Volcanism On Unimak Island, Alaska, Usa: A Petrologic Focus On Shishaldin And Fisher Volcanoes

Abstract

Volcanism on Unimak Island, Alaska represents a microcosm of Aleutian arc volcanism in general. This work focuses on two of the most significant features on Unimak Island, Fisher Caldera and Shishaldin Volcano. Despite frequent activity and potential for violent, hazardous eruptions, these volcanoes have been relatively unstudied. The present work details the processes occurring within Shishaldin and Fisher volcanoes, and highlights the complexities of their magma storage systems. Fisher Caldera began as a scattered series of independent stratocones formed from small, independent, non-communicating reservoirs. The 100 km 3 caldera-forming eruption (CFE) resulted from injection of three chemically distinct magmas, one being the largest magma batch to have passed through this system. Extensive fracturing during the CFE destroyed the pre-caldera infrastructure, and subsequent magmatism formed a single mixed reservoir. Post-caldera activity, stemming from this centralized chamber, produced several structurally controlled stratocones that erupted into the newly formed caldera lake. A tsunami generated by an explosive intra-caldera eruption catastrophically drained the caldera lake. Current activity is largely hydrothermal. The progression through which the Fisher system developed is similar to those seen in other caldera systems, yet has not been put forth in the literature as a common process. I suggest the Fisher sequence is an end-member in the spectrum of worldwide caldera formation, and present this process in a global context. Shishaldin Volcano has been formed through the concurrent activity of two separate magma systems, the products of each of which are compositionally distinct. Parental magmas for each series are both basalt, but have different trace-element signatures that require separate protoliths. Furthermore, distinct paths of subsequent chemical evolution are also required. One series shows evidence of ponding at high pressure prior to final ascent, whereas the magmas of the other series are directly emplaced in several small, shallow reservoirs. Results from both volcanoes tend to support a view involving complex magma storage: discrete magma batches with limited interaction rather than simple differentiation in a central chamber.* *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Adobe Acrobat; Microsoft Office.