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The Eruptive History of Mount St. Helens

by Donal R. Mullineaux and Dwight R. Crandell


DISCUSSION

One of the most interesting features of Mount St. Helens' history is the change in eruptive behavior that occurred about 2,500 yr ago. Eruptions of dacite had characterized the volcano for more than 35,000 yr. Then, with virtually no interruption in eruptive activity, andesite and basalt began to alternate with dacite, and not always in the same order. The chemical composition of eruptive products changed gradually during some episodes and abruptly during others. Thus, basalt followed dacite and dacite succeeded basalt; andesite followed dacite of considerably different SiO2 content, and vice versa. Some of these changes in composition of eruptive products are not adequately explained as results of eruption of cyclic sequences of compositionally different magmas derived from successively deeper levels in a larger magma body that differentiated at shallow depth, as proposed by Hopson (1971) and Hopson and Melson (19800. An alternative explanation that fits the stratigraphic record better, suggested by R.E. Wilcox (oral commun., 1974), is that some changes resulted from repeated contributions from more than one magma body, or from different parts of an inhomogeneous magma.

Explosive eruptions of volumes on the order of 0.1 to 3 km have occurred repeatedly at Mount St. Helens during some eruptive periods in the past. This record suggests that a similar sequence could occur during the present period of activity and could result in one or more explosive magmatic eruptions of similar or larger volume than the eruption of May 18. If the lengths of the last two eruptive periods are a valid guide to the future, we might expect intermittent eruptive activity to continue for several decades.

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