Micro-Textural Controls on Magma Rheology and Vulcanian Explosion Cyclicity

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.

Time-series experiments of pyroxene crystal nucleation and growth in basaltic magmas and implications for magma rheology

<p>Basaltic volcanism is strongly influenced by magmatic viscosity, which, in turn, is controlled by magma composition, crystallisation, oxygen fugacity and vesiculation. We developed an environmental cell to replicate the pressure and temperature during magma ascent from crustal storage to the surface, while capturing crystallisation using in-situ 4D X-ray computed microtomography. Crystallisation experiments were performed at Diamond Light Source, using monochromatic 53 keV X-rays, a pixel size of 3.2 μm, a sample to detector distance of 2000 mm, 1440 projections per 180 deg, an acquisition time of 0.04 s, and a rotation velocity of 3.125 deg.s-1. The redox conditions were controlled using an oxidised nickel disk for each experiment. Our starting materials were samples made of crystal-free glass cylinders (Ø 3 mm) from the 2001 Etna eruption with 0.9 and 0.8 wt. % water content. In the experiments, samples and crucibles were sealed initially by applying ~10 N loads. All samples were then heated up above glass transition (between 800 °C and 900 °C) in order to allow sample homogenisation while preventing volatiles exsolution. We then pressurised each sample by applying uniaxial loads (between 80 and 380 N), using high-degree alumina pistons, in order to generate enough internal pressure to maintain bubble-free samples when the desired high temperature was reached. Once at the initial high temperature, we began experiments via dropping the temperature to different target isothermal (from 1210 to 1130 °C or 1180 to 1110 °C) and isobaric conditions (8 and 10 MPa, respectively). For the whole duration of the experiments, we were able to observe directly and record pyroxene crystal nucleation and growth. Specifically, we were able to observe pyroxene nucleation on bubbles at small undercooling (∆T) and epitaxial growth of pyroxene at large ∆T. An increase of ∆T (up to 50 °C) can be associated with a decompression of a magma chamber or a decompression during magma ascent in the conduit. As ∆T = 30 - 50 °C can be reached in most of the basaltic volcanic systems on Earth, our results provide a feasible explanation of which mechanisms control nucleation and growth of pyroxene crystals in hydrous basaltic magmas. In addition, epitaxial growth promotes a faster increase of the crystal volume. As a larger crystal content translates into a higher viscosity, our results have important implications for magma rheology, and are extremely important to improve our understanding of magma ascent dynamics during volcanic eruptions, and our capacity to predict eruptions and mitigate volcanic hazards.</p>

A Simple Procedure for Measuring Magma Rheology

In this study, a procedure to measure the viscosity of multi-phase magma at high temperatures (>1000°C) was developed by using a simple apparatus comprising a commercially available desktop furnace and viscometer. In particular, the use of a disposable container enabled observations of the microstructure of an entire sample. The procedure was applied to basaltic andesite magma of the 1986 Izu–Oshima fissure eruption, Japan. The results show that reliable data, consistent with previous studies, were obtained and that the magma rheology became non-Newtonian with decreasing temperature, showing clear shear-thinning behavior. The rheological properties of the magma at 1180°C are quantitatively described as a function of shear rate based on three simple non-Newtonian fluid models. Sectional views of the sample confirm that plagioclase and Fe–Ti oxide crystals were nearly uniformly dispersed in the sample. The mean crystal volume fraction of 0.14 enabled crystal interactions inducing changes in crystal arrangement, affecting the rheology.

Analisis Tipikal Erupsi Gunung Lokon Periode Erupsi 2012-2013 Berdasarkan Karakterisasi Mikrostruktur Abu Vulkanik

Gunung Lokon yang berada di lengan utara Sulawesi adalah salah satu gunung api paling aktif di Indonesia. Perilaku erupsinya telah dipelajari melalui analisis mikrostruktur abu vulkanik. Tujuan dari karakterisasi mikrostruktur adalah untuk mengestimasi nilai dari viskositas dan permeabilitas magma. Karakterisasi mikrostruktur menggunakan XRD, FTIR, SEM/EDS/XRF dan µCT. Abu vulkanik Lokon adalah mineral polimorf yang banyak mengandung kristal plagioklase. Abu Lokon mempunyai kandungan air 0,3 -0,6 % berat dan massa dasarnya terdiri dari partikel vesikular dan non vesikular. Viskositas dari magma Lokon adalah sekitar 107Pa.s pada 10000C dan fraksi volume kristal sekitar 0,45-0,5. Hasil-hasil ini menunjukkan bahwa reologi magma Lokon adalah bersifat non Newtonian dan mekanisme fragmentasinya adalah brittle fragmentation. Berdasarkan pada permeabilitas dan porositas yang dikuantisasi dengan µCT dapat disimpulkan bahwa fragmentasi magmanya tidak dipicu oleh outgassing. Dinamika erupsi eksplosif dari Gunung Lokon pada 2012-2013 adalah erupsi vulkanian pada skala sedang.Lokon volcano where located on North arm of Sulawesi is one of the most active volcanoes in Indonesia. Behaviour of its eruptions have been learned through microstructure analysis of volcanic ash. The goal of microstructure characterization is estimate value of magma viscosity and permeability. Characterization of microstructure using XRD, FTIR, SEM/EDS/XRF and µCT. Lokon volcanic ash is a polymorph minerals which contains many plagioclase crystal. Ash has water content between 0.3 – 0.6 % wt and its groundmass contains vesicular and non vesicular particles. Viscosity of Lokon magma is about 107Pa.s at 10000C and fraction of crystal volum between 0.45-0.5. These results showed that magma rheology of Lokon is non Newtonian and the mechanism of its fragmentation is brittle fragmentation. Based on permeability and porosity that quantified by µCT, it is concluded that the brittle fragmentation is not triggered by outgassing. Dynamics of explosive eruption of Lokon volcano at 2012-2013 is moderate vulcanian eruption.