What is the name of the cycle that generated pyroclastic flows and lava domes from vents on the northern, southern, eastern, and southeastern sides of the volcano?
South Sister, also known as "Charity", is the tallest volcano of the trio, standing at 10,363 feet (3,159 m). The eruptive products range from basaltic andesite to rhyolite and rhyodacite. It is a predominantly rhyolitic stratovolcano overlying an older shield structure. Its modern structure is no more than 50,000 years old, and it last erupted about 2,000 years ago. Although its first eruptive events from 50,000 to 30,000 years ago were predominantly rhyolitic, between 38,000 and 32,000 years ago the volcano began to alternate between dacitic/rhyodacitic and rhyolitic eruptions. The volcano built a broad andesitic cone, forming a steep summit cone of andesite about 27,000 years ago. South 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 0.25 mi (0.40 km) 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 3,600 feet (1,100 m). On the volcano's northern flank, below the summit peak, erosion from these glaciers exposed a headwall about 1,200 feet (370 m) high. During the Holocene, smaller glaciers formed, alternating between advance and retreat, depositing moraines and till between 7,000 and 9,000 feet (2,100 and 2,700 m) 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.
Devils Hill eruptive cycle