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The Eruptive History of Mount St. Helens

by Donal R. Mullineaux and Dwight R. Crandell

SUGAR BOWL ERUPTIVE PERIOD

During the next 1,200 yr, the only eruptions recorded at Mount St. Helens are those associated with the formation of Sugar Bowl, a dome of hypersthene-homblende dacite at the north base of the volcano. During extrusion of the dome, a directed blast carried rock fragments laterally northeastward in a sector at least 50 degrees wide and to a distance of at least 10 km. The resulting deposits are as much as 50 cm thick and consist of ash, lapilli, and breadcrusted blocks of dacite from the dome, fragments of charcoal, and stringers of material eroded from the underlying soil. A single fragment of charcoal from within the deposit has a radiocarbon age of about 1,150 yr, whereas a sample of wood charred and buried by the deposit has an age of about 1,400 yr (Hoblitt and others, 1980, p. 556). We provisionally assign an age of about 1,150 yr to the blast deposit; the older date may have been obtained from a fragment of a mature tree that was overridden by the blast.

A pyroclastic flow deposit of breadcrusted blocks, as well as prismatically jointed blocks of dacite of the same composition as the dome, was found on the north slope of Mount St. Helens downslope from Sugar Bowl; this pyroclastic flow may have occurred at the time of the lateral blast. Three lahars containing breadcrusted blocks of similar dacite were formerly exposed in the North Fork Toutle River valley west of Spirit Lake. These lahars may have been caused by melting of snow by the lateral blast or by the pyroclastic flow.

East Dome, a small dome of hypersthene-homblende dacite at the east base of the volcano, may have been formed at about the same time as the Sugar Bowl dome. East Dome is overlain by tephra of the Kalama period but not of the Castle Creek period, and could have been formed any time between the Castle Creek and Kalama eruptive periods, a time span of about 1,200 yr.

Continue to KALAMA ERUPTIVE PERIOD

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