Mount St. Helens Possible Future Behavior

Most earth-science studies are concerned with past events, and the axiom that "the present is the key to the past" is fundamental to these studies. In recent years, as earth scientists have been asked repeatedly to look forward in time, the axiom that "the past and present are keys to the future" has become increasingly significant. Clues to the possible future behavior of Mount St. Helens are gleaned from its past eruptive history. During the past 50,000 years, Mount St. Helens has experienced nine "eruptive periods," not counting the activity of the 1980s. The term eruptive period is informally used by geoscientists for a segment of a volcano's eruptive history encompassing a series of eruptive episodes closely associated in time and/or type of eruptive processes or products; such periods are separated by dormant intervals, generally of longer duration.

The most recent and best known of the pre-1980 eruptive periods began with a major explosive eruption in 1800 A.D. 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. Such activity could include the outpouring of lava flows (not observed to date), as well as renewed dome growth and small-to-moderate explosive events. The chance of another catastrophic landslide and blast comparable to that of May 18, 1980, is exceedingly low. The past history of the volcano suggests, however, that one or more explosive eruptions with heavy ash fall comparable to that of the May 18, 1980, eruption might occur before Mount St. Helens returns to a dormant state. 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.

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 cubic kilometer of eruption deposits) 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-year 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-meter- (-880- feet) 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, 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.