Abstract
Understanding the relationship between degassing, crystallization processes and eruption style is a central goal in volcanology, in particular how these processes modulate the magnitude and timing of cyclical Vulcanian explosions in intermediate magmas. To investigate the influence of variations in crystal micro-textures on magma rheology and eruption dynamics, we conducted high-temperature (940°C) uniaxial compression experiments at conditions simulating a shallow volcanic conduit setting on eight samples of high-crystallinity andesite with variable plagioclase microlite populations from the 2004 to 2010 Vulcanian explosions of Galeras volcano, Colombia. Experiments were conducted at different strain rates to measure the rate-dependence of apparent viscosities and assess the dominant deformation processes associated with shear. Variations in plagioclase micro-textures are associated with apparent viscosities spanning over one order of magnitude for a given strain rate. Samples with low numbers of large prismatic microlites behaved consistently with published rheological laws for crystalline dome samples, and displayed extensive micro-cracking. Samples with high numbers of small tabular microlites showed a lower apparent viscosity and were less shear-thinning. The data suggest a spectrum of rheological behavior controlled by concurrent variations in microlite number, size and shape. We use previously published micro-textural data for time-constrained samples to model the apparent viscosity of magma erupted during the 2004–2010 sequence of Vulcanian explosions and compare these results with observed SO2 fluxes. We propose that variations in magma decompression rate, which are known to produce systematic textural differences in the plagioclase microlite cargo, govern differences in magma rheology in the shallow conduit. These rheological differences are likely to affect the rate at which magma densifies as a result of outgassing, leading to magmatic plugs with a range of porosities and permeabilities. The existence of magmatic plugs with variable physical properties has important implications for the development of critical overpressure driving Vulcanian explosions, and thus for hazard assessment during volcanic crises. We suggest a new conceptual model to explain eruption style at andesitic volcanoes based on micro-textural and rheological differences between “plug-forming” and “dome-forming” magma. We advance that existing rheological laws describing the behavior of andesitic magma based on experiments on dome rocks are inappropriate for modeling large Vulcanian explosions (∼106 m3), as the magma involved in these eruptions lacks the characteristics required to form exogenous lava domes.
Highlights
Hazardous sequences of Vulcanian explosions are a common feature during periods of activity at arc volcanoes
We investigate the effect of varying crystal microtextures on magma rheology by conducting a series of hightemperature (940°C) deformation tests on eight bomb samples ejected during Vulcanian explosions from Galeras volcano during the 2004–2010 period of activity
The apparent viscosity computed for each applied condition decreased linearly as a function of applied strain rate (Figure 4B), and this characteristic shear-thinning behavior was observed for all samples
Summary
Hazardous sequences of Vulcanian explosions are a common feature during periods of activity at arc volcanoes These explosions are commonly associated with the development of overpressure beneath or within low-permeability, highcrystallinity magmatic plugs in the upper conduit (e.g., Sparks, 1997; Stix et al, 1997; Voight, 1999; Mueller et al, 2005; Clarke et al, 2007; Wright et al, 2007; Mueller et al, 2011a; Lavallée et al, 2015). The degassing, crystallization and densification processes responsible for plug and overpressure development may be modulated by differences in magma composition, ascent rate and decompression path, petrological characteristics, rheological evolution, outgassing and shear history (e.g., Calder et al, 2015) This results in a spectrum of physico-chemical evolution pathways that promotes a variety of repose times between explosions, a range of ejected volumes and transitions from explosive activity to effusive dome-building behavior (e.g., Cassidy et al, 2018; Wallace et al, 2020)
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