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.
Future eruptions are certain.
Although we do not know when the next one will occur, it should be planned for. It updates previous assessments, taking into account both recent experience at erupting volcanoes and topographic, hydrologic, and geologic changes initiated at Mount St. Helens by the 1980 eruptions.