(excert from)

The Eruptive History of Mount St. Helens

by Donal R. Mullineaux and Dwight R. Crandell


Multiple explosive eruptions of the Smith Creek eruptive period, which began about 4,000 yr ago, initiated at least 700 yr of intermittent and at times voluminous eruptive activity. Three coarse pumice layers at the base of tephra set Y are overlain by layers of denser, somewhat vesicular tephra. Deposition of these units was followed by an interval during which a soil began to develop on the tephra. The next eruption of the period produced the most voluminous and widespread tephra deposit of the last 4,000 yr; it is one of the largest, if not the largest, in the history of the volcano, and has an estimated volume of at least 3 km. The resulting pumice layer, Yn, has been found nearly 900 km to the north-northeast in Canada (Westgate and others, 1970, p. 184). The formation of this layer was followed shortly by another voluminous eruption of tephra, which resulted in layer Ye (Mullineaux and others, 1975, p. 331), then by a pumiceous pyroclastic flow and a coarse lithic pyroclastic flow. The lithic pyroclastic flow was accompanied by clouds of ash that spread at least a kilometer beyond the sides of the flow and as much as 2 km beyond its front. Many smaller eruptions of lithic and moderately vesicular ash and lapilli followed, perhaps within a few years or tens of years.

Lahars and pyroclastic flows of Smith Creek age formed a fan north of the volcano, and lahars extended down the North Fork Toutle River at least as far as 50 km downvalley from Spirit Lake. An ancestor of the lake probably came into existence at this time, dammed in the North Fork valley by the fan of lahars and pyroclastic-flow deposits. It is not known if the lake ever existed before Smith Creek time.

A dormant interval of apparently no more than a few hundred years followed the Smith Creek eruptive period.


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