USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Pine Creek Volcanic Assemblage at Mount St. Helens, Washington
--
Dwight R. Crandell and Donal R. Mullineaux, 1973,
Pine Creek Volcanic Assemblage at Mount St. Helens, Washington;
U.S. Geological Survey Bulletin 1383-A, 23p.
Abstract
The Pine Creek volcanic assemblage consists of the deposits of hot pyroclastic
flows and lahars that are interbedded with alluvium. The assemblage was formed
during two or more postglacial episodes of explosive andesitic and dacitic
volcanism at an old Mount St. Helens volcano which coincided with, but predated,
the modern cone. The assemblage forms a sloping flat-topped to convex valley
fill locally more than 100 meters thick and 1-4 kilometers wide. The fill
extends from the base of the modern volcano, where it emerges from under a broad
apron of younger lahars, lava flows, and alluvium, 16 kilometers southeastward
to the Lewis River valley and thence 70 kilometers downstream. In the Lewis
River valley, however, it is mostly concealed by three large reservoirs. Hot
pyroclastic-flow deposits in the assemblage are found as far as 16 kilometers
from the volcano, lahars reached at least 55 kilometers downvalley, and the
Lewis River transported volcanic rock debris downstream to its confluence with
the Columbia River. Clouds of ash accompanying the hot pyroclastic flows formed
deposits 5-100 centimeters thick and 1-3 kilometers wide which are interbedded
with tephra along both sides of the Pine Creek assemblage as far as 11
kilometers from the volcano. Radiocarbon dates indicate that the oldest Pine
Creek eruptive episode occurred before about 12,000 years ago and the youngest
between 3,000 and about 2,500 years ago.
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02/27/02, Lyn Topinka