Geophysical Investigations of Magma Plumbing Systems at Cerro Negro Volcano, Nicaragua by Patricia Grace MacQueen B.Sc., University of Oregon, 2011 Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science in the Department of Earth Sciences Faculty of Science Patricia Grace MacQueen 2013 SIMON FRASER UNIVERSITY Fall 2013 Approval Name: Patricia Grace MacQueen Degree: Master of Science (Earth Sciences) Title of Thesis: Geophysical Investigations of Magma Plumbing Systems at Cerro Negro Volcano, Nicaragua Examining Committee: Chair: Doug Stead Professor Glyn Williams-Jones Senior Supervisor Associate Professor Gwenn Flowers Supervisor Associate Professor Jackie Caplan-Auerbach External Examiner Associate Professor, Geology Western Washington University Date Defended/Approved: November 27, 2013 ii Partial0B Copyright License iii Abstract Cerro Negro near Léon, Nicaragua is a very young (163 years), relatively small basaltic cinder cone volcano that has been unusually active during its short lifespan (recurrence interval 6-7 years), presenting a significant hazard to nearby communities. Previous studies have raised several questions as to the proper classification of Cerro Negro and its relation to neighboring Las Pilas-El Hoyo volcano. Analysis of Bouguer gravity data collected at Cerro Negro has revealed connected positive density anomalies beneath Cerro Negro and Las Pilas-El Hoyo. These findings suggest that eruptions at Cerro Negro may be tapping a large magma reservoir beneath Las Pilas-El Hoyo, implying that Cerro Negro should be considered the newest vent on the Las Pilas-El Hoyo volcanic complex. As such, it is possible that the intensity of volcanic hazards at Cerro Negro may eventually increase in the future to resemble those pertaining to a stratovolcano. Keywords: Cerro Negro; Las Pilas-El Hoyo; Bouguer gravity; magmatic plumbing systems; potential fields; volcano iv Dedication This thesis is dedicated to Kristen Drechsler, Ivanka Mitrović, and my family, without whose love, support, and – most importantly – irrepressible sense of humor, none of this would have been possible. v Acknowledgements Although it will be my name alone on the cover of this thesis, in reality my name should share that space with the names of countless others whose aid, advice, encouragement, and sanity stability checks ensured that this work ever reached this stage. I thank my advisor, Glyn Williams-Jones, for first enchanting me with his siren song of volcanoes and gravity over two years ago, and generally avoiding (almost) all the pitfalls so commonly observed in other academic advisors. I am constantly reminded how lucky I am to have an advisor as enthusiastic about his students as he is about his science. And yes, you were right about the boots. Thank you, Gwenn Flowers, for asking all the really awkward questions in my committee meetings that often opened my eyes to areas of my work that proved crucial later on. I almost certainly would have perished several times over the course of my first year of graduate studies and beyond had it not been for my fellow graduate student Jeff Zurek, without whose amazing field skills and patient explanations I would have graduated in four years instead of two and a third. Special thanks to Megan Dewit, for timely help with DEM data and the random ARC-GIS requests I’d throw at her from time to time, and above all for her companionship that kept me happy and cheerful in our lovely windowless lab! Of course, I would never have even reached graduate school had it not been for those who inspired me and pushed me during my undergraduate years at the University of Oregon. I am deeply indebted to Marli Miller, who encouraged me to come as a high school student and was always there rooting me on for the rest of my time at Oregon, David Schmidt, whose clear advice as one of my senior thesis advisors influenced so much about how I organized and viewed my work during my master’s research, and Kathy Cashman, who brought me into the world of volcanology by inviting me to work in her lab, and encouraged me to check out this crazy thing I had heard about putting gravity meters on volcanoes, and who remains an inspiration for me to this day. I am also very thankful to Mike Poland of the Hawaiian Volcano Observatory for giving me my first chance to play with and fall in love with gravimetry as a volunteer at HVO, and for guiding me and supporting me during my time there and after during my undergraduate senior thesis. Several people in the wider world of science beyond the walls of Simon Fraser aided me with equipment, data, advice, and/or general camaraderie in the field. Thank you to Pete La Femina for sharing his continuous GPS data with us, and to Halldor for vi so patiently helping us access it! To Hazel Rymer I must express my undying gratitude for the loan of her gravity meter (which I actually rather enjoyed using after I got used to it!) when ours was discovered to be broken just before my second field season. I am grateful to Saskia Van Manen, Bethan Burson, and Hilary Erenler for the wonderful camaraderie in Nicaragua. Thank you to Samer Naif for his timely paper recommendations that sparked many good ideas. I am also grateful to Julie Roberge for her excellent explanations of melt-inclusion data that were incredibly helpful to my understanding of existing geochemical work on Cerro Negro. I am strangely fortunate to have inadvertently followed my father in my own career path, and I know I would have been utterly lost at several junctions were it not for the hours-long Skype conversations and even a week-long GPS and gravity data processing “boot-camp” that brought my sanity, and my thesis, back from the brink on multiple occasions. I feel incredibly fortunate to have done my graduate degree in a department with such wonderfully supportive, enthusiastic students with such a zest for life. Before I was even a student here they folded me in without question, and have been my anchor here through the exultant highs and stormy lows of the thesis process. Thank you for listening and offering useful insight on multiple iterations of the same “gravity blobs”, and, just as importantly, keeping up a steady stream of distractions to ground me in the real world and allow me to return to my work enthused. I would especially like to thank Ivanka Mitrovich, Hazel Wong, Sarah Hashmi, Francesca Furlanetto, Janisse Vivas, Anja Franck, and Mary Ann Middleton for much-needed sanity boosts and unforgettable memories. My family and friends outside of Vancouver, though far away, were no less important to the success of this degree. I am eternally grateful to Joyce and Joe Frost for becoming my Seattle aunt and uncle, taking me in at a moment’s notice, feeding me, and always treating me and my friends to the best of Seattle. I am constantly undone by your generosity. I believe I have mentioned the state of my sanity several times in this acknowledgement, and the prize for “most helpful in maintaining sanity” would have to go to Kristen Drechsler and my family, for dealing with what I’m sure were bewildering swings between screams and tears of frustration and crows of exaltation and joy in life. Kristen, I have yet to meet anyone with your talent for cheering me up – wouldn’t have made it without you and your constant moose-related surprises. Without my family I would not exist, and without you, Mom, Dad, Katie, Donald, and Wally (BITE!), I would never have made it in Vancouver, were it not for your much-needed financial support, sage advice, unconditional love, and of course all those puppet shows on Skype. vii Table of Contents Approval .............................................................................................................................ii Partial0B Copyright License .................................................................................................. iii Abstract .............................................................................................................................iv Dedication ......................................................................................................................... v Acknowledgements ...........................................................................................................vi Table of Contents ............................................................................................................ viii List of Tables .....................................................................................................................xi List of Figures................................................................................................................... xii List of Equations .............................................................................................................. xiv List of Acronyms ...............................................................................................................xv 1. General Introduction and Literature Review ........................................................ 1 1.1. Introduction .............................................................................................................. 1 1.2. Location and Geologic Setting ................................................................................. 3 1.3. Eruptive Style and Significant Eruptions .................................................................. 5 1968 Eruption ..............................................................................................