Where are California’s ‘high-threat’ volcanoes? Closer to Fresno than you may think

Californians have gotten used to the ground beneath their feet flinching spasmodically from earthquakes that occasionally ripple from seismic faults.

Far less common are another geologic hazard confronting the state: the prospect of volcanoes belching lava, ash and gases.

From the Oregon border to the north to the southern border with Mexico, California’s landscape is dotted with craters and cones that mark a tumultuous geologic history.

Now, a recent report suggests that several of these volcanic zones – including the Long Valley Caldera just over the crest of the Sierra Nevada range from Fresno and Madera counties – represent a “very high” threat to life and property, should they happen to belch forth.

Based on a pattern of eruptions over the past 100,000 years in the Long Valley volcanic region, geologists say they believe there is about a 2.5 percent chance of an eruption in the next 10 years and a 22.5 percent change in the next 100 years.

Despite the ominous-sounding threat label, an eruption “is not imminent,” said Keith Putirka, a professor of volcanology at Fresno State. “But we don’t want to pretend it’s not there.”

The Long Valley region, which also encompasses Mammoth Mountain, Mono Lake and Mono Craters, is one of seven areas that are considered “active” volcanoes because they have chambers of molten rock or magma underneath. Two other active volcanoes that are deemed very high threats are Mount Shasta and Lassen Volcanic Center, both in the northern reaches of the state.

Geologists with the U.S. Geological Survey and the California Geological Survey who wrote the report said the threat rankings “are not simply based on the probability of an eruption occurring,” but “reflect the potential for adverse impact should an eruption occur.”

Threat factors include the number of people in the surrounding population, communication and utility sites, emergency facilities, transportation routes and agricultural or crop acreage that could be affected by an eruption and accompanying lava, gases or ash.

There’s no need for Valley residents to rush right out for hard hats or filter masks, however. Putirka said that if Long Valley produces anything, it’s more likely going to be a relatively small eruption compared to what has happened in the past.

“We don’t expect a very large eruption anytime in the near future,” Putirka said. “It already happened more than 700,000 years ago. What we’ve had since is a series of very small eruptions, from Mammoth Mountain and then trending north through the Inyo Craters and Mono Craters.”

The Long Valley Caldera sits about 75 miles northeast of Fresno, on the eastern side of the Sierra Nevada.

Geologists say the caldera – a basin measuring roughly 20 miles wide – was formed by what they describe as a “cataclysmic” eruption almost 770,000 years ago.

Although the surrounding region has experienced volcanic activity for about 4 million years, the big eruption ejected about 150 cubic miles of magma from its underground chamber and scattered ash over much of the western U.S. The basin itself is the result of the emptied magma chamber collapsing on itself, Putirka said.

“The system has been active for a very long time. The big eruption 700,000 years ago was really the culmination of activity that was going on for millions of years,” he added.

“But in no time for as long as Homo sapiens has been on the planet (about 200,000 years) has there been anything as big as Long Valley” or other gigantic “supereruptions” like the ones that formed the Yellowstone Caldera in Wyoming, Putirka said.

The most recent eruptions in the Long Valley region were about 300 years ago in the Mono Lake volcanic field north of the caldera.

Long Valley - Mono Craters satellite.JPG
Tim Sheehan The Fresno Bee

Despite the caldera’s explosive ancient history, “no one should be worried, merely aware that they live in a volcanically active area and that they might have to consider how to deal with volcanic hazards in the future,” said Jessica Ball, a geologist with the U.S. Geological Survey’s California Volcano Observatory in Menlo Park and one of the report’s authors.

Ball and Putirka both said the Long Valley area is festooned with a range of sensors that track seismic activity and ground changes that would likely offer plenty of warning if some kind of eruption was anticipated.

“While it has experienced seismic swarms and ground deformation in the past that would have been cause for increased attention from our scientists, we have not seen any precursors that would indicate that any kind of eruption is near,” Ball said.

“If we did, we would issue information statements and change the alert level at the volcano.”

