What are the "Three Sisters" and which is the oldest?
The ages of most volcanoes in the Three Sisters area are not precisely known. North Sister, a basaltic andesite pyroclastic and lava cone that rests on a shield volcano, is the oldest of the Three Sisters and postdates the approximately 0.3-million-year-old Shevlin Park Tuff. Middle Sister is intermediate in age between North and South Sister and, like South Sister, is compositionally diverse. Broken Top volcano is also younger than Shevlin Park Tuff and is older than South Sister, but its age relation to Middle and North Sister is not known. The relative degree of erosion of Broken Top is a complex composite cone of dominantly basaltic andesite that intermittently erupted andesite, dacite, and rhyolite as lava flows, pyroclastic flows, and pyroclastic falls. Cayuse Crater, which is located between Broken Top and the Cascades Lakes Highway, and two nearby vents on the southwest flank of Broken Top erupted during the earliest Holocene or lates Pleistocene time, but these events were probably unrelated to the long-inactive Broken Top system.
South Sister is the youngest composite volcano of the Three Sisters-Broken Top center and has erupted lavas ranging from basaltic andesite through rhyolite. Although not dated directly, most, if not all, of South Sister is probably of late Pleistocene age. ... The cone of basaltic andesite that forms the summit of South Sister is probably of latest Pleistocene age; its crater is still closed and is filled with 60 meters of ice and snow. Le Conte Crater, a basaltic andesite scoria cone on the south flank, is between about 15,000 and 6,850 years old. The youngest eruptions recognized on the volcano occurred at a series of vents on the south and northeast flanks that erupted rhyolite tephra and lava flows and domes between about 2,200 and 2,000 years before the present.
Sources of Information:
Hoblitt, R.P., Miller, C.D., and Scott, W.E., 1987, Volcanic Hazards with Regard to Siting Nuclear-Power Plants in the Pacific Northwest: U.S. Geological Survey Open-File Report 87-297, 196 p.
Scott, W.E., and Gardner, C.A., 1990, Field trip guide to the central Oregon High Cascades, Part 1: Mount Bachelor-South Sister area: Oregon Geology, September 1990, v.52, n.5, p. 99-117.