![]() ![]() 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. 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. 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. 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. 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 SO 2 fluxes. The data suggest a spectrum of rheological behavior controlled by concurrent variations in microlite number, size and shape. Samples with high numbers of small tabular microlites showed a lower apparent viscosity and were less shear-thinning. Samples with low numbers of large prismatic microlites behaved consistently with published rheological laws for crystalline dome samples, and displayed extensive micro-cracking. Variations in plagioclase micro-textures are associated with apparent viscosities spanning over one order of magnitude for a given strain rate. Experiments were conducted at different strain rates to measure the rate-dependence of apparent viscosities and assess the dominant deformation processes associated with shear. To investigate the influence of variations in crystal micro-textures on magma rheology and eruption dynamics, we conducted high-temperature (940☌) 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. 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. 5Observatorio Vulcanológico y Sismológico de Pasto, Servicio Geológico Colombiano, Pasto, Colombia.4Observatorio Vulcanológico y Sismológico de Manizales, Servicio Geológico Colombiano, Manizales, Colombia.3Department of Geography, Edge Hill University, Ormskirk, United Kingdom.2School of Environmental Sciences, Department of Earth, Ocean & Ecological Sciences, University of Liverpool, Liverpool, United Kingdom.1School of Geosciences, University of Edinburgh, Grant Institute, Edinburgh, United Kingdom.Cortés 3 Gloria Patricia Cortés 4 Diego Gómez Martinez 5 Roberto A. Kendrick 1,2 Anthony Lamur 2 Yan Lavallée 2 Eliza S. Hungry for some science, but you don’t have time for a full-course research plate? Then check out USGS Science Snippets, our snack-sized science series that focuses on the fun, weird, and fascinating stories of USGS science.Amelia A. Next EarthWord: Our next word sounds like what the British might call an outhouse. In determining what types of oil and gas are present, as well as how much is technically recoverable, viscosity of the petroleum is very important. ![]()
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