(excert from)

The Eruptive History of Mount St. Helens

by Donal R. Mullineaux and Dwight R. Crandell


The second eruptive period probably began about 20,000 yr ago, and was characterized by the eruption of small volumes of pumiceous dacite tephra; it also produced lahars, pyroclastic flows of pumiceous and lithic dacite, a few lava flows of dacite or high-silica andesite (C.A. Hopson, written commun., 1974), and perhaps one or more dacite domes. Several different eruptive episodes can be identified during the period. At least one pumiceous pyroclastic flow moved southward to at least 16 km from the center of the present volcano about 20,350 yr ago (Hyde, 1975, p. B11-B13). Two sequences of air-fall tephra that followed (sets M and K) are separated by a two-part deposit of fine air-laid sediment that locally is a meter or more thick, and that contains at least one weakly developed soil. After another quiet interval during which there was a small amount of soil development, at least two more pyroclastic flows moved south and southeast from the volcano between about 19,000 and 18,000 yr ago. The Cougar eruptive period occurred during the Frasier Glaciation when alpine glaciers in the Cascade Range were at or near their maximum extents, and the products of eruptions generally are poorly preserved.

One lahar that apparently occurred early in the Cougar period is of special interest because of some similarities to the debris avalanche of May 18, 1980, that swept down the North Fork Toutle Valley. The lahar of Cougar age consists of an unsorted and unstratified mixture of gray dacite fragments in a compact matrix of silt and sand as much as 20 m thick. Locally, it contains discrete texturally similar masses of red dacite many meters across. The iron-magnesium mineral content of rocks in the lahar is similar to that of the Ape Canyon period, suggesting that the lahar might have been derived from older parts of the volcano. The lahar was recognized in the Kalama River drainage 8 km southwest of the center of the modern volcano, and on both walls of the Lewis River valley near Swift dam (Hyde, 1975, p. B9-B11). It has not been recognized elsewhere; thus, little is known of its original extent. Its local thickness and heterolithologic character suggest that the lahar might have originated in a large slope failure on the south side of the Mount St. Helens of early Cougar time.

There is no stratigraphic record of volcanism at Mount St. Helens between about 18,000 and 13,000 yr ago.


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