Geology and petrology of ejecta from the 1999 eruption of Shishaldin Volcano, Alaska

摘要: Shishaldin Volcano erupted repeatedly during April and May 1999, with major eruptive events on 19 23 April. Tephra deposits >20 cm thick were distributed across the southern flanks hyperconcentrated debris flows ran down northern slope. All analyzed clasts basaltic in composition, abundant (~29 vol%) phenocrysts of plagioclase, olivine, magnetite. Glass-inclusion analyses indicate low volatile contents ( 900 to 58 ppm, respectively). Comparisons sulfur glass inclusions groundmass glasses suggest significant degassing occurred even prior eruption. Trace-element studies that magma reservoir below may not be a closed, long-lived chamber, U–Th–Ra isotopic short crustal residence times (on order at most few thousand years) for 1999 products. Comparison from this eruption other similarly sized eruptions suggests explosive event should classified as sub-Plinian Such are rare. The was largest last 175 years. However, petrology, chemistry deposit thickness distribution resemble those least 11 scoria 9,000 years, suggesting type is common Shishaldin.