The Viscous to Brittle Transition in Crystal- and Bubble-Bearing Magmas

Abstract

The transition from viscous to brittle behavior in magmas plays a decisive role in determining the style of volcanic eruptions. While this transition has been determined for one-or two-phase systems, it remains poorly constrained for natural magmas containing silicic melt, crystals, and gas bubbles. Here, we present new experimental results on shear-induced fracturing of three-phase magmas obtained at high-temperature (673-1023 K) and high-pressure (200 MPa) conditions over a wide range of strain-rates (5 6 1 3 1During the experiments bubbles are deformed (i.e., capillary number is in excess of 1) enough to coalesce and generate a porous network that potentially leads to outgassing. A physical relationship is proposed that quantifies the critical stress required for magmas to fail as a function of both crystal (0.24-0.65) and bubble volume fractions (0.09-0.12). The presented results demonstrate efficient outgassing for low crystal fraction (<0.44), whereas high crystal fractions (>0.44) promote gas bubble entrapment and inhibit outgassing. The failure of bubble-free, crystal-bearing systems is enhanced by the presence of bubbles that lower the critical failure stress in a regime of efficient outgassing, while the failure stress is increased if bubbles remain trapped within the crystal framework. These contrasting behaviors have direct impact on the style of volcanic eruptions. During magma ascent, efficient outgassing reduces the potential for an explosive eruption and favors brittle behavior, contributing to maintain low overpressures in an active volcanic system resulting in effusion or rheological flow blockage of magma at depth. Conversely, magmas with high crystallinity experience limited loss of exsolved gas, permitting the achievement of larger overpressures prior to a potential sudden transition to brittle behavior, which could result in an explosive volcanic eruption.

Syn-Emplacement Fracturing in the Sandfell Laccolith, Eastern Iceland—Implications for Rhyolite Intrusion Growth and Volcanic Hazards

“…Such a rheological transition of the magma from viscous flow to jamming may be related to a viscosity increase induced by crystallization and/or porosity reduction, degassing and cooling (Cashman and Blundy, 2000;Sparks et al, 2000;Giordano et al, 2008;Costa et al, 2009;Cordonnier et al, 2012;Pistone et al, 2012Pistone et al, , 2013Pistone et al, , 2015Pistone et al, , 2016Heap et al, 2015;Kennedy et al, 2016;Kushnir et al, 2017). Anderson and Fink (1992) described vesicular bands ("striations") in an obsidian lava flow, which they attributed to cooling, because they formed perpendicular to cooling joints.…”

“…Sparks et al, 2000;Cordonnier et al, 2012;Pistone et al, 2012Pistone et al, , 2013Pistone et al, , 2015Pistone et al, , 2016Galetto et al, 2017;Kushnir et al, 2017). Transport of the Sandfell rhyolite magma from a deeper source to the emplacement location most likely entailed a pressure change.…”

On the kinematics and dynamics of crystal‐rich systems

“…See the supporting information for methods and values used to plot data fields. Calculation details in supporting information [Androvandi et al, 2011;Caricchi et al, 2008Caricchi et al, , 2014Castruccio et al, 2010;Champallier et al, 2008;Cimarelli et al, 2011;Coetzee and Els, 2009;Dufek and Bergantz, 2005;Forien et al, 2011Forien et al, , 2015Girard and Stix, 2009;Grossmann and Lohse, 2001;Jellinek and DePaolo, 2003;Le Mével et al, 2016;Okumura et al, 2016Okumura et al, , 2015Parks et al, 2012;Petrelli et al, 2016;Pistone et al, 2012Pistone et al, , 2015Rutherford, 2008;Walker et al, 1999].…”

Volcanic magma reservoir imaged as a low-density body beneath Aso volcano that terminated the 2016 Kumamoto earthquake rupture

“…Furthermore, the mixture of gas, melt, and crystals that causes the low density of the SLDB also contributes to the aseismicity. The presence of gas and melt controls the viscous-brittle transition in solids (Cordonnier et al 2012;Pistone et al 2015), and a magma reservoir with a high melt fraction behaves as a viscous fluid (Lejeune and Richet 1995). The rheology of a melt-rich magma reservoir is also consistent with the aseismicity within the SLDB.…”