Clues to the possible future behavior of Mount St. Helens are gleaned from its past eruptive history. For the next 57 years, this event was followed by intermittent relatively small explosive eruptions, lava flows, and the extrusion of a lava dome. Assuming that Mount St. Helens behaves as it did in the 19th century, the present activity could continue intermittently for years, possibly decades. This history of intermittent activity is one of the most important reasons why scientists continue to monitor the volcano to detect the intensive, sustained seismic activity and ground deformation that can be expected to accompany any massive infusion of new magma required to break the pattern of dome building in the 1980s and to feed an explosive eruption of major proportions. Two of the major eruptions were separated by only 2 years.
Volcanoes commonly repeat their past behavior.
Thus, it is likely that the types, frequencies, and magnitudes of past activity will be repeated in the future. Among the possibilities for renewed eruptive activity at Mount St. Helens are resumption of dome growth, eruption of basaltic or andesitic tephra and lava flows, or explosive eruptions of dacitic tephra and pyroclastic flows in volumes that could be as large as or even larger than the volume erupted in 1980. Lahars may also be generated without an eruption by intense storm runoff over erodible sediment, by landslides, or by failure of the Castle Lake impoundment as a consequence of an earthquake or heavy rains. Sufficient time has elapsed since the last dome-building eruption in October 1986 for magma in the conduit beneath the dome to crystallize and form a plug. Several scenarios for renewed eruptive activity notwithstanding, a conservative approach to hazards assessment requires us to assume, until there is specific evidence to the contrary, that the next eruption will be explosive and as large as or larger than the eruption of May 18, 1980.