(excert from)
The Eruptive History of Mount St. Helens
by Donal R. Mullineaux and Dwight R. Crandell
SWIFT CREEK ERUPTIVE PERIOD
The third eruptive period was characterized by repeated explosive eruptions that initially produced
many pyroclastic flows as
well as pumiceous air-fall tephra deposits, some of which had
large volumes and extended at least as far east as central Washington.
These eruptions of dacite pumice were followed by many
lithic pyroclastic flows, which are believed to have been derived from
domes; at least one of these
pyroclastic flows reached a point 21 km from the center of the present
volcano. The pyroclastic flows were followed, in turn, by another
series of explosive eruptions that produced the voluminous tephra set
J. One coarse pumice layer of set J extends
west-southwest from
Mount St. Helens, and is as much as 20 cm thick as far as 20 km from the
volcano. The layer represents the only coarse and thick pumice known to
have been carried principally in a westerly direction. The sequence of
explosive eruptions that formed set J apparently ended the Swift
Creek
eruptive period sometime before 8,000 yr ago, and was followed by a
quiet period of at least 4,000 yr.
Continue to SMITH CREEK ERUPTIVE PERIOD
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