The latest eruptions on South Sister, which occurred in two closely spaced episodes about 2,000 years ago, illustrate a relatively modest scale of eruptive activity. Initial explosive eruptions produced small pyroclastic flows and tephra fallout from several aligned vents low on the south flank. Tephra fallout deposits more than 2 meters (7 feet) thick, composed of pumice, rock fragments, and ash, blanketed areas within 2 kilometers (1 mile) downwind of vents; at 13 kilometers (8 miles) about 10 centimeters (4 inches) fell. Less than one centimeter (0.5 inch) of ash fell at least as far as 40 kilometers (25 miles) south of the vents (at Cultus Lake) and east of the vents (at Bend). Following tephra eruptions, lava emerged from two vent areas, forming a large lava flow, Rock Mesa, and several small lava domes. Decades to a few centuries later, a similar eruptive sequence occurred along a zone of vents that extended from just north of Sparks Lake to high on the southeast flank of South Sister, as well as along a shorter zone on the north flank near Carver Lake. Some of the lava flows and domes of that episode are shown in the accompanying photograph. Similar-style eruptions, but up to about ten times larger in terms of volume of ejecta, occurred during and just before the last ice age, about 30,000 to 15,000 year ago. Sisters77_south_sister_lava_flows_09-77.jpg Aerial view of the south flank of South Sister composite volcano showing numerous blocky lava flows erupted about 2,000 years ago. Early eruptions formed Rock Mesa (just above center, far left), a broad flat flow emplaced on nearly level ground. Subsequent eruptions formed a line of lava domes and flows that extend from Sparks Lake meadow (lower right) to Green Lakes (just above center, far right). Several small lava domes were also formed on the northeast flank, out of view. Cratered cone in lower left is Talapus Butte, a basaltic scoria cone. USGS Photograph taken in September 1977 by C.Dan Miller. [medium size] ... [large size] ... [TIF Format, 25 M] The geologic record shows that even much larger eruptions with much wider impact have occurred in the Three Sisters volcanic center. At least four times in the past 700,000 years, explosive eruptions that were probably sited near the present location of Broken Top and Three Sisters produced pyroclastic flows that swept over a broad area from Sisters to south of Bend. A tephra fallout deposit as thick as 13 meters (42 feet) composed largely of fist-sized and smaller white pumice clasts from one of these eruptions is exposed in numerous pumice quarries. Distal tephra deposits from this event have even been found in northern California and in cores from the northeast Pacific Ocean. Such an event today would be catastrophic for Deschutes County, but, fortunately, events of this magnitude are infrequent. Furthermore, there is no evidence that the large volume of magma necessary to drive such an eruption is present in the Three Sisters region today, nor would such a volume likely be generated in the near future.

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