Explosive magma-water interaction (MWI) during hydromagmatic eruptions generates substantially more fine-grained volcanic ash than equivalent 'dry' eruptions. However, the social and economic disruption caused by recent subglacial eruptions in Iceland highlighted gaps in our knowledge of the mechanisms responsible for fine fragmentation during MWI and the resulting implications for modelling ash dispersion. By developing new ways to characterise and interpret volcanic ash properties, this thesis shows that (a) magmatic vesiculation prior to MWI is the rule rather than the exception, (b) rapid quenching and brittle disintegration of vesicular melt (or pyroclasts) can explain the high fragmentation efficiency of hydromagmatic activity, and (c) the bubble population at the time of MWI is a fundamental control on the physical characteristics of the resulting pyroclasts. Covariation in the size, shape, and texture of hydromagmatic ash particles reflects interaction between the size distributions of particles and bubbles during brittle fragmentation, such that the length-scale and geometry of fracturing is controlled by the size and spatial distribution of bubbles. This result questions the validity of many of the 'diagnostic' particle properties used to distinguish magmatic from hydromagmatic deposits, and emphasises the need to analyse multiple grain size fractions to infer fragmentation mechanisms. I hypothesise that high thermal stress in glassy vesicular pyroclasts quenched during MWI is an important driver of secondary brittle fragmentation, a process that is compatible with, but not fully explored in, existing models of hydromagmatic processes. Importantly, the dynamics of vesiculation prior to MWI strongly influence the size and shape distributions of erupted material, both of which are critical input parameters for models of ash dispersion and resuspension
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:702943 |
| Date | January 2016 |
| Creators | Liu, Emma J. |
| Contributors | Cashman, Katharine V.; Rust, Alison C. |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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