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. -- Scott, 1987 |