<P> 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 980 - foot (300 m) - wide volcanic plug . The dikes and plug were exposed by centuries of erosion . At one point, the volcano stood more than 11,000 feet (3,400 m) 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 . </P> <P> 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 4.3 miles (7 km) from its base . </P> <P> With an elevation of 10,052 feet (3,064 m), 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 Collier Glacier, which contains 700 million cubic feet (20 × 10 ^ m) 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 . </P> <P> 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 . </P>

Where does the name three sisters originate from