Critical Eruptive Controls of an Intra-plate Volcano: Ascension Island, South Atlantic

Davies, Bridie (2022) Critical Eruptive Controls of an Intra-plate Volcano: Ascension Island, South Atlantic. Doctoral thesis, University of East Anglia.

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Understanding what drives transitions in eruptive style is an important challenge in volcanology. Existing descriptions of small-volume trachytic eruptions often record transitions in eruptive behaviour, including and outwith explosive-effusive transitions, but detailed ascent and eruption dynamics reconstructions are rare. Historically, poor deposit preservation and exposure and unconstrained trachytic melt physical properties have contributed to a dearth of relevant literature. Here, I first make use of recent advances in understanding trachytic melt properties to reconstruct ascent and eruption dynamics of a particularly well-preserved and -exposed small-volume eruption on Ascension Island, South Atlantic - the Echo Canyon eruption (EC). Forensic stratigraphy, petrographic analysis, reconstruction of bulk magma properties and quantitative textural analysis reveal: the EC eruption underwent several transitions in eruptive behaviour, driven by rapid ascent, decompression and vesiculation. Further, peak explosion intensity equivalent to volcanic explosivity index (VEI) 6 eruptions was transiently achieved before transition to effusive activity by rapid evolution of permeable vesicle networks in conduit margin shear zones. Next, I use 2D and 3D textural analyses of a stratigraphically and compositionally well-constrained basalt-rhyolite ‘Mingled Fall’ deposit to assess how mingling impacts vesiculation during ascent. I find vesiculation in mingled clasts’ basaltic and rhyolitic regions progressed independently with little–no shear. In the basaltic regions, connectivity development was only slightly inhibited relative to scoriaceous clasts. I show 2D and 3D studies in texturally complex samples return different vesiculation histories. The 3D vesicle size distribution studies are more reliable for complex clasts, whereas 2D shape descriptors are better constrained and more useful for inter-eruption comparisons. Finally, I demonstrate how frameworks developed throughout offer insights into the evolution of the volumetrically comparable La Soufrière St Vincent, 2020-21 eruption. This thesis makes a useful contribution to understanding ascent and eruption dynamics of small-volume, but potentially high-impact events, common to isolated ocean island settings.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Environmental Sciences
Depositing User: Nicola Veasy
Date Deposited: 22 Jun 2023 09:16
Last Modified: 22 Jun 2023 09:16


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