Dynamic petrology and rheology of ascending magma during lava dome eruptions : effusive-explosive activity

Lava dome eruptions are one of the most hazardous forms of intermediate volcanic activity, with their ability to switch from benign effusive to catastrophic explosive eruption style with little precursory warning. Such behaviour can be attributed to a complex interplay between deep and shallow magma...

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Bibliographic Details
Main Author Wallace, Paul A
Format Dissertation
LanguageEnglish
Published University of Liverpool 2019
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Summary:Lava dome eruptions are one of the most hazardous forms of intermediate volcanic activity, with their ability to switch from benign effusive to catastrophic explosive eruption style with little precursory warning. Such behaviour can be attributed to a complex interplay between deep and shallow magmatic processes which alter the rheological, physical and chemical state of the magma, and its propensity to erupt. A comprehensive understanding of such processes is thus paramount for deciphering causes for different eruptive behaviours. This thesis aims to address three distinct processes, all of which are deemed a common phenomenon at lava dome volcanoes and can have an integral role in magma ascent dynamics in shallow volcanic conduits: (1) magma shearing, (2) fault friction and (3) magma recharge. During the ascent of high-viscosity magma, crystallisation and volatile exsolution favours a non-Newtonian response, which promotes localisation of stress and strain near the conduit margins, leading to the formation of discrete shear zones. The impact and role of strain localisation during magma ascent was investigated through a detailed petrological survey of a shear zone developed at the margins of the 1994–1995 lava spine at Unzen volcano (Japan). A multidisciplinary approach combining field observations, microtextures, crystallography and magnetics was deployed, which was supplemented by the first high-temperature, highvelocity rotary shear experiment to simulate the processes involved during shear. I show that crystals can effectively monitor stress conditions during magma ascent through the viscous– brittle transition, with viscous remobilisation, crystal plastically and comminution all systematically increasing with shear intensity at the conduit margins. I suggest evidence for a thermo-mechanical response due to strain localisation (shear/frictional heating) which triggered disequilibrium conditions leading to mineral reactions, altered rock magnetic properties and compacted the original porous network. The range of deformation textures recorded can have a profound effect on magma outgassing and the style of surface activity. Strain localisation can lead to seismogenic magma failure proceeded by faulting and slip along fracture planes. In particular, frictional sliding near the conduit margins can cause localised melting (by frictional heating) and impose an important rheological control on slip dynamics, acting as a lubricant or viscous brake that controls magma ascent in the upper conduit. Frictional melting is a non-equilibrium process, involving the selective melting of mineral phases and the softening of a glass at the glass transition. I present a detailed, systematic experimental investigation demonstrating the importance of host-rock mineralogy on the mechanical, geochemical, textural and rheological evolution of a frictional melt. The presence of hydrous minerals, such as amphibole, have the most influential role due to their lower melting point, generating heterogeneous mafic melts with lower viscosities that promote slip. In contrast, when amphibole is absent, frictional melts are more compositionally homogenous with higher viscosities resulting in higher shear resistance during slip. Understanding frictional properties with respect to mineralogy may aid in constraining flow behaviour in the shallow volcanic conduit that can impact activity at the surface. Regular, small-to-moderate gas-and-ash explosions at Santiaguito dome complex (Guatemala) have previously been attributed to shear and frictional processes. However, in 2015–2016, eruptive activity intensified to larger, less frequent ash-rich explosive activity. I present a unique, multiparametric dataset that integrates petrological and geochemical signatures of the eruptive products with a rich geophysical dataset to assess the cause for a shift in eruption style. I provide evidence for a magma recharge event, resulting in changes in ash componentry, mingling textures, bulk compositional changes, alteration of groundmass microlite textures that occur concordantly with a deeper explosion source in the conduit revealed by seismicity. The integrated observations imply a switch from shallow shear-driven fragmentation to deeper gas overpressure fragmentation, intensifying local hazards. These observations not only aid in understanding Santiaguito's ongoing dynamic activity, but may also assist in deciphering complex shifts in eruption style at many active volcanoes.
Bibliography:0000000485055691
DOI:10.17638/03046850