LASSEN VOLCANIC NATIONAL PARK — When it comes to volcanoes, the U.S. National Park System offers explosive drama, famously blasting with regularity from a couple of high-profile parks. Both have lately been rumbling away.

Though most do not compete with Kīlauea’s familiar fireworks, or Yellowstone’s crowd-pleasing geysers, it turns out that the National Park Service lists 21 parks as active volcano sites, Lassen Volcanic National Park in Northern California among them.

As its name attests, Lassen’s volcanic features as well as its spectacular blow-the-lid-off eruption in 1915 are what led to its national park designation in 1916, the same year that the Park Service itself was founded.

Like most other volcanic parks, Lassen is not eruptive like Hawai'i Volcanoes National Park’s Kīlauea, one of the world’s most active volcanoes where magma lately has been on the move and earthquakes, including a 1,500-quake swarm over one six-day period, abruptly increased after a June 3 eruption.  

Nor do the less heralded parks mimic Mauna Loa, the world’s biggest active volcano and Kīlauea’s sibling. It perked up in 2022 after decades of calm, its third eruption since 1950, pouring a lava lake into its caldera, spewing lava 200 feet skyward, and sending fiery orange entrails slithering down its slopes.

The 1915 eruption of Lassen Peak, seen in the two images above, were captured by Chester Mullen and B.F. Loomis/B.F. Loomis Collection

As well, other volcanic parks don’t match the visitor-thrilling, world-class geysers of Yellowstone, whose massive hydrothermal system also sparked a violent explosion just this summer when water boiling below Biscuit Basin’s hot blue pools blasted steam and debris hundreds of feet skyward, destroying a section of boardwalk and flinging boulders across the terrain.

The 21 parks in fact are among 91 park units the Park Service lists as having volcanoes or volcanic rocks. But only those 21 are designated “active/recent,” the caveat being that “recent” here is not what human-timescale minds might envision. To be on the list, a volcano must have erupted in the past 11,700 years years (the Holocene, which is the youngest geologic division of time), or like Yellowstone be considered potentially active, according to the Park Service definition.  

But even if demure in comparison to the biggies, Lassen and its peers have no shortage of fiery history and future potential, with secret lives beneath the surface and a trove of mysteries yet to be solved. They retain visible evidence of collapsed mountaintops, hurled boulders and denuded terrain. In some cases it’s only telltale rocks or ancient lava flows that betray their volcanism. But Lassen by contrast boasts hydrothermal features that on a daily basis offer a hot, burbling glimpse into nature’s vast plumbing that ultimately, scientists say, will trigger a new eruption.

A “Mini-Volcanic Theme Park”

Modest in size compared to its California cousins — Death Valley, the biggest national park in the lower 48, and Yosemite, which logged nearly 4 million visitors through its gates last year — Lassen draws some 400,000 visitors a year to its hiking trails, snow fun, and impressive natural wonders.

Unlike Mount Rainier or Mount St. Helens, which are solo volcanic mountains, Lassen is a broad “volcanic center,” an intricate geologic plumbing system of long-lived volcanism atop a magmatic source that erupts from numerous volcanos over a prolonged time.

The Lassen Volcanic Center’s birth 825,000 years ago was the latest stage of a 3.5 million-year evolution that saw four earlier volcanic centers leave their marks on the land before fizzling out.

Sprawling today across roughly 200 square miles within and outside the park boundaries, the center encompasses 30 distinct volcanic domes, along with massive boulder jumbles and gurgling hydrothermal areas. Part of the landscape, including remnant peaks and steamy, muddy works, are left where Mount Tehama, also known as Brokeoff Volcano, once measured eight miles in diameter — until it was weakened by hot acidic hydrothermal fluids and eroded away by glaciers after 200,000 years of eruptive activity.

”In my mind, it’s just such an underappreciated park because you've got these really accessible volcanic features and a really big variety of them all in one place,” said Jessica Ball, volcano hazards specialist and spokesperson at the California Volcano Observatory operated by the U.S. Geological Survey.

Hydrothermal areas with names like Bumpass Hell and Sulphur Works are a big visitor draw along the park’s 30-mile highway, noted Ball. “That is unusual, because for a lot of volcanoes you have to travel a long way and do a lot of work to get to what you want to see,” the roadless volcanic parks of the Alaskan Arctic being a prime example. “And at Lassen, it's just sort of this mini volcanic theme park for volcano science.”  Among others, the park’s visual offerings include:     

• One of the world’s largest volcanic domes, Lassen Peak, whose 1914-1917 eruption series marks the second-most recent in the Cascades. Mount St. Helens blew in 1980 to become the most recent. Topping out at 10,457 feet, Lassen Peak is one of the park’s 30 distinct dome volcanoes that have erupted in the last 300,000 years.
• A hydrothermal system of burping mudpots, steaming pools and smelly vents emitting sulfurous gases, all propelled to the surface by steam from boiling water that’s heated by magma underground. It is the largest hydrothermal field in the Cascade Range, and the most impressive, USGS scientists say.
• Diverse volcano types, differentiated by the way they erupted, some sending miles-long lava flows across the landscape, others tossing massive boulders around like Tinker Toys, still others pushing lava up but not spilling down the slopes, instead piling up debris and lava within a caldera.
• The “Devastated Area,” with its thousands of strewn boulders that speak to the might of the 1915 eruption that threw hot boulders onto the snow and sent a rock and pyroclastic flow some 4 miles downhill.

