Open-File Report 95–497
Mount St. Helens remains a potentially active and dangerous volcano, even though it is now (1995) quiescent. In the last 515 years, it is known to have produced 4 major explosive eruptions (each with at least 1 km3 of eruption deposits, fig. 1) and dozens of lesser eruptions. Two of the major eruptions were separated by only 2 years. One of those, in 1480 A.D., was about 5 times larger than the May 18, 1980 eruption, and even larger eruptions are known to have occurred during Mount St. Helens’ brief but very active 50,000-yr lifetime. Following the most recent major eruption, on May 18, 1980, there were 5 smaller explosive eruptions over a period of 5 months. Thereafter, a series of 16 dome-building eruptions through October 1986 constructed the new, 270-m- (880-ft-) high, lava dome in the crater formed by the May 18, 1980 eruption.
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, 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.
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First posted ~2010
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Wolfe, E.W., and Pierson, T.C., 1995, Volcanic-hazard zonation for Mount St. Helens, Washington, 1995: U.S. Geological Survey Open-File Report 95–497, 12 p., 1 plate, scale 1:100,000, https://pubs.usgs.gov/of/1995/0497/.
Hazardous Geologic Processes
Monitoring and Warnings
References Cited and Suggested Reading