Living in Geologic Time: Every one of the Pacific Northwest’s volatile volcanoes is likely to erupt again before the range goes extinct.

From British Columbia in Canada to Northern California, an oddly straight line of snow-crowned stratovolcanoes towers thousands of meters above the landscape, visible from cities like Seattle, Wash., and Portland, Ore. In the past 200 years, several of these behemoths have erupted. Most spectacular was Mount St. Helens in 1980, in a blast that scattered ash across 11 states.

So how many of these volcanoes will erupt in the next few thousand years? According to geologists, all of them.

How to Climb a Stratovolcano

Some of the world’s most iconic and dangerous volcanoes are stratovolcanoes, including Mounts Fuji, Pinatubo, Vesuvius, and Kilimanjaro. Stratovolcanoes are identified by steep-sided cones formed by many layers of successive eruptions.

I suspect that many mountaineers earn their ice axes on stratovolcanoes. The first really big mountains I ever climbed were stratovolcanoes: 5,790-meter Cayambe and 5,897-meter Cotopaxi, both in Ecuador. I suspect that many mountaineers earn their ice axes on stratovolcanoes. The climbs tend to be relentless trudges up loose, steep slopes, although some routes require glacier travel skills using ropes, crampons, and ice axes. Vertical rock climbing on stratovolcanoes is rare; the rock is too loose and dangerous.

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The Cascades have enough peaks to keep even the most prolific mountaineer busy for years. Dozens of volcanic features march north to south for over 1,100 kilometers, from the Silverthrone Caldera in southwestern British Columbia, to Lassen Peak in Northern California. The snow-capped stratovolcanoes—including Mounts Baker, Rainier, St. Helens, Hood, and Shasta—are the most visible features in the range, but dozens of calderas, cinder cones, lava domes, and shield volcanoes also occur along this volcanic arc.

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All these diverse volcanic features have one thing in common: They are driven by the Cascadia Subduction Zone just 95 kilometers off the Pacific coast. Here the Gorda, Explorer, and Juan de Fuca tectonic plates are diving under the North American plate at the rate that fingernails grow. As these oceanic plates descend under the continent, the escalating heat and pressure produce hot, buoyant magma that rises and collects in chambers underlying the inland volcanic features.

The existing Cascade volcanoes sit atop the eroded remains of older volcanoes, explained Tom Sisson, a geologist with the U.S. Geological Survey (USGS) in Menlo Park, Calif. “The current Cascade volcanoes are mostly younger than half a million years, and many of them sit atop older volcanoes,” Sisson said. “It’s somewhat arbitrary how we define where the old volcano ends and the younger one begins, but for whatever reason, they seem to form in the same location and have a life span on the order of half a million years.”

Eavesdropping on Sleeping Stratovolcanoes

The volcanoes are relatively active. “If you average things out, there are roughly two eruptions per century in the Cascades, often multiple-year-long eruptions,” Sisson said. Mount St. Helens has hogged the limelight in the past few decades with multiple eruptions in the 1980s and early 1990s, and from 2004 to 2008, but “it would not surprise me if another volcano raised its hand during our lifetimes,” he noted.

In 2018, USGS scientists designated eight volcanoes in Washington and Oregon as “very high threat” based on their eruptive history and proximity to populated areas: Glacier Peak, Mount Baker, Mount Rainier, Mount St. Helens, Mount Hood, Three Sisters, Newberry, and Crater Lake.

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Eruptions in the Cascades have the potential to cause extensive damage and loss of life because of their explosive natures, their years-long eruptive cycles, and their proximities to populated areas. Lahars (mudslides caused by rapidly melting snow and ice) can bury everything downstream. Ash plumes ejected high into the stratosphere from even the most remote peaks can travel vast distances and snarl air traffic.

No climbers were on Mount St. Helens during the 1980 eruption, but three rope teams of mountaineers witnessed the event from near the summit of Mount Adams, another stratovolcano 56 kilometers east of St. Helens. “First it was a little puff at the top of the mountain [St. Helens]. Then, within 2 or 3 seconds, it appeared that the north side of the mountain just blew out. The whole top of the mountain was engulfed in the column of smoke. It rose like an atomic explosion…with sort of a shock wave that went to the north. It reminded me of the pictures you see on late-night TV of the world blowing up,” climber Fred Grimm recounted to the American Alpine Club.

