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The present dormant state of Mount St. Helens began in 1856, and no way is now known of determining when the volcano will erupt again. Mount St. Helens' behavior pattern during the last 4,500 years has been one of spasmodic periods of activity, separated by five or six dormant intervals of a little more than 2 to about 5 centuries' duration. In addition, 12 dormant periods 1 or 2 centuries in length have been identified, and many intervals of a few years or a few decades surely occurred during prolonged periods of intermittent eruptive activity. The volcano's behavior pattern suggests that the current quiet interval will not last as long as a thousand years; instead, an eruption is more likely to occur within the next hundred years, and perhaps even before the end of this century.
It has been suggested that all three rock types erupted at Mount St. Helens in the past have been derived from an andesitic parent magma, and that a typical eruptive period starts with the appearance of an explosive gas-rich dacite which forms pumice (Hopson, 1971). This is followed successively by the formation of a dacite dome and lava flows of andesite and possibly basalt. Hopson implied that such a succession would occur over a period of a few years or several decades. If such a sequence is followed during future eruptions, the greatest potential danger will exist at or soon after the onset of volcanic activity. However, not all past eruptions at Mount St. Helens have followed ths idealized succession, and it is by no means certain that all future eruptions will. It is quite possible that any of the three rock types will be erupted singly or in some combination in the future. One possible sequence could start with the eruption of a small amount of an andesite or basalt tephra from a vent on the volcano's flank, followed by a lava flow of similar composition from the same vent. Or it is conceivable that dacite tephra could be erupted from one vent as andesite or basalt tephra and lava flows were being erupted at another vent if magmas of different composition were present beneath the volcano at the same time.
Because of the variable behavior of the volcano, we cannot be sure whether the next eruption will produce lava flows, pyroclastic flows, tephra, or volcanic domes, or some combination of these. If the next eruptive period is like the last, which continued from about 1831 to 1856, intermittent activity on various scales and of various kinds can be expected over a period of several decades.
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