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 km3 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 km3 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 SiO2 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.

Bimodal volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and tectonic implications of age‐progressive rhyolites

“…and 1100 C.E. Little Glass Mountain and Glass Mountain flows of the Medicine Lake Volcano yielded about 10% by volume (DRE) of tephra [Heiken et al, 1978;Donnelly-Nolan et al, 1990]. Using these eruptions as proxies for HLP-NWBR volcanism would add an estimated 12-100 km 3 to the volume of the silicic domes in the region.…”

Tectonic Versus Volcanic Origin of the Summit Depression at Medicine Lake Volcano, California

“…Open ground cracks striking NNE to NNW with E-W opening directions are common on the lower flanks and the upper NW flank of the volcano (Donnelly-Nolan, 1990;Donnelly-Nolan et al, 2008). These cracks and volcanic vent alignments that are typically parallel or subparallel to regional normal faults, suggest they are related to regional extension (Anderson, 1941;Donnelly-Nolan, 1990;Donnelly-Nolan et al, 1990, 2008.…”

Automated identification of lava flow structures using local Fourier spectrum of digital elevation data

“…It has an estimated volume of about 0.08 km 3 [Donnelly-Nolan et al, 1990] and dimensions of $1.6 km east-west by 2.1 km north-south ( Figure 3). The MLD has well-developed com- pressional ridges and crease structures that have been documented in previous studies [Anderson and Fink, 1992;Anderson et al, 1998;Fink, 1980a].…”

“…Eruption temperatures ranging from 943°to 1007°C and oxygen fugacities from À11.6 to À10.0 have been estimated using Fe-Ti oxide geothermometry [Gerlach and Grove, 1982]. The MLD also contains fine-grained andesitic, granitic, and gabbroic inclusions [Donnelly-Nolan et al, 1990;Grove and Donnelly-Nolan, 1986].…”

“…Typical ratios of flow to crust thickness for basaltic flows are )100 [Gregg et al, 1998]. Using an estimated flow thickness of 35 m for MLD [Donnelly-Nolan et al, 1990] and the observed relationship of H $ 0.12 L results in crust to total thickness ratios ranging from 4 to 58 (for L = 67 m and L = 5 m, respectively). The similarity of the folding geometry between the Figure 9.…”