Afoot or by car, these features are not hard to access. Day hikes in the park reveal craters, lava tubes, lava beds, hydrothermal vents and ponds, and various types of volcanic rocks from a turbulent past.

“I didn't know something like that existed. It was a real wake up moment for me. It was like, ‘The earth is alive, the earth is changing. Things are happening under the ground that we don't see,’” said Sierra Coon, the Lassen park’s chief of interpretation, describing her first look at the roaring steam, mudpots and multicolored soils of Bumpass Hell. She was about 10, visiting the park with her dad. “It is an experience that a lot of people probably have going to a place like Lassen,” she told the Traveler. The profound impression stuck with her and influenced her decision to study geology in college, she added.

On a weekday early this summer a modest but steady flow of tourists wandered into the historic, stone-sided Loomis Museum near Manzanita Lake. There, they viewed remarkable black-and-white photos of Lassen Peak belching angry, dark volcano clouds across the sky. Video of the 1915 eruption is believed to be one of the first showing a volcanic eruption, and the availability of photos was likely tied to its relatively non-remote location within sight of populated towns, Coon said.  

“It does not have the explosive geysers Yellowstone has, and it may not be as tall as Mount Shasta,” said Andy Calvert, a geologist and scientist in charge at California Volcano Observatory, referring to Lassen’s ice and snow-wrapped neighbor rising from U.S. Forest Service land off Oregon-bound Interstate 5. “But it’s a fantastic place.” He added, “It’s significantly under-visited.”

            Celebrating Volcanoes in the National Parks

            National Parks on Volcano Highest Threat List

The Age Of Argon-40

Some 280 miles south of Lassen, along the San Francisco Bay in Sunnyvale, Calvert and Ball are on the USGS team that monitors and studies volcanoes in California and Nevada. The California Volcano Observatory is one of five such observatories, the others keeping an eye on volcanism in Hawaii, Alaska, the Pacific Northwest and Yellowstone. It is newly situated on the sunny campus of NASA Ames Research Center at Moffett Field after moving from its longtime Menlo Park address a few miles up the San Francisco Peninsula.

In his new, glass-walled lab — amid mass spectrometers, small vacuum chambers, pumps and lasers that melt crystals and rock fragments — Calvert figures out the ages of old rocks. His findings are important to volcanic research, especially at places like Lassen where clues to Earth’s machinations are held by volcanic rocks that erupted over millennia.

Calvert extracts and measures their argon content, and gamely gave the Traveler a primer about how potassium-40 in rocks decays into argon-40. It’s easier to date older rocks, he said, but his focus is “really young” rocks, meaning thousands of years old instead of billions.

“If we can figure out the past, that’s the key to the future,” he said.

High Threat

Calvert dubs Lassen “a capable volcano” and “an active place.”  But it might surprise people wandering along the Lily Pond Nature Trail or poking around the stinky Sulphur Works that it’s also a “high threat” volcano — a designation that bears explanation.

Mike Clynne talks about the earthquake and volcano dynamics of the Pacific where the Hawaiian Islands anchor the southeast end of a chain of volcanoes that began forming more than 70 million years ago. Each island began with one or more volcanoes erupting on the ocean floor and then emerging above sea level as continued eruptions built/Rita Beamish

In USGS parlance, high threat is not a forecast about the likelihood of eruption. Rather it focuses on the potential damage an eruption could inflict, explained Mike Clynne, a white-bearded research geologist at the observatory wearing a T-shirt adorned with the Periodic Table of the Elements. A USGS veteran of nearly 45 years, Clynne probably knows more than anyone about Lassen volcanism.

The high-threat nomenclature not only takes into account Lassen’s “recent” eruption and past, but it also “means that there's a significant chance of damage or death if there was an eruption that was not forecast,” Clynne told the Traveler.

In essence, it incorporates the human factor: “How many people live near the volcano? How close are they?” Ball explained. “Are there big pieces of infrastructure that we have to worry about? Is it hard for those people to evacuate if they needed to? So it's sort of a holistic view of the volcano and, what kind of damage it could do if it were to erupt again.”

Rural communities around Lassen, more populous than in 1915, would be a concern, not to mention park visitors, Clynne’s analysis shows. He anticipates Lassen would likely produce a small explosive eruption with violent ejection of hot lava blocks, and lava would flow a few kilometers. Heavy rain or snow could intensify mudflows and landslides moving rapidly downslope.  

The Big Question

So Lassen is a high-threat volcano. But will it erupt and when?

“It will certainly erupt again over the next hundreds of thousands of years, though there are no signs it’s happening soon,” said Calvert. “On the human timescale that’s a more difficult question. We watch it. Given strong evidence for magma (melted rock) beneath Lassen and other California volcanoes, we could see an eruption in our lifetimes, and we need to be ready.”  

