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Yes, Cascades volcanoes could blow — but not like Hawaii’s

Mount Rainier stands in the foreground, framed in the background by Mount Adams, far left, Mount Hood, second left, and Mount St. Helens, right, in this aerial view over the central Cascade mountains east of Seattle. (Elaine Thompson / Seattle Times)

As Mount Kilauea unleashes a world-class show of geologic force on Hawaii’s Big Island, some Northwesterners are eyeing the Cascade Range with a new wariness.

“Mental note: Don’t live near a volcano!” one Seattleite tweeted last week, pointing out the city’s view of Mount Rainier.

@MountRainier WA itself — or its anonymous social-media avatar — also took to Twitter with a warning for May, which is Washington State Volcano Preparedness Month: “Everybody go out and buy a gallon of water, a windup radio and all of the lava-resistant clothes you can find.”

It’s an empty threat at the moment, scientists say, but a good reminder that the Pacific Northwest is home to an 800-mile chain of active volcanoes that have erupted in the recent past and will erupt again in the future.

With their towering, snow-capped peaks, the Cascade volcanoes are very different beasts from Hawaii’s low-slung, shield volcanoes, says Seth Moran, scientist-in-charge of the U.S. Geological Survey’s Cascades Volcano Observatory. They don’t erupt as often and they don’t spew out as much magma — but when they do let loose, the result can be the type of explosive blast epitomized by Mount St. Helens’ 1980 cataclysm.

Nine Cascade volcanoes have the potential to erupt in our lifetimes, Moran said. (In Washington they are: Mount Rainier, Mount St. Helens, Glacier Peak, Mount Baker and Mount Adams. In Oregon: Crater Lake, Mount Hood, Newberry Volcano and the Three Sisters.)

All are currently quiet, but the eruption that blew off Mount St. Helens’ top in 1980 and the dome-building eruptions that followed in 2004-2008 show how quickly the giants can stir to life.

“Those two eruptions came out of nowhere, and we had about a week’s worth of earthquakes before the first explosion,” Moran said. “We’ve got nine volcanoes that really could erupt tomorrow, and we need to treat that as a serious possibility.”

The most likely scenario, though, is that any eruption will be preceded by days to weeks of seismic rumblings, ground movement and gas emissions that signal the movement of magma — and which can be detected by ongoing monitoring at all the region’s volcanoes.

“It’s our job not to be surprised,” Moran said.

Given its restless nature, geologists say Mount St. Helens is the odds-on favorite to erupt next. But six other Cascade volcanoes have been active in the past 300 years, including steam eruptions at Glacier Peak and Mount Rainier and a 1915 blast at Mount Lassen, in California, that destroyed nearby ranches.

But the type of scenes playing out in Hawaii, with rivers of lava snaking through neighborhoods and sprouting fountains, are highly unlikely in the Pacific Northwest, Moran said. Cascade volcanos produce a thicker, more viscous type of lava than Hawaiian volcanoes, so it doesn’t run out as far. Also, very few people live on the flanks of our volcanoes.

The closest settlement to a volcanic crater is probably Government Camp, gateway to the ski resorts on Mount Hood, Moran said. Lava could conceivably reach the town, but the greater threat there is that an eruption could trigger a pyroclastic flow — a fast-moving cloud of hot ash and gas.

By far the greatest danger to people and property from Cascades volcanoes are lahars — volcanic mudflows triggered when eruptions collapse slopes and melt glaciers. The resulting slurry can roar down valleys at immense speeds and travel for dozens of miles.

Black-and-white film from 1980 showed bridges, logging trucks and houses being carried away in churning rivers of mud and debris from Mount St. Helens. A massive lahar off Mount Rainier 5,600 years ago flowed all the way to Puget Sound and left thick deposits in the Puyallup River valley, atop which the communities of Puyallup, Orting, Buckley and Enumclaw were later built.

Seattle is probably safely out of reach of any future volcanic lahars, Moran said. For communities in harm’s way along the Puyallup River, a warning system was installed in 1998 and will be upgraded soon, he added.

The system uses seismometers and trip wires to detect the roar of an approaching lahar and trigger sirens and phone alerts so residents can evacuate to high ground. But there’s no similar warning system in towns like Darrington or Arlington, downstream of Glacier Peak.

“There have been some pretty big lahars that have come down there thousands of years ago,” Moran said.

The most far-reaching effect of a Cascade volcano eruption would be the massive amounts of ash that can be lofted into the air. It’s mostly a nuisance, though volcanic ash suspended in air can choke airplane engines. It’s not healthy to breathe, either.

The plume from Mount St. Helens rose more than 80,000 feet. Prevailing winds deposited thick, gritty layers across Central and Eastern Washington and carried a fine dusting to the East Coast. Within two weeks, tendrils extended around the globe.

The Cascades Volcano Observatory (CVO) posts weekly status reports for the region’s volcanoes. All currently register “Normal.”

But that doesn’t stop those who operate unofficial Twitter accounts in the volcanoes’ names from trash talking and boasting. @GlacierPeak describes itself as “10,541 feet of potentially explosive doom, topped with glaciers and attitude.”

@MountStHelensWA claims to be “more badass than the mountain to the north.” @MtBakerWA takes potshots at St. Helens for exploding sideways.

The volcanologists at CVO say they don’t know who’s behind the pseudonymous accounts. Their attention is riveted on the real drama unfolding in Hawaii.

Several scientists from the Northwest traveled there to help, while those left behind are following the developments with the fascination of experts in a discipline built around events that don’t occur very often.

“If all of us who work with volcanoes were brutally honest with ourselves, this is why we do it,” Moran said. “This is an amazing event. It’s also tragic, to see homes destroyed. But watching something like this evolve so fast, it’s just so rare that you can see the Earth do something like that.”