Middle Sister also known as "Hope," is a stratovolcano consisting primarily of basalt but also has erupted andesite, dacite and rhyodacite. Last erupting approximately 50,000 years ago, it is considered extinct. The mountain's form is that of a cone which has lost its east side to glaciation. The Hayden and Diller glaciers continue to cut into the east face while the Renfrew Glacier is located on the northwestern slope. The large but retreating Collier Glacier descends along the north side of Middle Sister and cuts into North Sister's west side.
Middle Sister is the smallest and most poorly studied. It is also the middle in age, but only somewhat older than South Sister, with the most recent flows dated to 14,000 years ago.
Read more about this topic: Three Sisters (Oregon)
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