THE 2005 CRATER LAKE FORMATION, LAHAR, ACIDIC FLOOD, AND CAUSTIC AEROSOL AND GAS RELEASE FROM CHIGINAGAK VOLCANO, ALASKA

A 400-m-wide, 100-m-deep crater lake developed in the formerly snow-and-ice-filled crater of Mount Chiginagak volcano sometime between August 2004 and early summer 2005, presumably due to increased heat flux from the hydrothermal system, or possible magma intrusion and degassing. In early summer 2005, an estimated 3 million cubic meters of sulfurous, clay-rich debris and acidic water, with an accompanying caustic gas plume and aerosol mist, exited the crater through tunnels in the base of a glacier that breaches the south crater rim. Over 27 kilometers downstream, the acidic waters of the flood reached approximately 1.3 meters above current water levels inundating an important salmon spawning drainage, acidifying Mother Goose Lake from surface to depth (approximately 480 million cubic meters in volume at a pH of 2.9 to 3.06) and preventing the annual salmon run. During the sampling period August-September 2005, the pH of the crater lake outlet stream (5 km downstream of the crater, mixed with glacial meltwater), ranged from 3.06 to 3.22. A sulfate concentration of 506 ppm was measured at the crater lake outlet stream in August 2005. The flood was accompanied by a simultaneous release of caustic volcanic gas and aerosols that followed the flood flow-path, causing defoliation and necrotic leaf damage to vegetation in a 30 square kilometer area along and above affected streams, to heights of over 150 meters in elevation above flood level. This previously undocumented phenomena of simultaneous vegetation-damaging aerosol and gas release accompanying a crater lake draining event has important implications for the study of hazards associated with active volcanic crater lakes.