Post-11,000-year volcanism at Medicine Lake Volcano, Cascade Range, northern California

Abstract

Eruptive activity during the past 11,000 years at Medicine Lake volcano has been episodic. Eight eruptions produced about 5.3 km 3 of basaltic lava during an interval of a few hundred years about 10,500 years B.P. After a hiatus of about 6000 years, eruptive activity resumed with a small andesite eruption at about 4300 years B.P. Approximately 2.5 km 3 of lava with compositions ranging from basalt to rhyolite vented in nine eruptions during an interval of about 3400 years in late Holocene time. The most recent eruption occurred about 900 years B.P. A compositional gap in SiO 2 values of erupted lavas occurs between 58 and 63%. The gap is spanned by chilled magmatic inclusions in late Holocene silicic lavas. Late Holocene andesitic to rhyolitic lavas were probably derived by fractionation, assimilation, and mixing from high-alumina basalt parental magma, possibly from basalt intruded into the volcano during the early mafic episode. Many basaltic to andesitic lavas contain iron-rich crystals and have high FeO*/MgO (characteristics caused by mixing of high-alumina basalt with ferrobasalt liquid produced by fractionation of parental high-alumina basalt). When ferrobasalt and high-alumina basalt are contaminated with a granitic crustal component, a calc-alkaline trend is produced. Some eruptions have produced both tholeiitic and calc-alkaline compositions. The eruptive activity is probably driven by intrusions of basalt that occur during east-west stretching of the crust in an extensional tectonic environment. Vents are typically aligned parallel or subparallel to major structural features, most commonly within 30° of north. Intruded magma should provide adequate heat for commercial geothermal development if sufficient fluids can be found. The nature and timing of future volcanic activity cannot be predicted from the observed pattern, but eruptions high on the edifice could produce high-silica products that might be accompanied by explosive activity, whereas eruptions lower on the flanks are likely to vent more fluid mafic lavas.