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South Sister, Oregon, USA


U.S. Geological Survey photo by Lyn Topinka, 1984 (fair use policy)

South Sister is the tallest of the Three Sisters volcanoes of central Oregon, by about a hundred meters. This and other nearby volcanic structures form an unusually dense cluster along the crest of the High Cascades, a beautiful western backdrop to the growing city of Bend.

South Sister is a composite cone, a tall, long-lived structure built of a mixture of flows and ash beds. Hundreds of similar volcanoes run along the western edge of North America between northern California and the Alaska Peninsula, then across the northernmost Pacific Ocean to the far tip of the Aleutian Islands. All of them arise from the subduction of oceanic crust into the mantle beneath the North American plate.

There has been no volcanic activity here for at least 1000 years. But in the spring of 2001, satellite interferometry measurements detected a deep-seated bulge just a little west of South Sister. In the last five years, that area has risen as much as 10 centimeters, part of the slow, obscure movement of magma in the crust far beneath us. No one can yet say whether this uplift is typical or not—until the satellite technique was devised, this sort of activity was very difficult to observe. Keep up with developments at at the Cascades Volcano Observatory site.

Pictures from the Cascades Range

Plate Tectonics in a Nutshell

Back to the Gallery of Peaks

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