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The Eruptive History of Mount St. Helens

by Donal R. Mullineaux and Dwight R. Crandell


The first stratigraphic evidence of the existence of Mount St. Helens consists of voluminous dacitic deposits of slightly vesicular to pumiceous air-fall tephra and pyroclastic flows, and at least one pumice-bearing lahar. These deposits overlie extensively weathered glacial drift formed during the next-to-last alpine glaciation of the Cascade Range. The volcanic deposits were formed during at least four episodes, separated by intervals during which very weak soils developed. The entire eruptive period may have extended over a time span as long as 5,000 yr. One pumiceous tephra deposit produced during the period probably had a volume as great as that of any subsequent tephra erupted at Mount St. Helens.

The Ape Canyon eruptive period was followed by a dormant interval which may have lasted from about 35,000 to 20,000 yr ago. Most of this 15,000-yr interval coincided with climates which, at times, were evidently somewhat cooler than those of the present (Alley, 1979, p. 233).


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