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The Eruptive History of Mount St. Helens

by Donal R. Mullineaux and Dwight R. Crandell

CASTLE CREEK ERUPTIVE PERIOD

The next period of activity marked a significant change in eruptive behavior and variety of rock types being erupted at Mount St. Helens. During the Castle Creek eruptive period, both andesite and basalt were erupted as well as dacite, and these rock types evidently alternated in quick succession. The overall sequence includes, from oldest to youngest, andesite, dacite, basalt, andesite, dacite, basalt.

Thus, the stratigraphic sequence of Castle Creek time is complex, and not all stratigraphic units are represented on all sides of the volcano. Northwest of Mount St. Helens, in the Castle Creek valley, the sequence preserved includes the following:

Lava flow of olivine basalt (youngest)
Lava flow of hypersthene-augite andesite
Tephra deposit of olivine-augite andesite scoria (layer Bo)
Pyroclastic-flow deposits of hypersthene-dacite pumice
Tephra deposit of hypersthene-augite andesite scoria (layer Bh)
Lava flow and lahars of hypersthene-augite andesite (oldest)

The pumiceous pyroclastic-flow deposits have a radiocarbon age of 2,000-2,200 yr. Deposits and rocks of Castle Creek age on the south and east flanks of the volcano include pahoehoe basalt lava flows whose radiocarbon age is about 1,900 yr, and pumiceous dacite tephra whose age is about 1,800 yr (layer Bi.). East of the volcano, layer Bi overlies a pyroclastic-flow deposit of pyroxene andesite, and directly underlies thin olivine basalt lava flows which probably are correlative with the uppermost unit in the Castle Creek valley. The Dogs Head dacite dome was extruded before those thin olivine basalt flows, probably during the Castle Creek eruptive period. Layer Bu is the youngest tephra of Castle Creek age; it underlies a deposit whose radiocarbon age is about 1,620 yr. This tephra is basaltic and probably was formed when thin olivine basalt lava flows were erupted near the end of the Castle Creek period.

Castle Creek time marked the start of eruptions that built the modern volcano. It is interesting to note that the change in eruptive behavior from that of the preceding 35,000-plus years did not follow a long period of dormancy like several that occurred during Mount St. Helens' earlier history. The dormant interval that followed Castle Creek time apparently lasted about 600 yr.

Continue to SUGAR BOWL ERUPTIVE PERIOD

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