A few minutes after the initial eruption, the sky turned black and the rope teams on Mount Adams were pelted with searing ash and pebbles, forcing them to take cover. None of the climbers was seriously injured, but elsewhere in the world, mountaineers have been killed during volcanic eruptions by noxious hot gas, falling debris, and landslides; in 2014 the Ontake stratovolcano in Japan erupted with little to no warning, killing 56 people on the flanks of the mountain.

Only a few of the Cascade volcanoes are sufficiently instrumented due to a combination of logistical and regulatory barriers. Stratovolcanoes have a sneaky reputation for awakening after hundreds or even thousands of years of silence, but there is usually some warning. Seismic, GPS, and other instruments can often detect magma moving deep underground from days to hours before an eruption. However, only a few of the Cascade volcanoes are sufficiently instrumented due to a combination of logistical and regulatory barriers, said Seth Moran, the scientist-in-charge at the Cascades Volcano Observatory in Vancouver, Wash.

“Mount St. Helens is the gold standard for the Cascades, with 25 seismic stations, 20 GPS stations, and other kinds of equipment,” Moran said. “It’s one of the best monitored volcanoes in the world.”

At the other end of the spectrum is Glacier Peak in northern Washington, “which by some measures is the second most explosive volcano in the Cascades, and yet it has only one seismic station,” said Moran.

Glacier Peak is the most remote of the Cascade volcanoes, and therein lies the problem: The mountain lies entirely within land officially designated as wilderness. “That poses a very substantial permitting problem,” Moran said. The Wilderness Act states that no permanent structures can be erected in designated wilderness areas, but the National Volcano Early Warning and Monitoring System Act authorized by Congress in March 2019 may help overcome that barrier. Last fall the USGS won permission to add monitoring stations within the wilderness zone on Mount Hood, after a 5-year battle. But even with a green light from Congress, logistical issues still abound.

“We can’t just plop down equipment wherever we want it due to the challenging terrain and unforgiving climate,” Moran said. “All of the Cascade volcanoes are [at] fairly high elevation, and winter is rough on our equipment. It’s not uncommon for instruments to get buried under [10 meters] of snow.”

Installing and maintaining equipment in remote, high-altitude settings sounds like a job for mountaineers, especially in the winter. Some researchers have mountaineering skills, whereas others work with specialists in the National Park Service, such as the Mount Rainier Climbing Rangers, Moran says. Snow presents unique challenges, but because the rocky slopes of stratovolcanoes tend to be dangerously loose during the summer, experienced climbers often aim their expeditions in the winter or early spring, when they can negotiate smooth snow on crampons or skis.

The Cataclysmic Cascades: How Explosive Will the Future Be?

So far, I’ve stood on top of about half of the major high points of the Cascades, and I intend to keep climbing. This spring I’m aiming to ski Mount Shasta! After all, there’s no telling how long the Cascade volcanoes will be gracious hosts. “The eight volcanoes that are considered to be the highest threat have all erupted in the [past] 7,000 years, and we would expect them all to erupt again within that kind of time frame,” Moran said.

On longer timescales, however, the fate of the Cascades will be tied to the evolution of the diminishing Cascadia Subduction Zone, said Ray Wells, a structural geologist with the USGS in Portland. “You need subduction to produce the Cascade Range, and the subduction zone is getting progressively smaller over geologic time.”

As it converges at a rate of 50 millimeters per year, the subduction zone is being shortened by 100 kilometers every 2 million years. “We don’t really know how fast it will wink out,” Wells said. “But in 10 million years, I would expect that both the Cascadia Subduction Zone and the Cascade Range will be very different animals.”

—Mary Caperton Morton (@theblondecoyote), Science Writer

Living in Geologic Time is a series of personal accounts that highlight the past, present, and future of famous landmarks on geologic timescales.