Subterranean Eruption Clues Volcano monitoring relies upon detecting changing physical conditions around an active source, such as increased seismicity or ground deformation; however, relating these behaviors to magmatic processes below ground remains challenging. The geochemical signatures of erupted magmatic crystals, including chemically distinct zones formed during growth and cooling, provide clues as to the conditions of the magma chamber prior to the eruption. Through a combination of chemical analysis of orthopyroxene crystals and chronometry based on diffusion, Saunders et al. (p. 1023 ) were able to link the magmatic processes and seismic events leading up to and during the 1980–1986 series of eruptions at Mount St. Helens, USA. The seismic events corresponded to earthquakes caused by magma degassing and ground movement—events of which are routinely detected with geophysical instruments.