Geology of Kasatochi Volcano, Aleutian Islands, Alaska

Kasatochi is a small, isolated island volcano in the center of the Aleutian Island chain. It consists of a roughly circular cone ~3 km in diameter with a lake-filled central crater that is 1.2 km in diameter and extends from the highest point on the island to sea level. The earliest unit recognized is a thick series of mid-Pleistocene glaciovolcanic deposits consisting of autobrecciated lava, lahars, and volumetrically minor lava masses that we believe to have been emplaced underneath a regional ice cap. This unit is unconformably overlain by several massive Holocene lavas, above which lies a thick sequence of latest- Holocene pyroclastic deposits likely deposited during the crater-forming eruption. The 2008 eruption enlarged the preexisting crater, and produced pyroclastic density currents, surges, and fall that blanketed the entire island except for steep, seaward-facing cliffs on the flanks and the crater wall. 2008 deposits initially extended the shoreline seaward by up to 500 m.

Kasatochi lava and scoria are porphyritic basalt, basaltic andesite, and andesite, all of which bear trace-element evidence for prolonged crustal residence and equilibration with an amphibole-rich gabbroic residue. Lavas from individual effusive eruptions have limited compositional range, whereas juvenile scoriae from explosive eruptions span the majority of the compositional range of the entire volcano. 2008 pyroclastic deposits contain texturally diverse amphibole gabbro clasts and smaller, less abundant, plagioclase-free pyroxenitic and peridotitic cumulate inclusions. We infer that the gabbroic inclusions are from the margins of regions of crustal magma storage and evolution and that equilibration with the amphibole plays an important role in the evolution of mafic and intermediate magmas.