Strain‐Dependent Rheology of Silicate Melt Foams: Importance for Outgassing of Silicic Lavas

Abstract

Outgassing of volcanic systems is a ubiquitous phenomenon. Yet the mechanisms facilitating gas escape from vesiculating magmas and lavas remain poorly understood. Pervasive outgassing is thought to depend on the efficient and abundant formation of permeable pathways. Here we present results from experiments designed to identify the conditions and mechanisms needed to form such permeable pathways. We use a foamed silicate melt (FOAMGLAS®) in our experiments as a proxy for natural vesicular melts. FOAMGLAS® cores are compressed under a range of temperatures and strain rates, and results are evaluated against the state of melt relaxation (parameterized using the Deborah number). We find that foam microstructure and rheological and outgassing behaviors evolve with strain and as a function of melt relaxation state. Relaxed melt foams harden during deformation but remain impermeable. As foams become less relaxed at higher strain rate and/or melt viscosity, they show complex responses to deformation (strain weakening and hardening) yet remain impermeable. Strain localization and formation of high‐permeability bands occur only in highly strained, unrelaxed foams. However, these bands are thin and oriented perpendicular to the principal stress, resulting in limited outgassing. Permeable pathways do not readily form in foams; rather, high‐porosity melts are “persistently impermeable.” Our results imply that vesicular silicic lavas may not outgas as efficiently as previously thought. Instead, sustained impermeability allows the lava to maintain low effective viscosities, and to flow extended distances. In addition, pore pressures may rise in deforming impermeable lavas, perhaps priming the lava for later explosive behavior.

The Permeability Evolution of Tuffisites and Implications for Outgassing Through Dense Rhyolitic Magma

“…Unprecedented observations of recent subaerial rhyolite eruptions in Chile have demonstrated that effusive extrusion of rhyolitic lava can be coincident with intermittent explosions of ash, lapilli, and bombs (Castro et al, ; Lara, ; Schipper et al, ). Such hybrid activity demands reappraisal of existing paradigms of eruptive style transitions (Eichelberger et al, ) and highlights that outgassing mechanisms are of critical importance (e.g., Chevalier et al, ; Collinson & Neuberg, ; Farquharson et al, ; Gonnermann & Manga, ; Kushnir et al, ; Ryan et al, ). Chaitén volcano (Chile) exhibited prolonged hybrid activity in 2008 (Castro et al, ), whereas longer‐lived hybrid (nine months) activity occurred at Cordón Caulle (also in Chile) during 2011–2012 (Schipper et al, ).…”

“…For example, the photograph taken during the 10 January 2012 activity at Cordón Caulle ( t = +4 s) shows that localized outgassing occurred via pathways with fracture (i.e., plane) geometries (Figure a), rather than outgassing through permeable foam (e.g., Eichelberger et al, ). Indeed, recent experimental work by Ryan et al () has shown that it is difficult to create permeability in initially impermeable, high‐porosity foams, describing them as “persistently impermeable.” Dense obsidian bombs emitted during hybrid activity characteristically hosted tuffisites, which are centimetric fractures infilled with pyroclastic material (Castro et al, ; Heiken et al, ; Saubin et al, ; Stasiuk et al, ; Tuffen et al, ; Figure a). Tuffisites have been interpreted to record the transient opening and occlusion of the permeable pathways that provide fleeting escape routes for pressurized gas prior to eventual blockage and violent ejection (Saubin et al, ).…”

Magma / Suspension Rheology

“…4.7A). The simplest approach to calculate the apparent viscosity of the sample in uniaxial compression parallel plate experiments is to derive it directly from the ratio of the recorded stress (s, i.e., load over surface area in contact with the piston) and engineering-strain-rate ( e e ; i.e., variation of the sample length (∆l/l) per unit time) (Quane and Russell 2005;Avard and Whittington 2012;Heap et al 2014;Ryan et al 2019a):…”

“…The rheology of a bubbly melt can change drastically across this transition due to self-stabilization of the bubble walls as they transition from curved bubble walls to more straight melt lms that intersect at plateau boarders in sets of three. This structure is not stable at high strain and can thus result in rapid breakdown of the foam through bubble wall failure and gas escape (Ryan et al 2019a). A constitutive description of ow in this regime has, however, not been published to date.…”

Conduit system, degassing, and flow dynamics of a rhyolite lava: A case study of the Shiroyama lava on Himeshima Island, Japan

“…Potential hazards such as surface explosions and flow front collapses have also been pointed out from ancient rhyolite lavas [Castro et al 2002a;Fink and Manley 1987]. Because such hazards are potentially caused by the overpressure of inner volcanic gasses, the degassing and outgassing systems of rhyolite lavas have been examined [Furukawa et al 2010;Ryan et al 2019;Shields et al 2016]. The flow dynamics of rhyolite lavas have been deduced by researchers based on textural analyses, magnetic experiments, and numerical studies [Bullock et al 2018;Castro et al 2002a;Furukawa and Uno 2015;Magnall et al 2017].…”