Elucidating Shallow Volcanic Structures and Dynamics using Geophysics and Numerical Modelling

  • Nicky Young

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Our understanding of volcano dynamics and structures is frequently measured by how accurately we can model and reproduce volcanic signals. However, both forward and inverse models of volcano movement and mass distributions are inherently non-unique, which means that the best fit model may not necessarily be the closest to reality. It is imperative that we consider models
of volcanoes in the context of plausible geological conditions and integrate these conditions as limiting constraints. With the computing power and sophistication of numerical modeling software available today, options for exploring volcanic processes and structures are endless. I focus on three different volcano types (restless caldera, explosive stratovolcano and persistently active complex volcano) undergoing contrasting activity. I use the most suitable available data to numerically model the mechanics shallow crust at each volcano and retrieve details of fluid distributions and subsurface structures.

First, I process and invert Bouguer gravity data at Campi Flegrei caldera, which has been experiencing restless behaviour symptomatic of hydrothermal fluid perturbations. Using numerical modelling software GROWTH2.0 I retrieve the subsurface locations of low gravity anomalies within the caldera, and show that their position is related to shallow crustal structure.

Next, I use volumetric strain data from a magmatic explosion at Soufri`ere Hills Volcano to investigate conduit dynamics and the shallow structure of Montserrat, through progressively more complicated 2D axisymmetric and 3D models. Collectively, these models demonstrate that the shape of the island’s surface and internal heterogeneities, including a weakened halo around the conduit and stiff relict volcanic core, have a quantifiable impact on strain partitioning within the shallow crust and are necessary to reproduce the data at strain benchmarks during the eruption. Migration of this work from 2D axisymmetric to 3D also highlights that the conduit at Soufri`ere Hills Volcano cannot be cylindrical, with implications for magma flow dynamics both here and at other volcanoes. Lastly, at Masaya Volcano I use InSAR data to find the source of deformation during a phreatic event and reconstruct a joint magmatic and hydrothermal explosion instigator.

The results highlight that shallow structures such as faults and mechanical heterogeneity are key to strain partitioning in the shallow crust and thus the storage of fluid, both magmatic and hydrothermal.
Date of Award23 Jan 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorJoachim H Gottsmann (Supervisor) & James M Wookey (Supervisor)

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