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

Abstract.

Shishaldin Volcano erupted repeatedly during April and May 1999, with major eruptive events on 19 April and 23 April. Tephra deposits >20 cm thick were distributed across the southern flanks and hyperconcentrated debris flows ran down the northern slope. All analyzed clasts were basaltic in composition, with abundant (~29 vol%) phenocrysts of plagioclase, olivine, and magnetite. Glass-inclusion analyses indicate low volatile contents (<1.5 wt%), oxygen fugacity of NNO+1, and highly variable S levels ranging from near saturation to highly undersaturated (>900 to 58 ppm, respectively). Comparisons of sulfur contents in glass inclusions and groundmass glasses suggest significant degassing occurred even prior to eruption. Trace-element studies suggest that the magma reservoir below Shishaldin may not be a closed, long-lived chamber, and U–Th–Ra isotopic analyses indicate short crustal residence times (on the order of at most a few thousand years) for the 1999 products. Comparison of the deposits from this eruption with other similarly sized eruptions suggests the 19 April explosive event should be classified as a sub-Plinian basaltic eruption. Such eruptions are rare. The 19 April explosive event was the largest at Shishaldin during the last 175 years. However, the petrology, chemistry and deposit thickness and distribution resemble those of at least 11 scoria deposits in the last 9,000 years, suggesting that this type of eruption is common for Shishaldin.