrest including earthquake swarms, ground deformation, and CO2 gas emissions continue in our area, the chances of an eruption occurring in the near future will remain somewhat increased. However, unrest can persist for decades or even centuries without leading to an eruption.
But how much of a stretch is it to think that we will see some resurgence of volcanic activity from the Long Valley Caldera and surrounding craters and volcanoes any time soon? “There is a lull in activity now which is giving us time to look at the enormous amounts of data we acquired when major deformation was going on,” Hill said. “When things were so active in the 1980s and 90s we didn't have time to do much analyzing of the data but now that it's peaceful we can look back and analyze the active processes.”
How long will it stay peaceful? No one knows. The chance of an eruption in any given year is small, but an eruption in the Long Valley area will occur again. USGS scientists are closely monitoring the activity in the region and local and State authorities have developed procedures to alert the public in a timely fashion before an eruption.
Since 2000, “things” have been stable and there hasn't been much movement. “We keep track of ground deformation and our data hasn't shown any additional change since 1979 and 1990's where there was 2-feet of uplift,” Hill said. “Very little has been happening in the last eight years.”
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Last Updated ( Thursday, 19 June 2008 )
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