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The Eruptive History of Mount St. Helens

by Donal R. Mullineaux and Dwight R. Crandell

KALAMA ERUPTIVE PERIOD

Most of the rocks visible at the surface of the volcano before eruptions began in 1980 were formed during the Kalama eruptive period. Although the range in radiocarbon dates and ages of trees on deposits of Kalama age suggest that the eruptive period lasted from nearly 500 to 350 yr ago, all the events described here probably occurred during a shorter time span, perhaps less than a century.

The Kalama eruptive period began with the explosive eruption of a large volume of dacite pumice (layer Wn) which forms the basal part of tephra set W. Layer Wn was deposited northeastward from the volcano across northeastern Washington and into Canada (Smith and others, 1977, p. 209) and was followed by additional pumice layers. At about the same time, pyroclastic flows of pumiceous and lithic dacite moved down the southwest flank of the volcano. The relative timing of these events is poorly known because most of the air-fall tephra was carried eastward and northeastward, whereas the pyroclastic flows have been found only on the southwest flank of Mount St. Helens.

A short time later, scoriaceous tephra of andesitic composition was erupted. In addition, andesite lava flows extended down the west, south, and east slopes of the volcano, and andesite pyroclastic flows moved down the north, west, and south flanks.

These eruptions of andesite were followed by the extrusion of the dacite dome that formed the summit of the volcano before the May 18, 1980, eruption. Avalanches of hot debris from the dome spilled down over the upper parts of the preceding lava flows, and some of this hot debris partly filled channels between levees of the andesite lava flows on the south side of the volcano (Hoblitt and others, 1980, p. 558). Late in this eruptive period, a pyroclastic flow of pumiceous dacite moved northwestward from the volcano down the Castle Creek valley and covered lahars of summit-dome debris. Charcoal from the pyroclastic-flow deposit has a radiocarbon age of about 350 yr (Hoblitt and others, 1980, p. 558).

The Kalama eruptive period was characterized by frequent volcanism of considerable variety; rock types being erupted alternated from dacite to andesite and back to dacite, and the volcano grew to its pre-1980 size and shape. The eruptive period was followed by a dormant interval of about 200 yr.

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