USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Upper Pleistocene Pyroclastic-Flow Deposits and Lahars South of Mount St. Helens
Volcano, Washington
--
Jack H. Hyde, 1975,
Upper Pleistocene Pyroclastic-Flow Deposits and Lahars South of Mount St. Helens
Volcano, Washington:
U.S. Geological Survey Bulletin 1383-B, 20p.
Abstract
The Swift Creek volcanic assemblage consists chiefly of deposits of pyroclastic
flows, lahars, and alluvium which are interbedded with tephra. These deposits
and the glacial drift of late Pleistocene age associated with them provide the
best available record of geologic events at Mount St. Helens during the period
between about 36,000 to 12,000 years ago. The assemblage was formed during
intermittent episodes of andesitic and dacitic volcanism and includes some of
the oldest known volcanic deposits from Mount St. Helens. The Swift Creek
assemblage forms a valley fill that extends 14 kilometers down the Swift Creek
valley and into the Lewis River valley, where it is as much as 200 meters thick.
Pyroclastic flows reached the mouth of Swift Creek and lahars extended at least
24 kilometers farther. Glaciers which originated on the volcano during the
Fraser Glaciation were apparently small and did not extend very far down
valleys. The small size of glaciers, together with the nonvesicular andesitic
and dacitic rock debris in the volcanic assemblage, suggests that the old Mount
St. Helens eruptive center consisted mostly of low domes or a succession of
domes rather than a large, high volcano. The large volume of rock debris
transported into the Lewis River valley during eruptions caused the river to
aggrade; downcutting occurred when the influx of material from the volcano
decreased.
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02/27/01, Lyn Topinka