South Sister is the southernmost and tallest of the Three Sisters. Early eruptions began between 50 to 45 ka, when South Sister produced numerous rhyolite lava flows and domes. Between 37 and 30 ka, the broad cone was built of rhyolite to andesite domes and flows, and construction of the cone ended with an explosive eruption that left pyroclastic deposits greater than 200 m (650 ft) thick.
An intense period of eruptive activity occurred between 30 and 22 ka and was almost entirely centered on the summit. These included mildly explosive eruptions of andesite as lava fountains, which fed lava flows that built up the steep radial apron of the cone and developed an 800-m-wide (2,600-ft) summit crater. Between about 25 and 23 ka, dacite lava flows filled and overtopped the former summit crater and subsequent explosive eruptions produced material that make up much of the uppermost 150 m of the summit cone of today’s South Sister. A eruption producing a thin cap of basaltic andesite lava and scoria ended the summit-building sequence around 22 ka.
The volcano had been quiet for about 20,000 years when two episodes of rhyolite eruptions occurred. The first, about 2.2 ka, began explosively sending ash at least as far as 30 km (20 mi) from the vent. Subsequent lava flows produced several small domes as well as the Rock Mesa coulee at the southwest toe of the volcano. At about 2.0 ka, a similar eruptive sequence occurred along two linear zones that opened on the southeast and north flanks of South Sister. These eruptions formed the Devils Chain, a 5–km–long (3–mi–long) chain of 20 domes and coulees to the southeast, and a 1.2-km-long similar chain of minor vents to the north. Sixty percent of the magma released during the younger, Devils Chain, episode issued from a vent at 2,400 m (7,800 ft) elevation on the South Sister edifice, which formed the large Newberry flow.
