Hydrovolcanism in Okmok caldera, Alaska

Abstract

Hydrovolcanic activity in Okmok Caldera predominated on the crater floor during approximately the first 775 years after the caldera collapsed at 2050 yr. B.P. Interactions between rising magma and shallow water (<100 m) controlled the development of lithofacies observed in the early post-caldera deposits. The distinctive lithofacies reflect the eruptive processes active as Cone D, a composite tuff, lava flow, and cinder cone, breached the surface of a lake which once covered the caldera floor. Three phases of eruptive activity constructed Cone D: first, a subaqueous cycle; second, emergent; and finally a purely subaerial strombolian and hawaiian phase built the edifice to its current height. Radiocarbon dates provide constraining ages for a catastrophic flood that emptied the 4.3 x 10⁹ m³ caldera lake and exposed the subaqueous lithofacies. An effusion rate of 2.7 x 10⁶ m³yr⁻¹ for this early eruptive period is calculated using eruptive volumes determined from a 5-m resolution DEM, based on AirSAR data. The prehistoric effusion rate determined for Cone D is on the same order of magnitude as the calculated historic effusion rate of 5.3 x 10⁶ m³yr⁻¹ from Cone A, based on mapped extents and thicknesses of lava flows and the cone itself.