Clynne’s analysis, looking at the 100,000-year record, puts probability of a Lassen eruption in any given year at about 1 chance in 7,150. Violent eruptions are driven by magma rising to punch through the surface; magma’s ability to move like that depends on its chemical and mineral composition, which accounts for its density, combined with a temperature hot enough to get the magma fluid enough to move — its viscosity. The magma now beneath Lassen is too viscous to erupt — in other words, it’s more or less stuck — unless hotter magma, perhaps activated by tectonic movement, mixes into it, Clynne said.

Crags Lake is reached by a modest trail not far from the Manzanita Lake campground in Lassen National Park, below the Chaos Crags area that erupted some 1,100 years ago. A subsequent dome collapse created the mass of rocks known as Chaos Jumbles/Rita Beamish

In the past 100,000 years, Lassen Volcanic Center had 13 eruptions, averaging one every 7,000 years.

But the picture is more complicated. “In reality the events tend to happen in groups,” Clynne explained. “So a repose period can be much longer than the 7,000 years, and you might have five events in 1,000 years. As an example, there have been three events at Lassen in the last 1,100 years.”

Besides Lassen Peak’s 1914-17 series of eruptions, the Lassen Volcanic Center had another about 1,100 years ago and also one in 1666. Other than that, Lassen’s past 25,000 years have been “relatively quiet,” USGS says. But the three “recent” eruptions as well as its underground magma reservoir and deep seismicity related to magma and/or fluid movement still make it an active volcano, the geologists say. And at the surface, the bubbling, hissing hydrothermal features are a visible indicator that the volcano is not extinct, said Clynne.

No Need To Freak Out

The good news: An eruption won’t take us by surprise, thanks to intensified monitoring since the 1980 Mount St. Helens eruption, the USGS experts say. “Volcanoes tend to give you warning,” sometimes years in advance, explained Calvert.

USGS has installed earthquake-detecting seismometers and accelerometers in and around the park, and also relies on GPS equipment and satellite radar imagery that shows ground surface changes, specifically inflation and deflation caused by below-ground movement of magma.

“Volcanoes almost always inflate when they're getting ready to erupt, meaning they puff up … because magma is either coming up shallowly near the surface or actually into the edifice," said Clynne. Lassen currently is and has been deflating.

“Most likely we would have days to weeks to months, and potentially more warning,” Clynne said. The park would have time to evacuate and close, and regional alerts could go out.

Action In The Park

Though not akin to what’s been churning beneath Kīlauea, Lassen did have a recent burble of excitement: An earthquake swarm rippled through on June 24 for the first time since October 2022. Before that, the last swarm was in 2014. Among dozens of quakes detected in the recent swarm, the largest magnitude was 2.5 — nothing that would cause Californians to do anything other than roll over and go back to sleep, although they probably wouldn’t feel it at all.

Small quakes are frequent, but swarms “do not come all that often,” said Alicia Hotovec-Ellis, a volcano seismologist whose small office at the volcano observatory is decorated with her own vibrant artwork depicting volcanos. The seismicity “tends to come in clumps,” she told the Traveler. “It'll be really active for a few days, and then it'll just kind of be quiet for a while.” 

Why Does It Matter?

California is always shaking, mostly unremarkably, so what’s important about Lassen’s underground rattling? Lassen sits atop a convergence of tectonic plates, the Cascadia Subduction Zone, and when the plates shift, magma can rise, causing earthquakes as it moves through rock. Quakes thus can be an indicator of magma heading up to cause eruption.

This summer’s swarm was not such an occasion, the USGS team determined. “The likelihood of an eruption at Lassen right now is low,” they concluded.

“Lassen can be quiet, but is often very chatty,” said Hotovec-Ellis. Importantly, many of its quakes are not magma related but hydrothermally-caused, she said. “Basically hot water filtering through cracks makes lots of little earthquakes.”

Next Hot Spot?

The last eruption from Lassen Peak will not foreshadow the location of the next, USGS experts say. That’s because lava domes like Lassen Peak tend to close up after erupting once and are unlikely to host a new blast (although Lassen Peak itself in a rare exception that did have additional eruptions). “The conduit gets plugged up when the lava solidifies, and it's easier for new magma to come up in a different place,” explained Clynne.  

Where that different place might be is hard to divine, at least until the Earth starts sending out some hints.

The USGS team has its eye on Lassen and its broader region as well as other California volcanic centers where, Calvert says, “there’s lots of potential for activity.” They plan to expand the Lassen monitoring network, Hotovec-Ellis said, “to capture seismic activity associated with possible volcanism in the broader volcanic area away from Lassen Peak itself."

Traveler footnote: Lassen Volcanic National Park is in wildfire country, and is still recovering from the devastation of the 2021 Dixie fire, one of California’s largest on record. But one thing fires don’t affect is volcanic activity. “The volcano has no idea that there is a fire on the surface,” said Andy Calvert, scientist in charge at the California Volcano Observatory. Wildfire has the park on edge again this summer. With the fierce Park Fire bearing down not far from park boundaries, officials closed Lassen in late July as a precaution. However, on August 17 the north entrance to the park reopened.