VENT PROCESSES AND DEPOSITS OF A HIATUS IN A VIOLENT ERUPTION: QUILOTOA VOLCANO, ECUADOR By Joanne A. Best A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science in Geology Northern Arizona University December 2010 Approved: __________________________________ Michael Ort, Ph.D., Chair __________________________________ Katharine V. Cashman, Ph.D. __________________________________ Nancy Riggs, Ph.D. ABSTRACT VENT PROCESSES AND DEPOSITS OF A HIATUS IN A VIOLENT ERUPTION: QUILOTOA VOLCANO, ECUADOR Joanne A. Best The 800 BP eruption of Quilotoa volcano, Ecuador, produced two plinian eruptions separated by a short hiatus. Units 1 and 3 (U1 and U3) of the eruption correspond to the first and second Plinian eruptions, respectively, which produced fallout and pyroclastic density currents. Unit 2 (U2) records processes during the hiatus and consists of three subunits: U2a, U2b, and U2c. This study examines uppermost U1 through to lowermost U3, with particular attention paid to U2, to determine why a dual eruption occurred. The field relations of the units, and the grain-size distributions, componentry and surficial characteristics of the samples, were investigated. U2a is the product of pyroclastic surges, U2b a fallout unit, and U2c the result of a single vulcanian eruption immediately prior to the onset of the second plinian event (U3). The granulometry study revealed that the units share some common grain sizes, namely the modes at 1.0, 2.0 and 4.0 phi, which indicate a common fragmentation processes for all the units. Common crystal-size populations appear to be the most likely cause of these modal grain sizes. Componentry data show that the units are equally crystal rich. The volume fraction of lithic fragments remained low and the vent and conduit stable, except at the top of U1. ii Low glass vesicularity, as determined through the study of the surficial characteristics of the grains, indicate that the gases that drove the eruption of U2b came from depth, and not from the fragmentation of the U2b material itself. U2b is also covered in an orange dust, the product of hydrothermal alteration. Transport processes had a very limited influence upon the characteristics of the deposits, other than a general thinning with distance. Erosion of the country rock at the top of U1 blocked the conduit and ended the plinian eruption. The period of hiatus, as defined by less explosive, lower volume, discontinuous eruptions, began with the emplacement of U2a surges from the main vent. The vent was eventually plugged with a dome. Gas flux from depth oxidized the U2b material located in a fumarole or secondary vent, which was periodically erupted and deposited as fallout. The accumulation of exsolved gases beneath the dome pressurized the system. The release of this pressure resulted in the vulcanian eruption that produced U2c, and the second plinian eruption quickly ensued, depositing U3. With a body of eruptable magma at depth, which would later be deposited as U3, the erosion of the conduit was enough to prevent a continued eruption and force a hiatus. This has implications for hazard mitigation, where it is necessary to determine whether a cessation of eruptive activity marks the end of the eruptive event, or is merely a short hiatus prior to a subsequent eruption of substantial magnitude. iii ACKNOWLEDGEMENTS This project was primary funded by a National Science Foundation grant, with additional funding from the Friday Lunch Clubbe. Many individuals helped make this project possible. Firstly, I would like to thank my advisor Michael Ort for his assistance on this project, Kathy Cashman (University of Oregon) and Nancy Riggs for joining my committee and their valuable input: discussion was always lively, even if we didn‟t all agree! I am indebted to the rest of the research team, who not only made the field season possible but also immensely enjoyable. Huge thanks to Jorge Bustillos (Escuela Politecnica Nacional, Quito) for being my wingman and tackling those crazy mountain roads, to Patty Mothes (Escuela Politecnica Nacional, Quito) for sharing her knowledge of the area, to Andrea di Muro (Observatoire Volcanologique, Reunion) for his expertise and for making us laugh, and to Mauro Rosi (Universita di Pisa, Italy) for his legendary careful insight in the field. Once back from the Andes I had extensive support. Thanks to Minard (Pete) Hall (Escuela Politecnica Nacional, Quito) for ensuring first and foremost that I got to take my samples home. Thanks to John Donavan (University of Oregon) and Jim Wittke for training me on the SEM, to Caleb Schiff for getting me started on the Coulter Particle Size Analyser, to Dan Ruscitto (University of Oregon) and Colleen Donegan for their insight and training in the FTIR method, and to Isolde Belien (University of Oregon) for rescuing my samples! I owe a lot to many more people and here are a few of them. Many thanks to Susan Sabala-Foreman and Meredith Michele Poggi-Jenkins for their departmental iv support. You both work immensely hard, helped me navigate a multitude of obstacles, and are the glue that keeps the whole operation together. Thanks to my housemates, Megan Beach, Chris Kassel, and Keri Thornton for their patience with me at the end of each long day, and to my officemates Tenielle Gaither and Rob Ross for putting up with me during work hours! Many thanks to Matt Schmidt for getting me through that first semester, to Colleen Donegan for her wisdom and humour, to Mallory Zelawski for emergency coffees and innumerable rides home, and to the entire department for being my home for the last couple of years. My final thanks have to go to the cornerstone that is my family. Most live many miles away but they have always encouraged me for as long as I can remember. My greatest thanks to my brother Andrew who has greatly supported me throughout this endeavour, and to my Mum and Dad, two people whom I am very fortunate to have in my life. v TABLE OF CONTENTS ABSTRACT ...................................................................................................................... II ACKNOWLEDGEMENTS ........................................................................................... IV TABLE OF CONTENTS ............................................................................................... VI LIST OF TABLES .......................................................................................................... XI LIST OF FIGURES ...................................................................................................... XII CHAPTER 1 ...................................................................................................................... 1 INTRODUCTION AND BACKGROUND ................................................................................................. 1 INTRODUCTION ...................................................................................................................................... 1 GEOLOGIC SETTING .............................................................................................................................. 3 The Andes ............................................................................................................................................... 3 Quilotoa .................................................................................................................................................. 5 BACKGROUND: ERUPTION DYNAMICS ......................................................................................... 13 The conduit ........................................................................................................................................... 13 The vent ................................................................................................................................................ 13 Fragmentation ....................................................................................................................................... 16 The role of water................................................................................................................................... 17 BACKGROUND: TRANSPORT PROCESSES ..................................................................................... 19 The Plume: grain-size distribution ........................................................................................................ 19 Fine-grained deposits ....................................................................................................................... 22 The Plume: componentry ...................................................................................................................... 23 Depositional processes ......................................................................................................................... 23 THIS STUDY ........................................................................................................................................... 25 Outline .................................................................................................................................................. 25 Significance .......................................................................................................................................... 27 CHAPTER 2 .................................................................................................................... 29 METHODS .................................................................................................................................................