The rise of caldera forming eruptions: refining tools for understanding magma ascent
Harris, Megan Ann
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The rate at which magma moves from the magma chamber to the surface influences the amount of degassing and crystallization that occurs, which in turn controls the style and intensity of the ensuing eruption. Thus, our understanding of magma ascent rates is crucial to understanding and mitigating future volcanic hazards. The bulk of this dissertation revolves around using the diffusion of water through melt-filled pockets (embayments) in quartz crystals as an ascent speedometer, coupled with geochemical and textural analysis of co-erupted material. In Chapter Two, I apply and refine the water diffusion speedometer to establish timescales of ascent for the two eruptions that formed the modern-day Valles Caldera, with the aim of understanding whether these rates change during subsequent eruptions from the same caldera. In Chapter Three, I apply the diffusion speedometer to the opening behavior of the 1991 eruption of Mount. Pinatubo and compare the results to ascent rates obtained using independent petrologic methods (bubble number density and microlite number density). This chapter seeks to reconcile the several orders of magnitude offset in ascent rates produced by various geospeedometers. Finally, in Chapter Four, I explore the mechanisms by which embayments are formed in magmatic systems. I do this by conducting a survey of cathodoluminescence images of crystals taken from five volcanic systems to determine how the embayments interact with the internal zoning of the crystal. I then attempt to form embayments experimentally using a cold-seal pressure vessel under variable magmatic conditions. The culmination of this work emphasizes that embayments are robust and faithful recorders of a magma's journey from its source to the surface and may be a critical piece of evidence for unraveling the magmatic history leading to eruption. This dissertation includes both previously published and unpublished co-authored work.