the Late Pleistocene. North Sister possesses more dikes than any similar Cascade peak, caused by lava intruding into pre-existing rocks. Many of these dikes were pushed aside by the intrusion of a -wide volcanic plug. The dikes and plug were exposed by centuries of erosion. At one point, the volcano stood more than high, but erosion reduced this volume by a quarter to a third. The plug is now exposed and forms North Sister's summits at Prouty Peak and the South Horn. Middle Sister Middle Sister, also known as "Hope", is a basaltic stratovolcano that has also erupted more-viscous andesite, dacite, and rhyodacite. The smallest and least studied of the three, Middle Sister began eruptive activity 48,000 years ago, and it was primarily built by eruptions occurring between 25,000 and 18,000 years ago. One of the earliest major eruptive events (about 38,000 years ago) produced the rhyolite of Obsidian Cliffs on the mountain's northwestern flank. Thick and rich in dacite, the flows extended from the northern and southern sides. They stand in contrast to older, andesitic lava remains that reach as far as from its base. With an elevation of , the mountain is cone-shaped. The eastern side has been heavily eroded by glaciation, while the western face is mostly intact. Of the Three Sisters, Middle Sister has the largest ice cover. The Hayden and Diller glaciers continue to cut into the east face, while the Renfrew Glacier sits on the northwestern slope. The large but retreating , which contains of ice and is the thickest and largest glacier in the region, descends along the north side of Middle Sister, cutting into the west side of North Sister. Erosion from Pleistocene and Holocene glaciation exposed a plug near the center of Middle Sister. South Sister South Sister, also known as "Charity", is the tallest volcano of the trio, standing at . 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