(excert from)

The Eruptive History of Mount St. Helens

by Donal R. Mullineaux and Dwight R. Crandell


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.


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