The Three Sisters area contains 5 large cones of Quaternary age-- North Sister, Middle Sister, South Sister, Broken Top, and Mount Bachelor. North Sister and Broken Top are deeply dissected and probably have been inactive for at least 100,000 years. Middle Sister is younger than North Sister, and was active in late Pleistocene but not postglacial time. South Sister is the least dissected; its basaltic andesite summit cone has a well preserved crater. Most of South Sister predates late Wisconsin glaciation and is therefore older than 25,000 years; however, eruptions of rhyolite from flank vents have occurred as recently as 2,000 years ago. -- Hoblitt, et.al., 1987
Details:
- North Sister, 3,074 meters (10,085 feet)
- Middle Sister, 3,062 meters 10,047 feet)
- South Sister, 3,157 meters, (10,358 feet)