(excert from)
The Eruptive History of Mount St. Helens
by Donal R. Mullineaux and Dwight R. Crandell
PINE CREEK ERUPTIVE PERIOD
Although only a short time elapsed between the Smith Creek
and Pine Creek periods, eruptive products of Pine Creek age contain an
iron-magnesium phenocryst
assemblage that is distinctly different from
those of Smith Creek age. During the Pine Creek eruptive period, large
pumiceous and lithic pyroclastic
flows moved away from the volcano in
nearly all directions. The lithic pyroclastic flows,
some of which
extended as far as 18 km from the present center of the volcano, are
believed to have been derived from dactic domes. Eruptions of
dactic airfall tephra were of
small volume, but at least four formed
recognizable layers as far away as Mount
Rainier (Mullineaux, 1974, p. 36).
During this time, lahars and fluvial deposits aggraded the
valley
floors of both the North and South Fork Toutle River, and created the
basin of Silver Lake 50 km west-northwest of the volcano by locking a
tributary valley (Mullineaux and Crandell, 1962). Similar deposits also
formed a contiguous fill across the floor of the Cowlitz River valley
near Castle Rock that was about 6 m above present river level; this fill
probably extended 209 km farther to the mouth of the Cowlitz River.
Lahars and fluvial deposits
formed a similar fill in the Lewis River
valley which, near Woodland, was about 7.5 m higher than the present
flood plain (Crandell and Mullineaux, 1973, p. A17-A18).
The eruptions of Pine Creek time extended over a period of about 500
yr. No single eruption of very large volume has been recognized from
deposits of Pine Creek age, and the period seems to have been
characterized by many tens of eruptions of small to moderate volume and
the growth of one or more dacite domes. Some radiocarbon dates on
deposits of Pine Creek and Castle Creek age overlap,
and if the two
eruptive periods were separated by a dormant
interval, it must have been short.
Continue to CASTLE CREEK
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