Eruption History for Middle Sister
Middle Sister formed between 40 and 14 ka, but mostly between 25 and 18 ka, closely in time with South Sister. It has erupted basaltic andesite, andesite, and dacite lavas from its central-vent area, as well as dacites from six flank vents and andesites from three more. An early eruption about 38-ka produced the Obsidian Cliffs lava flow, one of the rare high-silica rhyolites erupted in the Cascades and an important source of obsidian stone tools for Native Americans. Prior to the growth of Middle Sister, there was a period of more than 100,000 years that had little or no volcanic activity. An older, buried edifice of chiefly basalt to andesite lavas that erupted about 180 to 160 ka underlies Middle Sister. Glaciation has removed much of the east half of the volcano, including most of the summit-vent complex, and has sharply steepened its east face.
Several notable features are evident to hikers near Middle Sister and are illustrated in accompanying photographs. The upper two thirds of the volcano is a stack of hundreds of thin basaltic andesite flows erupted in a short episode about 20 ka. From 27 to 18 ka, the largest dacite flank vent built a substantial pile of at least five thick lava flows that filled the saddle between North and Middle Sister; the saddle complex is informally called "Prouty Point," "Black Hump," or "Step Sister." Another notable view is from South Sister of the 14-ka dacite of Irving Glacier with its pronounced levees, little-modified surface, and evidence of its having flowed into glacier ice in the Middle/South saddle.