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Mount Hood, Oregon, an archetypal subduction zone stratovolcano, is dominated by extrusive eruptions of lava flows and domes, coupled with a high degree of homogeneity in erupted lava compositions. Over the last ~500,000 years— the age of the current edifice—the volcano has repeatedly erupted crystal-rich andesites and low SiO2 dacites, with SiO2 contents largely between 55 and 65 weight percent. Lavas also show similar phenocryst mineralogy, compositions, and textures, and are dominated by plagioclase together with pyroxene, amphibole, and occasional olivine. The presence of quenched mafic inclusions, bimodal populations of plagioclase and amphibole, mineral zoning, and a range of other evidence also shows that Mount Hood magmas are produced by quasi-binary mixing between relatively mafic (basaltic) and silicic (rhyodacitic-rhyolitic) parental magmas. Mineral zoning shows that magma mixing occurred very late in the petrogenetic history, within weeks to months of eruption. Mount Hood is a volcanic system driven by mafic recharge, where hot mafic magmas ascending from the mantle or lower crust interact with silicic magmas to produce mixed intermediate compositions. Evidence suggests that the silicic parental magma is stored within the shallow crust (3–6 kilometers) beneath the volcano as cool, crystalrich mush for long periods (>>10 ka) prior to eruptions. Mafic recharge provides both the impetus to erupt and produces the intermediate compositions, resulting in the long-term eruptive output of a homogeneous series of intermediate magmas.

Field-trip guide to Mount Hood, Oregon, highlighting eruptive history and hazards