Following the dramatic 1980 eruption of Mount St Helens, geophysicists rushed to study the movement of magma beneath the volcano’s surface. They concluded that magma builds up in shallow chambers 3–12 km within the crust. But where the magma originates in Earth's mantle, and thus how magma migrates to drive eruptions at Mount St Helens and similar volcanoes, remains poorly understood.
Now a study led by University of New Mexico researchers Steven Hansen and Brandon Schmandt refutes the idea that the volcano draws magma from melted mantle material directly below. As part of the Imaging Magma Under St Helens project, the researchers detonated an array of explosives near the summit crater. They monitored how the resulting seismic P waves reflected from the crust–mantle boundary and used the measurements to create detailed images of the deep crustal structure under the volcano. The lack of reflected waves beneath the crater suggests that Mount St Helens sits atop a nearly solid wedge of mantle made of cold serpentine rock that forms when mantle minerals react with water. Further to the east, reflected seismic energy increased 10–30%, suggesting a strong material contrast between the crust and an underlying warmer mantle.
The team proposes that magma is generated in the east, beneath other volcanoes in the same range, and migrates laterally in the mantle. Clusters of deep (23–44 km) long-period earthquakes, which are thought to be caused by moving magma, were detected southeast of the volcano and support the researchers’ conclusion. The observations will help scientists better understand subduction zones. (S. Hansen et al., Nat. Commun. 7, 13242, 2016.)