GEOLOGIC MAP OF THE 350 km3 BASALT-TO-DACITE VENIAMINOF VOLCANO, ALEUTIAN ARC

The geology of tholeiitic Mount Veniaminof, one of the largest and most active volcanoes in the Aleutian arc, has been mapped at 1:50,000 scale. The Veniaminof edifice has a basal diameter of ~40 km, a volume of ~350 km³, and an 8‑km-diameter ice-filled caldera. Over 100 Quaternary volcanic map units have been identified, resting on a basement of Tertiary sedimentary and volcanic rocks. The geologic map is supported by 600 chemical analyses of rocks and nearly 100 ⁴⁰Ar/³⁹Ar and K–Ar ages. Throughout its history, lava flows from Veniaminof recorded alternately ice/melt-water chilling or ice-free conditions that are consistent with independent paleoclimatic records. Exposures from deep glacial valleys to the caldera rim reveal a long history dominated by basalt and basaltic andesite from ≥260 ka to 150 ka that includes lavas with compositions as primitive as 9.4% MgO and 130 ppm Ni at 50% SiO₂. Basaltic andesite is common throughout Veniaminof's eruptive history. Repeated eruption of more differentiated melts from a shallow intrusive complex, represented by xenoliths in pyroclastic deposits, has featured virtually aphyric andesite since 150 ka and dacite (to 69.5% SiO₂) beginning ~110 ka. A large composite cone was present at least as early as 200 ka. Although asymmetric edifice morphology hints at early sector collapse to the southeast, coeval vents on northwest and southeast flanks and the distribution of extensive lava units indicate that a large cone (again) was present by 120 ka. Flank eruption of a wide variety of Veniaminof magmas was common from plate-convergence-parallel northwest-trending fissures from at least as early as ca. 80 ka. At 56 ka and at 46 ka, voluminous dacite lava erupted on both northwest and southeast flanks. A dacitic pyroclastic-flow deposit on the northwest flank, sandwiched between 46±1‑ka dacite and 28±4‑ka andesite lava flows, may record early caldera collapse as also evidenced by thick 33±6‑ka ice-chilled dacite lava that must have banked against ice in a depression spatially coincident with the northwest part of the present caldera. The modern caldera formed, or was enlarged, during two voluminous post-glacial explosive eruptions ~3700 ¹⁴C yr BP and >4700 ¹⁴C yr BP (Miller et al., 2002 Fall AGU, V11A-1376) that emplaced andesitic pyroclastic-flow deposits on Veniaminof's flanks and in surrounding valleys.