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.
