Sister remained dormant for 15,000 years, after which its composition shifted from dacitic to more rhyolitic lava. An eruptive episode about 2,200 years ago, termed the Rock Mesa eruptive cycle, first spread volcanic ash from flank vents from the south and southwest flanks, followed by a thick rhyolite lava flow. Next, the Devils Hill eruptive cycle consisted of explosive ash eruptions followed by viscous rhyolitic lava flows. Unlike the previous eruptive period, it was caused by the intrusion of a dike of new silicic magma that erupted from 20 vents on the southeast side and from a smaller line on the north side. These eruptions generated pyroclastic flows and lava domes from vents on the northern, southern, eastern, and southeastern sides of the volcano. These relatively recent, postglacial eruptions suggest the presence of a silicic magma reservoir under South Sister, one that could perhaps lead to future eruptions. Unlike its sister peaks, South Sister has an uneroded summit crater about in diameter that holds a small crater lake known as Teardrop Pool, the highest lake in Oregon. Its cone consists of basaltic andesite along with red scoria and tephra, with exposed black and red inner walls made of scoria. Hodge Crest, a false peak, formed between 28,000 and 24,000 years ago, roughly around the same time as the main cone. Despite its relatively young age, every part of South Sister other than its peak has undergone significant erosion due to Pleistocene and Holocene glaciation. Between 30,000 and 15,000 years ago, South Sister's southern flanks were covered with ice streams, and a small amount of ice extended below . On the volcano's northern flank, erosion from these glaciers exposed a headwall about high. During the Holocene, smaller glaciers formed, alternating between advance and retreat, depositing moraines and till between on the mountain. The Lewis and Clark glaciers have cirques, or glacial valleys, that made the outer walls of the crater rim significantly steeper. The slopes of South Sister contain small glaciers, including the Lost Creek and Prouty glaciers. Recent history and potential hazards When the first geological reconnaissance of the Three Sisters region was published in 1925, its author, Edwin T. Hodge, suggested that the Three Sisters and five smaller mountains in the present-day wilderness area were the remains of an enormous collapsed volcano that had been active during the Miocene or early Pliocene epochs. Naming this ancient volcano Mount Multnomah,