USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Holocene Rhyodacite Eruptions on the Flanks of South Sister Volcano, Oregon
--
Scott, W.E., 1987,
Holocene Rhyodacite Eruptions on the Flanks of South Sister Volcano, Oregon:
IN: Fink, J.H., 1987, The Emplacement of Silicic Domes and Lava Flows, GSA Special Paper 212
Abstract
Almost 0.9 cubic kilometers (dense-rock equivalent) of rhyodacite tephra, lava domes, and lava
flows erupted from numerous vents on the southwest, southeast, and northeast flanks of South
Sister volcano during late Holocene time. Eruptions occurred during two brief episodes between
2300 and 2000 carbon-14 years B.P., separated by a dormant interval of as long as several
centuries. The eruptions of each episode were probably fed by dikes, on the basis of the
following: the alignment of vents, the chemical uniformity of eruptive products, and
stratigraphic evidence that the eruptions of each episode occurred over a short interval of
time.
Each eruptive episode began with the explosive eruption of air-fall tephra. Small pyroclastic flows and hot
pyroclastic surges erupted from a few vents and traveled as far as 3 kilometers. Rapid snowmelt accompanied
the early phase of each episode and triggered small lahars. Each episode culminated with the extrusion of
lava domes and flows.
The distribution of late Quaternary mafic vents around the area of Holocene rhyodacite vents suggests that a
magma chamber with a maximum areal extent of 30 square kilometers may lie beneath the south flank of South
Sister. The chemical uniformity of the eruptive products of each episode is consistent with each having
tapped a relatively small homogeneous portion of a compositionally zoned magma chamber of much greater
volume than the erupted products. Alternately, if this chemical uniformity reflects the generation and
rapid ascent and eruption of a crustal partial melt, then a large magma chamber need not be present.
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04/10/01, Lyn Topinka