Magmatic precursors to the 18 May 1980 eruption of Mount St. Helens, USA

Perhaps the most difficult task facing volcanologists today is that of distinguishing between low-level volcanic restlessness and activity that presages a full-scale eruption. We illustrate these difficulties by reexamining the sequence of events that led to the 18 May 1980 eruption of Mount St. Helens, activity that is often presented as a classic example of early phreatic explosions leading to a magmatic eruption. This interpretation is problematic, however, given the known ascent of juvenile magma to within similar to250 m of the surface prior to the climactic event. Reexamination of ash samples from several precursory events shows that small amounts of juvenile material were erupted as early as 28 March 1980, just two weeks after the first seismic signals of reawakening, and that the juvenile content of the ash had probably increased by early May. The magmatic component of these eruptions was not recognized at the time because of the high crystallinity of the juvenile material-a signature of extensive degassing-induced crystallization during magma ascent. Identification of juvenile material in the precursory ash from Mount St. Helens has important implications for initiation of eruptive activity.