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 (sediment-rich floods in volcanic terrain) generated by snowmelt are likely to accompany any eruptive activity. 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. Neither a large debris avalanche nor a major lateral blast like those of May 18, 1980 is likely now that a deep, open crater has formed.
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. The pressure needed to overcome this blockage may exceed that of any eruption since May 18, 1980; therefore, the next eruption may be initially explosive owing simply to blockage of the conduit. 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. This report delineates areas that are likely to be at risk (hazard zones) during another major eruption. 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. These changes include (1) beheading of the summit, forming a truncated cone with a deep crater open to the north, (2) a large and growing volume of snow and ice in the crater, (3) the existence of a large, potentially unstable, debris-dammed lake (Castle Lake) in a tributary to the North Fork Toutle River, and (4) large volumes of erodible sediment in most of the river valleys draining the volcano.
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