6) suggests that the eastern limit of the antigorite stability field could extend to near Mount St Helens, therefore, we interpret the lack of PmP arrivals west of Mount St Helens as evidence for a serpentinized upper mantle. Most major volcanoes of the Cascade Arc sit neatly along a north-south line, where the wedging of the Juan de Fuca tectonic plate beneath the North American plate forces hot mantle material to rise. Mount St Helens therefore presents a thermal paradox because it lies directly adjacent to the cold mantle wedge and yet still produces arc derived magmatism which requires elevated temperatures. But instead of finding a hot mantle directly beneath the volcano, seismic data indicates a relatively cool wedge of serpentine rock. One way this dilemma can be resolved is if the lower crustal source region resides east of the volcano, towards the hotter axial region of the volcanic arc, and the ascending melts migrate laterally in the crust towards the forearc region. A local heat source is required to drive dehydration37 and this could be caused by the advection of heat associated with the arrival of mantle melts near the Moho beneath Mount St Helens (path B, Fig.
– Mount St. Helens already stands out as one of the most active volcanoes in the Cascade Arc and the deadliest in the US, since its 1980 eruption claimed nearly 60 lives. It's also an outlier in a literal sense, sitting 30 miles west of the volcanoes that neatly line the Cascade Arc from north to south. Now scientists are reporting in the journal Nature Communications that they've discovered another oddity: The volcano appears to be perched atop what Gizmodo calls "a cool wedge of serpentine rock"—dramatically unlike the fiery cauldrons of hot magma beneath other volcanoes. "We don’t have a good explanation for why that’s the case," Steve Hansen, a geoscientist at the University of New Mexico in Albuquerque, tells Gizmodo. His team drilled a couple dozen holes, filled them with explosives, and triggered minor earthquakes to watch seismic wave activity beneath Mount St. Helens, "a bit like a CAT scan," Hansen says. But their findings leave them with more questions, namely: What's the volcano's heat source, if it's not right below the volcano itself? Hansen surmises that it's coming from further east, but until his team does more research, it's what Science News is calling "a cold case." (Earlier this year, there were dozens of small quakes on the mountain every week.)