Building up to the next blast

Pete Caster/Lewiston TribuneIn this photo taken May 18, 2016, contrails follow a plane as it passes by Mount St. Helens in the Gifford Pinchot National Forest. Now 40 years to the day that St. Helens erupted, the mountain remains the most active in the Cascade Range.

Ask Seth Moran what’s new with Mount St. Helens, and he’ll tell you not much.

Forty years after its cataclysmic eruption put the Cascade volcano on the scientific map, St. Helens is outwardly calm and well-behaved. There’s no reason to think it presents an imminent danger.

The mountain “isn’t doing anything different from what it usually does,” said Moran, chief scientist at the U.S. Geological Survey’s Cascade Volcano Observatory.

The caveat, though, is that what Mount St. Helens usually does “is really interesting.”

“It has the highest background seismicity of any Cascade volcano,” Moran said. “It averages a couple of (small) earthquakes a day. Over the winter there was a lull, but about a month ago we started seeing an uptick to five or 10 events per day. It comes and goes.”

The Cascade Volcano Observatory was established in 1982, specifically to study and monitor activity at Mount St. Helens and the other volcanoes in the Cascade Range, which extends from British Columbia, Canada, south to Mount Shasta and Lassen Peak in Northern California.

Based on those nearly 40 years worth of observations, scientists interpret the small quakes as the signal of molten rock rising up from the underlying mantle into shallower magma reservoirs beneath the mountain.

“It’s likely some form of magma recharge to replace magma that was erupted (during Mount St. Helens’ last awakening) in 2004 to 2008,” Moran said. “It’s part of the life cycle of the volcano, as it builds to the next eruption.”

That next eruption, he said, “could be next year, or it could be a decade from now.”

Exactly what type of eruption it will be also remains a mystery. One of the notable characteristics about Mount St. Helens, Moran said, is that it displays a wider variety of eruptive behavior than almost any volcano in the Cascade Range.

“That’s one of the enigmas about Mount St. Helens,” he said. “It has had explosive eruptions like 1980 — and even (bigger ones) about 500 years ago — as well as Hawaiian-style lava eruptions.”

The 2004-08 eruptions, for example, extruded more than 92 million cubic yards of rather drab, gray lava. It oozed out onto the crater floor, creating a new dome about 1,500 feet high. However, there were no disruptions outside of the park.

The reasons for this varied behavior aren’t entirely clear, Moran said. It likely has something to do with the complex “plumbing” or magma circulatory systems beneath the mountain, as well as the speed at which magma rises from the mantle to the surface.

It’s a factor of time as well. While the Cascade volcanic arc has been active for about 37 million years, most of the major volcanoes people see today are relatively recent phenomena, less than 2 million years old.

The oldest volcanic rocks at Mount St. Helens, by contrast, are only about 275,000 years old — and most are much, much younger.

“The thing that’s really remarkable about Mount St. Helens is that most of what we see today was erupted in the last 4,000 years,” Moran said. “That takes the cake in the Cascade arc as far as recent eruptive volume.”

Combined with the ongoing signals of magma recharge, it’s one of the reasons why the Cascade Volcano Observatory lists Mount St. Helens as “the most active volcano in the Cascade Range, and most likely volcano in the contiguous United States to erupt again.”

“Six-thousand years ago, we wouldn’t have been talking about Mount St. Helens,” Moran said.

That may have been true even 41 years ago, before the 1980 eruption.

“That event really put the Cascades on the map,” Moran said. “Before that, only a handful of geologists were studying the Cascade volcanoes. After the eruption, the world came to Mount St. Helens.”

Scientists came to study the volcano, he said, chronicling the stages of the eruption and learning about the mudslides, landslides and other deposits produced during a major event.

“Then they went home and applied those lessons to their own volcanoes,” Moran said.

There are more than 4,000 volcanoes, fissures, cinder cones and volcanic domes in the Cascade Range. Most are small and relatively short-lived. Over the past 200 years, seven are known to have had at least minor eruptions, including Mount Rainier in the 1800s and Lassen Peak from 1914-17.

Moran said there are “occasional signs of activity” at other Cascade peaks, such as the surface deformation and uplift that took place at South Sister in Oregon in 2001. However, Mount St. Helens is the only volcano providing a clear signal of magma recharge.

That’s why ongoing monitoring of the mountain is so important, he said. It gives scientists some insight into when St. Helens is behaving “normally” and when it might be entering a new phase of its eruptive story.

Spence may be contacted at bspence@lmtribune.com or (208) 791-9168.

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