Three of California’s volcanoes remain considered a very serious threat, along with 15 others nationwide, according to the first update to the federal government’s high-priority assessment in more than a decade.
Mount Shasta in Siskiyou County, Shasta County, and the Long Valley Caldera, which includes the Mammoth Lakes area in Mono County, are in the highest risk category as defined by the United States Geological Survey.
The USGS list of 18 “very high threat volcanoes” remains the same as the last updated list in 2005. It includes two such volcanoes on the Big Island of Hawaii: Kilauea, which had a new eruption in early 2018 , which destroyed hundreds of homes; and Mauna Loa, considered the largest active volcano on the planet.
Four are in Oregon, including areas like Mount Hood and Crater Lake, and four are in Washington state, including Mount Rainier and Mt. St. Helens, which suffered a deadly eruption in 1980 and is the most active volcano in the Pacific Northwest. .
Five of them are in Alaska and in populated areas near economic infrastructure, or below busy air traffic corridors, the agency said.
There have been 10 eruptions in California over the last 1,000 years, and in any given year, the chance of a major volcanic eruption in the state is about the same as the risk of a major earthquake on the San Andreas fault.
Some of the most scenic places in California are threatened by volcanic activity.
More than 190,000 Californians live in a dangerous area; They include people who live or work in the Long Valley region, home to Mammoth Lakes, a favorite destination for California skiers, and other areas like Shasta Mount Shasta and Weed. Those cities are close enough to the volcanoes to be in danger in the next eruption.
Volcanoes in the Lassen, Shasta and Long Valley areas are capable of producing pyroclastic flows or surges when they erupt: flows of hot ash, rock and gas that travel rapidly down mountainsides, like the one that killed 57 people when Mount St. Helens erupted in 1980.
Those areas are considered to be in the second highest risk category, as is the Medicine Lake volcano in Siskiyou and Modoc counties.
But volcanic eruptions could have long-lasting repercussions that could affect all of California. Volcanic ash could bring down planes and disrupt hundreds of flights that pass daily through Northern California or the Mammoth Mountain area.
In 2010, the eruption of Iceland’s Eyjafjallajökull volcano forced the cancellation of 100,000 flights in a single week.
Volcanic ash can disrupt high-voltage lines that supply electricity to millions of California homes. The ash could disrupt travel on Interstate 5, the main route between California and Oregon, reducing visibility on windshields and making roads slippery and even impassable. It could contaminate water supplies in much of the state. The largest nature reserves in California are near the Shasta and Lassen volcanoes.
California volcanoes were most prolific in prehistoric times. Approximately 760,000 years ago, a powerful eruption occurred in what is now known as the Long Valley Caldera, blasting out some 140 cubic miles of magma covering much of central and eastern California, sending ash as far as Nebraska.
By contrast, Mt. St. Helens erupted and only released 0.05 cubic miles of material.
There is no sign that there is enough magma below the Long Valley to cause another super eruption.
About a century ago, the Lassen volcano had volcanic activity. In one of the most significant events of 103 years ago, Lassen Peak created a gigantic mushroom cloud that reached a height of 30,000 feet.
It could be seen as far away as Eureka and Sacramento, and it sent volcanic ash up to 280 miles away, reaching Elko, Nevada.
It was the first volcanic eruption in the 48 contiguous states since the founding of the United States, and the last until Mt. St. Helens erupted on May 18, 1980.
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