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.