The volcano is deemed “active” because it has an active hydrothermal system such as hot springs, “and it has been inferred that there is molten magma within the plumbing system that feeds eruptions in the caldera,” Ball said.

“However, we do not always expect a catastrophic eruption at a caldera volcano,” she added. “The most likely activity to see in the area is another dome eruption in the Mono-Inyo chain, rather than a huge explosion in the caldera.” Long Valley is dotted with dozens of small domes and cones that are the results of such smaller eruptions.

The report indicates that nearly 9,000 people live in the immediate vicinity of the volcano that could be subject to “near vent” hazards including fast-moving pyroclastic flows of hot ash and gases, ballistic debris such as rocks or boulders being blasted into the air, lava flows or flooding.

A much larger population – more than 63,000 people in eastern Fresno and Madera counties, as well as Mariposa, Mono and Inyo counties – resides in areas surrounding the caldera that could be affected by falling ash up to 2 inches deep.

volcano map still.JPG
The Long Valley Caldera, highlighted in red, and the Long Valley Volcanic Region including Mono Lake and Mammoth Mountain east of Fresno and Madera counties is the hub of a large area that could experience an ashfall of two inches or more if the volcano were to erupt. Map illustration by Tim Sheehan The Fresno Bee

The ashfall hazard area also includes more than 50 schools; dozens of emergency-service sites such as hospitals, fire and police stations; hundreds of communication towers; and more than 3 million acres in the San Joaquin and Tulare watershed basins.

“The biggest impact would be that you have a lot of ash blown up into the atmosphere,” Putirka said.

“You would have the health effects of people breathing all of this. It’s high in silica, so silicosis would be an issue for those who are heavily exposed.” The federal Occupational Safety and Health Administration indicates that silicosis is characterized by shortness of breath, fatigue and chest pain, and can increase a person’s risk for lung cancer, bronchitis, tuberculosis and other maladies.

Ash that reaches the upper atmosphere can also disrupt airline traffic, similar to what happened in 2010 when a volcanic eruption in Iceland produced enough ash to force airlines to cancel or reroute flights to and from Europe in what was characterized as one of the biggest disruptions to air service since the terrorist attacks of Sept. 11, 2001.

The USGS report indicates that of California’s active volcanoes, the Long Valley volcanic region has the highest volume of airline traffic passing through it on a daily basis – an average of more than 1,200 flights, primarily from the San Francisco Bay Area to destinations in the Midwest and eastern U.S., but also some north-south flights to and from airports in Southern California.

But unlike earthquakes, which typically strike without warning, volcanoes usually provide plenty of hints that something’s going on underground well before an eruption occurs, Putirka said.

“The monitoring that’s taking place, seismic and ground deformation, give us plenty of time to go out and get particle masks if an eruption is large enough to affect us on this side of the Sierra Nevada,” he said.

And, Ball added, earthquakes likely represent the greater threat to the San Joaquin Valley. Many people in the region have at least felt an earthquake, whether as close to Fresno as the Coalinga earthquake in 1983 or more distant temblors such as the Loma Prieta quake that rattled San Francisco and the Bay Area in 1989 or the Northridge earthquake in 1994.

On the other hand, the most recent volcano to blow its top in California happened more than 100 years ago when Lassen Peak erupted from 1914 to 1917.

“The Long Valley volcanic region has a 2.5 percent chance of experiencing an eruption in the next 30 years, while the probabilities of earthquakes in the central San Joaquin Valley are likely higher,” Ball said. “At the moment, since Long Valley is not showing any signs of unrest, an earthquake would probably be more likely to happen and cause damage.”

Lifelong Valley resident Tim Sheehan has worked in the Valley as a reporter and editor since 1986, and has been at The Fresno Bee since 1998. He is currently The Bee’s data reporter and covers California’s high-speed rail project and other transportation issues. He grew up in Madera, has a journalism degree from Fresno State and a master’s degree in leadership studies from Fresno Pacific University.