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Volcanism on Unimak Island, Alaska, Usa: a Special Focus Volcanism On Unimak Island, Alaska, Usa: A Petrologic Focus On Shishaldin And Fisher Volcanoes Item Type Thesis Authors Stelling, Peter L. Download date 07/10/2021 06:31:20 Link to Item http://hdl.handle.net/11122/8666 VOLCANISM ON UNIMAK ISLAND, ALASKA, USA: A SPECIAL FOCUS ON SHISHALDIN AND FISHER VOLCANOES A DISSERTATION Presented to the Faculty Of the University of Alaska Fairbanks In Partial Fulfillment of the Requirements For the Degree of DOCTOR OF PHILISOPHY By Peter L. Stelling, B.A. Fairbanks, Alaska May 2003 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: 3092299 UMI UMI Microform 3092299 Copyright 2003 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Ml 48106-1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. VOLCANISM ON UNIMAK ISLAND, ALASKA, USA: A SPECIAL FOCUS ON SHISHALDIN AND FISHER VOLCANOES By Peter L. Stelling RECOMMENDED: r,U:Advisory CompiffeeyChair x Head, Department of Geology and Ge^mysics , A (A APPROVED: v . y vy Dean, College of Sciences, Engineering and Mathematics Dean of the Graduate School Date Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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 ■a small, independent, non-communicating reservoirs. The 100 km caldera-formmg 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table of Contents Abstract .................................................................................................................................iii Table of Contents ...........................................................................................................................v List of Figures ................................................................................................ x List of Tables ...............................................................................................................................xii List of Other Materials.............................................................................................................. xiii List of Appendices ..................................................................................................................... xiv Acknowledgements ..................................................................................................................... xv Chapter 1: Introduction ................................................................................................................. 1 Chapter 2: Introduction to Fisher Caldera ................................................................................. 5 Chapter 3: The Eruptive History of Fisher Caldera, Alaska, USA.........................................7 3.1 Introduction ................................................................................................................ 7 3.2 Methods ....................................................................................................................... 8 3.3 Eruptive history......................................................... 18 3.3.1 Erosional and Eruptive Breccias .............................................................. 19 3.4 Mid-Pleistocene Eruptive Products ............... 21 3.5 Late Pleistocene Eruptive Products ..................................................................... 24 3.5.1 Northern and Westem Regions ................................................................ 24 3.5.2 Southeastern Region ..................................................................................31 3.5.3 Other Notable Late-Pleistocene Deposits .............................................. 33 3.6 Caldera-Forrning Eruption (CFE) ..................................... 34 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. vi 3.7 Holocene Eruptive Chronology ................... 41 3.7.1 Intra-Caldera Lake .................................................................................. 41 3.7.2 Polygenetic Vents ........................... 42 3.7.3 Monogenetic Vents ................................................................................... 45 3.7.4 Phreatomagmatic Eruptions ..................................................................... 46 3.7.5 Recent Activity .................................................... 47 3.7.6 Draining of the Intra-Caldera Lake ......................................................... 48 3.8 Discussion ...............................................................................................................49 3.8.1 Growth of Fisher Volcano and Formation of Fisher Caldera 49 3.8.2 Volume Estimates and Eruption Rates ...................................................54 3.8.3 Recurrence of Silicic Magma ............................................................... 56 3.9 Conclusions .............................................................................................................58 Chapter 4: Birth of Caldera Systems: Petrological and Structural Constraints on the Caldera-Forrning Eruption of Fisher Caldera, Alaska, USA ......................... 60 4.1 Introduction .............................................................................................................60 4.2 Location and Geologic Background ................................................................... 62 4.3 Methods and Techniques ......................................................................................64 4.4 Deposits of the Caldera Forming Eruption ........................................................65 4.4.1 Northern Dacite Pumice Fall Deposit .....................................................6 6 4.4.2 Northern Ignimbrite Deposit .................................................................... 71 4.4.3 Southern Dacite Pumice Fall and Flow Deposit ....... 75 4.5 Constraints on the Caldera-Forrning Eruption ....................................................78 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4.5.1 Timing of Eruptive Components ............................................................ 78 4.5.2 Vent Locations .......................................................................... 78 4.5.3 Volume Estimates ....... 80 4.6 Discussion ...............................................................................................................81 4.6.1 Genetic Relationships Between the CFE Magmas ............... 81 4.6.1.1 Northern Magmas ............................................................................81 4.6.1.2 Southern Dacite Pumice ..................................................................82 4.6.2 Shallow Magma Residence ...................................................................... 83 4.6.3 Structure of the Pre- and Post-Caldera System ......................................84 4.6.4 Implications for the Development of the Fisher System ..................... 8 6 4.6.4.1
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