We report the results of recent geologic mapping and
radiometric dating that add considerable detail to our understanding of the eruptive history of Mount St. Helens before
its latest, or Spirit Lake, stage. New data and reevaluation of
earlier work indicate at least two eruptive periods during the
earliest, or Ape Canyon, stage, possibly separated by a long
hiatus: one about 300-250 ka and a second about 160–35 ka.
Volcanism during this stage included eruption of biotite- and
quartz-bearing dacite domes and pyroclastic flows in the area
west of and beneath the present-day edifice, accompanied by
the deposition of set C tephras. Ape Canyon-stage rocks are
compositionally similar to younger Mount St. Helens dacite.
The Cougar stage, about 28-18 ka, was probably the most
active eruptive stage in Mount St. Helens’ history before the
Spirit Lake stage. During the Cougar stage, a debris avalanche
buried the area south of the present-day edifice, and voluminous pyroclastic flows, dacite domes, tephra, and a largevolume pyroxene andesite lava flow were erupted. Two tephra
sets, M and K, were deposited midway through this stage.
Swift Creek-stage deposits were emplaced in two phases,
beginning about 16 ka and ending about 12.8 ka. During the
first phase, set S tephras and three large fans and at least one
smaller fan of dacitic fragmental material were deposited on
the northwest, west, south, and southeast flanks of Mount
St. Helens. The fans are dominated by lithic pyroclastic-flow
deposits associated with dome building but include both primary and reworked material from pumiceous pyroclastic flows
and lahars. One Swift Creek-age dome on the west flank of the
volcano has been located, and others must have been nearby. During the second phase, set J tephras were deposited, but no
pyroclastic flows or domes are known to be associated with
the andesitic set J tephras.
Preliminary petrographic analysis of these older rocks
suggests that the volcano’s magmatic system was simpler during the Ape Canyon stage than during subsequent stages and
that the magmatic system has evolved from relatively simple to
more complex as the volcano matured. Compositional cycles
as envisioned by C.A. Hopson and W.G. Melson for the Spirit
Lake stage probably did not occur during the Ape Canyon stage
but developed later during the Cougar and Swift Creek stages.
Additional Publication Details
USGS Numbered Series
The Pleistocene eruptive history of Mount St. Helens, Washington, from 300,000 to 12,800 years before present: Chapter 28 in A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006