Mount St. Helens has a cold stone heart

The 1980 eruption of Washington's Mount St. Helens was the deadliest volcanic event in the history of the United States. But contrary to popular belief, there is no lake of magma within the explosive peak.

Instead, geologists say, Mount St. Helens has a cold stone center. In a study published Wednesday in the journal Nature Communications, researchers suggest that the volcano actually siphons magma from a distant source.

“The melt that supplies Mount St. Helens is probably formed to the east in the mantle wedge below Mount Adams and then moves west through the magmatic system somehow,” said lead author Steven Hansen, a postdoctoral researcher at the University of New Mexico, in a statement.

The study was borne of a collaborative research effort called “Imaging Magma Under St. Helens,” or iMUSH. The $3 million study seeks to map the inner workings of the 8,000-foot volcano and the surrounding Cascade Range.

By setting off explosives near Mount St. Helens, then tracking the seismic waves as they bounce back, researchers produce a kind of ultrasound image. After repeating this countless times, iMUSH discovered that the volcano sits atop a “cold mantle wedge,” not a blistering magma chamber.

Other volcanoes along the Cascadia arc sit on pools of magma that measure more than 1,400 degrees F. But Mount St. Helens’s peak, 30 miles west of that arc, has an internal temperature about 100 degrees cooler – in other words, it’s not hot enough to melt solid rock.

“Given the unusual location of Mount St. Helens, we think that this raises questions regarding the extent of the cold mantle wedge and the source region of melts that are ultimately responsible for volcanism,” Dr. Hansen said.

Exactly how the volcano gets its magma is still unclear. Researchers say it may come from Mount Adams, a peak that sits comfortably within the Cascadia volcanic arc. But chemical differences in volcanic rock from the two mountains may quash that theory.

Understanding the flow of magma through Mount St. Helens could help better understand its recharging process. Since its 1980 eruption, 130 small earthquakes have been detected in the region. But most had magnitudes of 0.5 or less, undetectable by humans.

In May, The Christian Science Monitor’s Corey Fedde reported:

No quakes have started close to the surface and no ground inflammation – the sign of an underground buildup of gas – has been detected.

In other words, say experts, all signs point a simple magma recharging process. As magma refills chambers below the volcano, the weight puts stress on the crust around and above it, causing these tiny earthquakes.

But without a hot magma chamber of its own, scientists are left wondering where Mount St. Helens is getting all this recharging magma is coming from – or if there's another explanation for the many deep, tiny earthquakes attributed to magma recharge.