Increases And Fluctuations In Thermal Activity At Mount Wrangell, Alaska (Volcano, Glacier)

Abstract

The objectives of this study were to document and interpret changes in thermal activity at two of three craters located on the rim of the ice-filled summit caldera of Mount Wrangell, an active glacier-clad shield volcano in south-central Alaska. The technique of "glacier calorimetry" was developed, through which changes in the volume of glacier ice in the craters and caldera were measured and related to changes in heat flow. Chemical analyses of gases and acid-thermal waters provided information on the underlying heat source. In 1965, thermal activity began increasing at both the North and West Craters. During the ensuing years, heat flow increased significantly at the North Crater, although in a highly fluctuating manner, while gradually declining at the West Crater. Pulses in heat flow at the North Crater occurred in 1966-68 and 1972-74, with both pulses followed by a four-year decline in activity. Increases in heat flow began again in 1978-79 and have continued unabated through the summer of 1983. Over 80 percent of the 4.4 x 10('7)m('3) ice volume within the crater in 1966 was melted by 1982, and the meltwaters have drained or evaporated from the crater. The subsequent rapid development of numerous fumaroles, the large dry-gas proportion of SO(,2) (27 percent), and the inferred presence of gaseous HCl indicate that a shallow degassing magma body is the source of heat driving the thermal system. Seismically induced fracturing above the magma body is hypothesized to explain the initial increases in thermal activity. The resulting massive influx of meltwaters into the subsurface is suggested as the cause of the fluctuations in heat flow. The continued increase in activity since 1979 suggests that the volume of meltwater being generated is no longer sufficient to quench the heat source beneath the crater.