Physical and geochemical systematics of the 2008 violent strombolian eruption of llaima Volcano, Chile
Ruth, Dawn Catherine Sweeney
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Arc basaltic volcanoes experience a wide range of eruptive styles and transition quickly from one type of behavior to the next. Violent Strombolian eruptions often occur with limited warning, increasing the risk of exposure to associated hazards. Current conceptual models for eruptive processes were developed at only a few well-studied volcanoes. This dissertation investigates the physical and chemical processes associated with the 2008 violent Strombolian eruption of Llaima volcano, Chile. A comprehensive geochemical and qualitative textural study of the erupted tephra was completed to ascertain the details of the physical and chemical environment prior to eruption onset. Volatile contents of olivine-hosted melt inclusions indicate the magmatic plumbing system can be traced from the conduit and shallow chamber (∼1.5-4 km) down to ∼14 km. During inter-eruption periods, passive degassing was driven by degassing of magma in the conduit and possibly the shallow chamber. Ascent, injection and mixing of a volatile-rich, deeply-sourced magma with a relatively degassed, shallow magma initiated the eruption sequence. Prior degassing and degassing associated with this mixing event caused significant microlite crystallization and an associated increase in viscosity, which may have reduced the permeability of the upper plumbing system. The introduction of additional volatiles from the injecting magma may have triggered a sufficient increase in system overpressure, ultimately exceeding the magma tensile strength to initiate the eruption. Kinetic modeling of Fe/Mg diffusion in olivine provides insight into the timescales of these magmatic processes. Diffusion derived-chronologies range from 56-468 days, with clusters at 100-160 and 206-222 days. Entrapment pressures derived from the volatile contents of melt inclusions show a general pressure decrease as the eruption drew nearer; this suggests magma ascent in the days prior to the eruption. These chronologies generally overlap with the timing of increases in seismic activity and ground deformation. Finally, a rare, uniformly thin tephra, here termed plate tephra was also described. Internal textures and external morphology suggest these tephra originated from the rupturing of the fluid walls of large gas slugs high above the summit vent. Their presence in the medial-distal tephra deposits alongside the scoria indicates plume transport, rather than ballistic transport. Concurrent deposition also suggests unsteady conduit dynamics, consistent with video recordings of the eruption. These tephra have been observed in deposits at other volcanoes, thus they may be indicative of violent Strombolian eruptions. This dissertation highlights the importance of multi-method investigations of eruptions. Because each method delivers information about different processes, a comprehensive understanding is only achieved when the approaches are combined. The result of this approach is that it is becoming more apparent that violent Strombolian eruptions share common characteristics, including general magma system geometry (vertically-extended, with a shallow, sub-edifice chamber), timescales to eruption, and conduit and vent processes (tephra types: texturally bimodal scoria and distally-dispersed plate tephra).