Medicine Lake Volcano
Geologic Setting and Eruptive History

During the past 11,000 years, eruptive activity at Medicine Lake volcano has been episodic. Eight eruptions produced about 5.3 cubic kilometers of basaltic lava during a time interval of a few hundred years about 10,500 years ago. That eruptive episode was followed by a hiatus that ended with a small andesitic eruption about 4,300 years ago. During the most recent eruptive episode between 3000 and 900 years ago, eight eruptions produced approximately 2.5 cubic kilometers of lava ranging in composition from basalt to rhyolite. Late Holocene lava compositions include basalt and andesite, but silicic lavas dominate (Donnelly-Nolan, et.al., 1989).

Medicine Lake caldera is a 7 x 12 kilometer depression in the summit area of the volcano. Anderson (1941) suggested that the caldera formed by collapse after a large volume of andesite was erupted from vents along the caldera rim. However, the distribution of late Pleistocene vents, mostly concentrated along the rim, suggests that ring faults already existed when most of the andesite erupted (Donnelly-Nolan, 1988). No single large eruption has been related to caldera formation. The only eruption recognized to have produced ash flow tuff occurred in late Pleistocene time, and this eruption was too small to account for formation of the caldera (Donnelly-Nolan and Nolan, 1986). Donnelly-Nolan (1988) concluded that Medicine Lake caldera formed by collapse in response to repeated extrusions of mostly mafic lava beginning early in the history of the volcano (perhaps in a manner similar to the formation of Kilauea caldera, Hawaii). She hypothesized several small differentiated magma bodies fed by and interspersed among a plexus of dikes and sills. In her model, late Holocene andesitic to rhyolitic lavas were derived by fractionation, assimilation, and mixing from high alumina basalt parental magma.