A multidisciplinary investigation of magmatic overpressure and its implications for volcanic eruption triggering

Thesis

Magmatic overpressure – pressure in excess of lithostatic – can be generated by processes such as thermal expansion, crystallisation or magma injection. We cannot observe these processes directly, yet the magnitude and timescale of overpressure build-up are both critical for understanding failure limits and forecasting volcanic eruptions. This thesis explores variations in the magnitude and rate of overpressure build-up with varying magmatic and crustal properties. It also assesses the use of electron backscatter diffraction as a new tool for probing reservoir overpressurisation and storage conditions.

Chapter 2 demonstrates that there are variations in the intrinsic overpressure-generating capacity of magmas according to their initial fluid content. Overpressure due to second boiling increases in magnitude with increasing XH2O, and is at a peak in magmas with initial H2O contents which are at the solubility limit. The implications of this variability in overpressure are discussed in the context of eruption style, and results are coupled with a thermal model to determine the associated timescales of overpressurisation. Chapter 3 continues with this novel coupling of thermodynamic and thermal models by quantifying the effect of changes in magma reservoir volume, geometry and crustal properties on timescales of overpressurisation. These timescales are discussed in the context of other overpressurisation mechanisms, including magma buoyancy and bubble rise. The importance of overpressurisation rate, as well as the absolute value, is highlighted in the context of eruption triggering. Chapter 4 explores overpressurisation from a different angle, by analysing deformation in natural and experimentally deformed volcanic samples to investigate physical signals of overpressurisation. Crystal plastic deformation is quantified using electron backscatter diffraction, and demonstrates potential for determining long-term magma storage conditions. Further experimentation to assess the effect of crystallinity, and analysis of samples from a range of explosive eruptions are the most useful next steps to achieve this goal.

Authors: Brookfield, A

• DOI: 10.5287/ora-1a7z8pveg
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: electron backscatter diffraction (EBSD); overpressure; relaxation timescales; crustal properties; thermal modelling; experimental petrology; magma; volcanic systems
• Subjects: Volcanology