Surficial Geologic Map of Mount Veniaminof Volcano, Alaska

Abstract:

Mount Veniaminof volcano is a >300 km³ andesite to dacite stratovolcano, characterized by an 8 x 11 km diameter ice-filled summit caldera. Veniaminof is one of the most active volcanoes in the Aleutian arc and has erupted at least 15 times in the past 200 years. The volcano is located on the Alaska Peninsula (56.1979° N, 159.3931° W) about 780 km SW of Anchorage. Our geologic investigations have documented two large (>VEI 5) caldera-forming or -modifying eruptions (V1, V2) of Holocene age whose eruptive products make up most of the surficial deposits around the volcano. These deposits and other unconsolidated glacial, fluvial, and colluvial deposits are depicted on the accompanying map. The the V2 eruption occurred 4.1–4.4 ka (cal 2-sigma age range) and produced an extensive landscape-mantling sequence of pyroclastic deposits >50 km³ in volume that cover or partly obscure older unconsolidated eruptive products. The V1 eruption occurred 8–9 ka and its deposits lie stratigraphically below the pyroclastic deposits associated with the V2 eruption and a prominent, widespread tephra fall deposit erupted from nearby Black Peak volcano 4.4–4.6 ka. The V2 pyroclastic-flow deposits range from densely welded, columnar jointed units exposed along the main valley floors, to loose, unconsolidated, blanketing accumulations of scoriaceous (55–57% SiO2) and lithic material found as far as 75 km from the edifice. Large lahars also formed during the V2 eruption and flowed as far as 50 km from the volcano. The resulting deposits are present in all glacial valleys that head on the volcano and are 10–15 m thick in several locations. Lahar deposits cover an area of about 800–1000 km², have an approximate volume of 1-2 km³, and record substantial inundation of the major valleys on all flanks of the edifice. Significant amounts of water are required to form lahars of this size, which suggests that an ice-filled summit caldera probably existed when the V2 eruption occurred.