One of America's Most Dangerous Volcanoes Isn't Where It Should Be—Now Scientists May Know Why

Mount St. Helens is well known for being one of the most dangerous volcanoes in the United States. But its location in Skamania County, Washington state, has long puzzled scientists. Essentially, the volcano isn't where it should be because it lies away from areas where magma is thought to bubble to the surface.

Now, a team led by U.S. Geological Survey scientists Paul Bedrosian and Jared Peacock have tried to address this mystery in a study published in the journal Nature Geoscience.

Mount St. Helens is part of the Cascade Arc, a line of volcanoes extending from British Columbia, Canada, to northern California, which are all formed by the convergence of the Juan de Fuca and North American tectonic plates—vast slabs of the Earth's crust. As they collide off the coast of Washington and Oregon, the Juan de Fuca plate is forced below the North American plate, giving rise to the volcanoes, Bedrosian told Newsweek.

"As the plate descends into the Earth's mantle, it heats up and releases water," he said. "This water causes the nearby rock to melt, and the molten rock rises through the crust, much like the 'lava' in a lava lamp. The part that reaches the surface forms the Cascade volcanoes, which have been built up over 40 million years into the mountain range we see today."

"Most Cascade volcanoes fall along a relatively straight line. St. Helens, however, sits about 40 miles west, in a region where scientists don't expect deep magma to surface," he explained. "Not only is St. Helens 'out-of-line', but it is also the most active volcano in the Cascades. Plus St. Helens erupts thick, sticky lavas, which we don't usually see from the other Cascade volcanoes."

For their research, the team used an advanced geophysical imaging technique to create a three-dimensional picture of the crust beneath southwest Washington. This provided them with clues as to how Mount St. Helens formed slightly away from the line of the other major Cascade Arc volcanoes.

"Our work reveals that Mount St. Helens sits directly above an ancient tectonic 'scar' where two plates were welded together 40-50 million years ago," Bedrosian said. "This scar is a weak point in the crust and it acts like a 'crustal valve' channeling deep magma to the surface.

"We also figured out why the vast region—35 times the size of Manhattan—between St. Helens and the expected volcanic 'line" hardly has any volcanic vents. We imaged a batholith—an immense volume of igneous rock that chilled in the crust some 20 million years ago. This batholith causes modern-day magma to pool beneath it rather than penetrating it."

According to the researchers, the study enables us to understand why Mount St. Helens is the most active and explosive volcano in the Cascades, while also having important implications for understanding the hazards posed by volcanoes which lie in unusual locations.

Mount Saint Helens in Washington state erupts in May, 1980. John Barr/Liaison Agency

"To reach the surface, magma takes advantage of weak spots in the crust," Bedrosian said. "Plate tectonics controls where magma forms deep in the Earth, but where it ultimately erupts is impacted by the geologic scars of the past. Worldwide, there are volcanoes in locations that aren't easily explained. Likewise, there are regions without volcanoes where we do expect them. This work tells us that mapping the complex, layered structure of the ancient crust will help us understand the hazards of these unusual volcanoes."

Mount St. Helens is best known for its cataclysmic eruption in 1980, which was the deadliest and most destructive volcanic event in the history of the United States. Since then, it has alternated between periods of moderate activity and dormancy.

The Cascade Arc is itself part of the "Ring of Fire"—a long, horseshoe-shaped, seismically active belt that extends around the Pacific Ocean basin—where many earthquakes and volcanic eruptions occur.

Correction: The image in this story has been updated with a picture of Mt St Helens. This article has been updated with comment from Paul Bedrosian.