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Geology and History Summary for Three Sisters

The cluster of glaciated stratovolcanoes called the Three Sisters extends for 20 km (12 mi) along the crest of the Cascade Range in Oregon, 35 km (20 mi) west of the city of Bend and 100 km (60 mi) east of Eugene. 

Map of the Three Sisters region with lakes, streams, and volcanic features labeled.
Map of the Three Sisters region, showing locations of notable features in the area.(Public domain.)

The broad Sisters span of the Cascade Arc extends about 40 km (25 mi) north and south of the Three Sisters cluster and contains at least 466 volcanoes of Quaternary age (chiefly the past one million years). Most of these erupted during short time periods (months to years), but many longer-lived mafic shields and stratocones of basalt to andesite are also present. At the latitude of the Three Sisters, the vent belt is about 35 km (22 mi) wide, extending about 12 km (7 mi) west and 22 km (13 mi) east of the Cascade crest. A central area focused on South and Middle Sister is composed of more evolved (andesite to rhyolite) and relatively young volcanic rocks that are surrounded by mafic (basalt to andesite) volcanoes. North Sister is part of this so-called mafic periphery and is almost entirely older than eruptive products of the other two Sisters. Broken Top, also a stratovolcano in the mafic periphery, completely preceded the Three Sisters in time.

The Three Sisters were formed at different times by different types of magma. North Sister (3,074 m or 10,085 ft) is a long-lived basalt-to-andesite volcano that last erupted about 55 ka. Middle (3,062 m or 10,047 ft) and South Sister (3,157 m or 10, 358 ft) began to grow around the same time, but South Sister erupted primarily rhyolite and dacite, whereas Middle Sister erupted primarily andesite and dacite except for an early high-silica rhyolite. The abundance of rhyolite lava at the two younger Sisters is unusual among Cascade stratovolcanoes.

Postglacial (past 15,000 years) eruptions of basalt are more common in the Sisters reach than anywhere else in the Cascade Range. The only postglacial eruptions within the central volcanic focus of Middle and South Sister were late Holocene rhyolite episodes at Rock Mesa and the Devils Chain (including Newberry flow) described in the South Sister section.

Three Sisters annotated photograph viewed from south to North from Broken Top, Oregon. (Public domain.)