Constraints on Eruption Dynanucs

Different models have been proposed for the "drumbeat" earthquakes that accompanied recent eruptive behavior at Mount St. Helens. Debate continues as to whether seismicity is related to brittle failure during the extrusion of solid dacite spines or is the result of hydrothermal fluids interacting with a crack buried in the volcanic edifice. My model predictions of steady-state conduit flow confirm the strong control that degassing exerts on eruptive behavior. I discuss the necessary role of degassing for extruded material to attain the high density (low vesicularity) of the observed spine material and discuss the implications for generating seismicity. A brittle-failure source of seismicity requires that the gouge elastic properties accommodate some strain, since the magma compressibility in the upper conduit is too low to do so on its own. I also report IV on a novel method for generating high-resolution digital elevation models of fault surface textures. VI ACKNOWLEDGMENTS There are many people to whom I am grateful. John Donovan was very helpful in providing technical assistance while using the SEM and MeX software. I am especially grateful to John Pallister of the USGS for his support in planning and leading field work during the summer of 2008. UCORE Fellow Naomi Mecham provided considerable assistance during the summer of 2009 with generating fault surface DEMs. I am incredibly grateful to my friends both in and out of the department, and in and out of Eugene for their ongoing support and love. There are too many of you all to name but I want to specifically acknowledge for their friendship over the last two years. I am also incredibly grateful for the support of my family throughout both this project and all of my endeavors, past, present and future. I would like to thank David for sparking my interest in geophysics, a relatively novel concept to a liberal arts student. lowe much recognition to Kathy for introducing me to the world of physical volcanology and for her overall enthusiasm for science. Finally, I am indebted to Alan, whose guidance allowed me to develop my appreciation for geophysical processes and numerical modeling. Without his patience and insight much of this project would not have been possible. I am also grateful for his financial support through research assistanceships and for the ability to attend conferences.