'Hidden' volcano near Seattle you probably don't know about

Glacier Peak. Courtesy: USGS
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Mount Rainier is considered the world's most dangerous volcano because of its size and how close it is to the population centers of Tacoma and Seattle.

But there's another mountain you've probably never seen that's finally getting attention for the risks it poses to our northern counties.

Glacier Peak lurks within the northern Cascade Mountains. Unlike most of the other Cascade volcanos viewable from I-5 or even Seattle, this is the mountain no one notices.

Yet Glacier Peak sits within the borders of Snohomish County and has a record of violent, even extreme eruptions.

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"So large, in fact, they've found ash in Irish Peat Bogs," said geologist Jim Vallance.

Vallance was a young field assistant on Mount St. Helens in the wake of the 1980 eruption. He remembers doing field work on St. Helens in 1979, when all was quiet.

"It was quiet, it was quiet. You may remember if you were an old timer in the Northwest, that Spirit Lake was a blue body of water with cabins all around," said Vallance. "That all changed dramatically in 1980."

"As impressive as it was, Mt. St. Helens was actually hundreds of feet shorter than Glacier Peak," Vallance points out. "The summit is right here."

Now his role at the Cascades Volcano Observatory is dedicated to understanding Glacier Peak.

Every year's brief field season is on foot or with the help of pack mules to bring out more samples that leads to more understanding.

"I'm working on a giant four-dimensional puzzle. I'm trying to work out, what happened in the past, when did it happen and how often," said Vallance.

When a volcano's glaciers melt during an eruption, it picks up massive amounts of fine dirt and debris. It becomes what's called a lahar.

In the case of Glacier Peak, the geological record shows lahars reaching as far away as Mt. Vernon, Burlington, Stanwood and Puget Sound by following the Skagit and Stillaguamish rivers.

Ben Pauk is geophysicist who works with sensing technologies.

In 2016, a sensitive seismometer, global positioning antennas and other sensors were installed on Glacier Peak, because more seismometers can tip off scientists to the first faint signals that magma's on the move.

"Most typical quakes around volcanoes are very small, very low magnitude, twos, ones, even less," said Pauk.

Then, as seen in the buildup to the 2004 St. Helens eruption, the quakes are constant.

"It's going to generate what's called volcanic tremor. So the ground is just constantly shaking," said Pauk. "And that gives us a really good indication of what type of eruption is going to occur."

Global Positioning Antennas measure when the mountain is actually starting to swell.

"By the time we get to Mt. Vernon, we're likely to see the problems of sedimentation rather than direct impact," said Pauk.

In other words, effects on flooding.

For Jim Vallance, his greater worry is for the town of Darrington, which is much closer to the mountain. But when could an eruption happen?

"It could be this year or a thousand years," said Vallance.

There's is no telling for a scientist who reflects back to that summer of 1979, when St. Helens seemed so quiet.

(Editor's note: This story was first reported in 2015 as part of coverage of the 35th anniversary of the Mount St. Helens eruption.)