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Research Papers

Issue date: 
Category: 
Volcanic activity
Paper number: 
4625

Volcanic architecture and unrest processes: Insights from static and time-varying potential field models (PhD thesis)

Craig Miller, University of Auckland (EQC funded project 14/U678)

A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philophy in Earth Sciences was accepted as a final report and is available on request.

Non-Technical Abstract

Knowledge of a volcano's architecture or internal anatomy, provides critical context for correctly interpreting volcano monitoring data.  In this thesis, I use measurements and mathematical 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 understand their internal structure and time-varying processes occurring within.  I reveal relationships between magma bodies, hydrothermal systems, and the surrounding rocks, and provide a quantitative basis for improving understanding of the causes of volcanic unrest.

Mathematical modelling of gravity data at Laguna del Maule volcanic field, reveals a shallow, gas rich, magma body, above a previously determined area of active magma injection, bound to the west by the regional scale Troncoso fault.  Magnetic models show NE trending features, parallel to the Troncoso fault, that are magnetised in a different direction to the present day magnetic field.  These magnetic features are interpreted as much older intrusions of magma along faults.  Further evidence of magma and fault interaction, from time-varying gravity changes, shows active magma injection produces stress changes on the Troncoso fault, allowing shallow hydrothermal water to migrate into it, resulting in mass addition detectable by precise gravity measurements.  Water flow into the fault zone may be further modulated by shaking from nearby earthquake swarms.

At Mt Tongariro, gravity and magnetic models constructed with the aid of geological maps, show large offset fractures in the rocks beneath the volcano, and delineate an extensive hydrothermal system.  Hot and acid water and gas circulating underground have destroyed magnetic minerals and lowered the density of the rocks they pass through. The 2012 eruption at Upper Te Maari crater depressurised the hydrothermal system, causing subsidence of the ground around the crater, as water and steam within the rock escapes through the crater.  Time-varying gravity models show shallow mass addition above the subsidence source, derived from a combination of water movement, condensation, cooling, and rainfall input, indicating the system is still repressurising. 

I show that the illumination of volcano architecture provides a richer, quantitative context, to better interpret volcanic unrest. I combine traditional geophysical methods and ground deformation data, with state of the art mathematical modelling techniques, and create a powerful and effective toolbox for the 21st century volcanologist.

Technical Abstract

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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