No abstract

Satellite SO 2 and ash measurements of Mount Spurr's three 1992 volcanic clouds are compared with ground-based observations to develop an understanding of the physical and chemical evolution of volcanic clouds. Each of the three eruptions with ratings of volcanic explosivity index three reached the lower stratosphere (14 km asl), but the clouds were mainly dispersed at the tropopause by moderate to strong (20-40 m/s) tropospheric winds. Three stages of cloud evolution were identified. First, heavy fallout of large (1500 mm) pyroclasts occurred close to the volcano (!25 km from the vent) during and immediately after the eruptions, and the cloud resembled an advected gravity current. Second, a much larger, highly elongated region marked by a secondary-mass maximum occurred 150-350 km downwind in at least two of the three events. This was the result of aggregate fallout of a bimodal size distribution including fine (!25 mm) ash that quickly depleted the solid fraction of the volcanic cloud. For the first several hundred kilometers, the cloud spread laterally, first as an intrusive gravity current and then by wind shear and diffusion as downwind cloud transport occurred at the windspeed (during the first 18-24 h). Finally, the clouds continued to move through the upper troposphere but began decreasing in areal extent, eventually disappearing as ash and SO 2 were removed by meteorological processes. Total SO 2 in each eruption cloud increased by the second day of atmospheric residence, possibly because of oxidation of coerupted H 2 S or possibly because of the effects of sequestration by ice followed by subsequent SO 2 release during fallout and desiccation of ashy hydrometeors. SO 2 and volcanic ash travelled together in all the Spurr volcanic clouds. The initial (18-24 h) area expansion of the clouds and the subsequent several days of drifting were successfully mapped by both SO 2 (ultraviolet) and ash (infrared) satellite imagery.

This page left intentionally blank. flows or surges probably would be limited to within the caldera, but ash fall could affect extensive regions downwind. Another voluminous eruption, such as Aniakchak II, is considered unlikely in the near future. Although the InSAR study indicates subsidence, the possibility that evolved magma may be accumulating beneath the central part of the caldera is worthy of consideration when interpreting future monitoring data. Pleistocene composite volcano that consists of basaltic andesite to dacite lava flows and fragmental rocks. Early in postglacial (Holocene) time, a voluminous eruption (Aniakchak I; Miller and Smith, 1987) left geochemically distinctive andesite pyroclastic-flow deposits (ignimbrite) in valleys and on the flanks of the edifice. This event was followed by dacite eruptions, at least one of which produced a widespread pumice fall deposit ca. 7,000 yr B.P. (VanderHoek, 2009). Unless otherwise noted, ages reported here are given in radiocarbon years before present (yr B.P., "present" being 1950 C.E.). This chronicle begins with a brief overview of Pleistocene ancestral Aniakchak and an account of the state of knowledge of postglacial, precaldera volcanism.

No abstract

No abstract