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Almost 0.9 km3 (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 14C yr 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 km. 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 area) extent of 30 km2 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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