POSTGLACIAL ERUPTIONS, MAGMA GEOCHEMISTRY, AND ONGOING SEISMICITY OF ANIAKCHAK CALDERA VOLCANO, ALEUTIAN ARC

Aniakchak is a Pleistocene to Holocene composite volcano of the Alaska–Aleutian arc that suffered at least one caldera-forming eruption in postglacial time and last erupted in 1931. The oldest recognized postglacial eruption, Aniakchak I, produced andesite ignimbrite distinctly rich in incompatible elements. Subsequently, the dacite–rhyodacite Black Nose Pumice falls were deposited ~7,000 ¹⁴C yr B.P. The ~3,430 ¹⁴C yr B.P. Aniakchak II eruption produced rhyodacite Plinian fall followed by rhyodacite and andesite ignimbrite that extended ≥50 km to the Bering Sea and Pacific coasts and resulted in collapse of the 10-km-diameter caldera. The rhyodacite (~70% SiO₂) is the most evolved and only known hornblende-phyric magma erupted from Aniakchak. Aniakchak II andesite (57.2–60.4% SiO₂) has low V and high Na₂O, Y, TiO₂, and, especially, P₂O₅.

Nearly all postcaldera vents are on the caldera ring-fracture system. The earliest of these extruded small dacite–rhyodacite domes into a deep lake and a related lava flow on the northwest flank. Three basaltic andesite–andesite tuff cones formed on the eastern caldera floor after catastrophic draining of the lake by ~200 m, one expelling the most primitive postglacial magma (52.3% SiO₂, 5.3% MgO). Dacite–andesite magmas issued from Vent Mountain and Half Cone starting as early as ~1,000 years ago. Plinian eruption at Half Cone ~400 ¹⁴C yr B.P. yielded widespread fall and local pyroclastic-flow deposits of dacite Pink and overlying andesite Brown Pumice. Strombolian eruption of basaltic andesite (~52.4% SiO₂) built Blocky Cone after Half Cone and most Vent Mountain activity. The most recent eruption, in 1931, yielded dacite–rhyodacite tephra followed by relatively voluminous andesite tephra and ended with basaltic andesite lava, spanning ~67 to 56% SiO₂.

Episodic seismicity indicates the magmatic system is active. The majority of earthquakes are long-period (LP) events. Epicenters for 135 (2009–2012; M_L ≤2.1) define a bow-tie pattern elongated parallel to plate convergence. Hypocenters identify events 14–28 km below sea level (bsl), few events 7–14 km bsl beneath the caldera, and events <10 km bsl concentrated under Vent Mountain. Recent seismicity is consistent with long-lived mush in the quiet zone, from which magma escapes to be stored ephemerally to within ~2.5 km bsl.