What controls mafic eruption style? The causes and consequences of coupled vesiculation and crystallization kinetics in ascending basaltic andesite magmas.
Explosive eruptions involving mafic magmas pose important implications for hazards near active volcanoes. In particular, Strombolian eruptions produce large volumes of fine ash, high ash plumes, and large dispersal areas that may severely impact surrounding populations. Numerous studies suggest that explosivity is related to expulsion of gases from ascending, decompressing magmas that in turn depends on porosity-permeability properties of the magma and fragmentation style. In an attempt to elucidate the degassing process and its effect on eruptive style, it is proposed to investigate experimentally the development of permeability in experimentally decompressed mafic silicate melts with particular focus on the role of crystallization in mafic magmas as a control on eruptive style. Specifically, how does syn-ascent microlite crystallization influence development of connected vesicle networks and the percolation threshold, and how do these factors influence gas separation in the conduit and transitions in eruptive style? Hypotheses to be tested include: [1] Are crystal number densities related directly to decompression rate (i.e., due to rapid crystallization of hydrous mafic melts during vesiculation)? 2) Is melt permeability inversely related to magma viscosity? 3) Does decompressionÂdriven crystallization change the percolation threshold (i.e., by limiting bubble expansion or by spatial control on bubble placement)? 4) Does decompression rate affect the style of mafic eruptions by controlling degassing efficiency and the depth and efficiency of synÂeruptive crystallization? These questions will be investigated via decompression experiments on synthetic basaltic andesite melts similar in composition to those erupted at Okmok volcano (Alaska), and with both pure water and mixed H2OÂCO2 fluid phase to examine the influence of CO2 on crystallization and vesiculation kinetics. The experiments will be compared with natural samples from the midÂHolocene Middle Scoria Okmok volcano, which produced violent strombolian to subplinian and vulcanian eruption deposits. A ?permeameter? will be constructed to measure permeabilities of both experimental and natural samples to estimate percolation thresholds, and the results will be used to model gas segregation and fragmentation during explosive mafic eruptions.
Broader Impacts: This project will support a PhD student, and undergraduate student assistant at UAF. A new link will be forged with the UAF Alaska Native Science and Engineering Program (ANSEP) with the aim to hire Alaska Native undergraduate student(s) to perform research for the project. The project will also involve international collaboration with volcanologist Kathy Cashman (University of Bristol). The PI?s association with the Alaska Volcano Observatory also facilitates application of this research to assessment of volcanic hazards related to violent mafic eruptions at active volcanoes in Alaska and Cascadia. Development of new research equipment in the UAF Experimental Petrology lab will also enhance the infrastructure at UAF and provide new equipment that could fill research needs beyond geology and volcanology studies, including applications in other physical sciences and engineering disciplines.
