Volcanic architecture and unrest processes: Insights from static and time-varying potential field models

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Hydrothermal system
Inverse modelling

Knowledge of a volcano's architecture or internal anatomy, provides critical context for correctly interpreting signals of volcanic unrest. In this thesis, I use measurements and models of Earth's gravity and magnetic fields, applied at two contrasting volcanoes, Laguna del Maule volcanic field, Chile, and Mt Tongariro, New Zealand, to model their architecture and time-varying processes occurring within. I reveal relationships between magma bodies, hydrothermal systems, basement and fault structures, and provide a quantitative basis for improving understanding of the causes of volcanic unrest indicators. Gravity inversion, constrained by thermodynamic modelling at Laguna del Maule volcanic field, images a shallow, volatile rich, silicic magma body, above a previously modelled inflating sill, bound to the west by the regional scale Troncoso fault. Magnetic models show NE trending, remanently magnetised features, parallel to the Troncoso fault, interpreted as dykes intruding along faults. Further evidence of magma and fault interaction, from time-varying gravity changes, shows the inflating sill produces stress changes on the Troncoso fault, allowing shallow hydrothermal system fluids to migrate into it, resulting in mass addition and positive gravity changes through time. Fluid flow into the fault zone may be further modulated by shaking from nearby earthquake swarms. At Mt Tongariro, geologically-constrained gravity and magnetic models map large faults cutting the basement beneath the volcano, and delineate an extensive hydrothermal system. The hydrothermal system is bound laterally by the basement faults, while the basement itself acts as a low permeability barrier. The 2012 eruption at Upper Te Maari crater depressurised the hydrothermal system, promoting subsidence from the evacuation of pore space. Time-varying gravity models show shallow mass addition above the subsidence source, derived from a combination of pore fluid migration, condensation, cooling, and meteoric input, indicating the system is still repressurising. I show that the illumination of volcano architecture provides a rich, quantitative environment, to better interpret volcanic unrest. I combine traditional potential field geophysical methods and ground deformation data, with state of the art modelling techniques, and create a powerful and effective toolbox for the 21st century volcanologist.

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This thesis may be printed or downloaded for non-commercial research and scholarly purposes. Copyright remains with the author.
Glyn Williams-Jones
Science: Department of Earth Sciences
Thesis type: 
(Thesis) Ph.D.