How dangerous are the Northwest’s volcanoes?

Mount St. Helens erupted on May 18, 1980.

If you live in the Pacific Northwest, chances are good you can see a volcano from where you stand.

In Oregon, Washington state and Idaho, magma has erupted out of the ground in at least 25 places in the last 10,000 years, a mere instant in the lifetime of volcanoes that can be hundreds of thousands of years old.

“There’s reason to believe it could happen again,” said Seth Moran, scientist in charge at the U.S. Geological Survey Cascades Volcano Observatory in Vancouver, Washington.

At 8:32 in the morning on May 18, 1980, the most famous Northwest volcano, Mount St. Helens, woke up, spewing ash for hundreds of miles, devastating the nearby landscape and killing 57 people.

The volcano stirred again in 2004 and 2008.

“Mount St. Helens is the poster child for a very tempestuous volcano,” Moran said.

But what about the other hot spots in the Cascades? Are we going to wake up one morning to a new Mount St. Helens?

Moran’s team, along with partners at the University of Washington, keep watch. They measure earthquakes, ground warping and gas emissions, all caused by magma from deep in the earth making its way to the surface. These volcanic vital signs let geologists compare what a volcano is doing today compared to what it normally does and has done in the past.

“That allows us to establish a range of potential behaviors,” Moran said.

Each volcano has its own personality. Mount Baker in Washington state emits a lot of gas, but doesn’t erupt very often. Mount Rainier, near Seattle, is the icy giant. It’s not likely to explode like Mount St. Helens, but could create large mudflows — think rivers of flowing concrete — caused by smaller eruptions melting the mountain’s ice.

Mount Hood is similar to Rainier, not subject to large explosions, but rather smaller eruptions on its sides. Because so many people live on and play on the mountain, Hood’s small eruptions, and the subsequent landslides and mudflows they create, can present a danger.

Mount St. Helens generally has fewer people around, but is far more active. It erupts once or twice a century, on average. It’s so active, the current mountain is only about 4,000 years old, compared to hundreds of thousand of years for other Cascade volcanoes.

The other volcanoes in Oregon vary widely. Mount Jefferson and Mount Bachelor in central Oregon have been relatively quiet for tens of thousands of years.

“They may be done,” Moran said.

South Sister erupts every couple of thousand years on average. Newberry Crater erupts more frequently and can generate rivers of lava, similar to Hawaii’s famous Kilauea volcano.

Then there’s Crater Lake in southern Oregon. What’s now a lake was once a mountain comparing in size to Mount Rainier.

Around 5600 B.C., an eruption 50 times the size of Mount St. Helens in 1980 leveled the mountain. Moran said that Crater Lake may not be done and is high on the observatory’s list of potential threats.

Three more volcanoes round out the Cascade range: California’s Lassen Peak, Mount Shasta and Medicine Lake.

Moran said these volcanoes don’t typically explode without warning. They have a baseline of earthquake, ground deformation and gas activity, even when they aren’t erupting. The pattern of that activity can be a clue to what will happen next. For example, an earthquake followed by a series of successively smaller earthquakes, might indicate normal seismic activity. But an earthquake followed by a series of larger earthquakes might indicate an eruption is brewing.

Moran said the goal is not to predict which volcano will erupt when, but rather to provide a forecast, similar to a weather forecast. There will always be a range of uncertainty with an active volcano he said, “but the goal … is to get that uncertainty to a range that society can live with.”


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