Permeability of compacting porous lavas

Abstract

The highly transient nature of outgassing commonly observed at volcanoes is in part controlled by the permeability of lava domes and shallow conduits. Lava domes generally consist of a porous outer carapace surrounding a denser lava core with internal shear zones of variable porosity. Here we examine densification using uniaxial compression experiments on variably crystalline and porous rhyolitic dome lavas from the Taupo Volcanic Zone. Experiments were conducted at 900°C and an applied stress of 3 MPa to 60% strain, while monitoring acoustic emissions to track cracking. The evolution of the porous network was assessed via X-ray computed tomography, He-pycnometry, and relative gas permeability. High starting connected porosities led to low apparent viscosities and high strain rates, initially accompanied by abundant acoustic emissions. As compaction ensued, the lavas evolved; apparent viscosity increased and strain rate decreased due to strain hardening of the suspensions. Permeability fluctuations resulted from the interplay between viscous flow and brittle failure. Where phenocrysts were abundant, cracks had limited spatial extent, and pore closure decreased axial and radial permeability proportionally, maintaining the initial anisotropy. In crystal-poor lavas, axial cracks had a more profound effect, and permeability anisotropy switched to favor axial flow. Irrespective of porosity, both crystalline samples compacted to a threshold minimum porosity of 17-19%, whereas the crystal-poor sample did not achieve its compaction limit. This indicates that unconfined loading of porous dome lavas does not necessarily form an impermeable plug and may be hindered, in part by the presence of crystals.

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

“…S-C fabric formation may also have led to compaction and porosity reduction in the magma perpendicular to the S plane, which would have increased its viscosity (cf. Ashwell et al, 2015;Kennedy et al, 2016). The orientation of the C plane parallel to the flow banding (and the contact) suggests that the S-C fabrics in the main body of the Sandfell laccolith are induced by the combined intrusive contact-perpendicular (compressive) stresses and flexural shear due to the inflation of the magma body causing σ 1 to be oriented 45-60 • to the contact.…”

“…The low vesicle content in coherent fracture-poor rhyolite exposed in the lower parts of Sandfell and the overall small vesicle sizes in fractured rock suggest that the Sandfell rhyolite was quite impermeable and therefore could not have effectively been degassing due to the transport from source to emplacement location ( Figure 5G). However, Ashwell et al (2015) showed in experiments on rhyolitic dome lavas that the porosity of the lava decreased at a constant shear rate. Reducing the porosity in magma by compaction, pore collapse and/or degassing increases the magma viscosity (Ashwell et al, 2015;Heap et al, 2015;Kushnir et al, 2017).…”

“…On the other hand, brittle deformation in magma moving through volcanic conduits has been shown to be a manifestation of strain-induced ductile-brittle transition and to regulate the eruptive behavior of magma by influencing its potential to degas (Gonnermann and Manga, 2003;Tuffen et al, 2003;Lavallée et al, 2007;Cordonnier et al, 2012;Kendrick et al, 2013Kendrick et al, , 2016Ashwell et al, 2015;Pistone et al, 2016;Kushnir et al, 2017). Fracturing, for example, decompresses the magma locally and creates permeability, which promotes degassing and affects the rheological behavior of magma (Nara et al, 2011;Castro et al, 2012;Pistone et al, 2013;Heap and Kennedy, 2016;Kushnir et al, 2017).…”

Geomechanical rock properties of a basaltic volcano

“…At volcanoes, the porous network comprises of vesicles and fractures that build the permeable network and which controls degassing (Ashwell et al, 2015). Porosity also controls the mechanical response of materials whereby their strength decreases and elasticity increases with porosity (e.g., Al-Harthi et al, 1999;Spieler et al, 2004;Paterson and Wong, 2005;Heap et al, 2014a,b).…”

From magma ascent to ash generation: investigating volcanic conduit processes by integrating experiments, numerical modeling, and observations

“…3D imaging via conventional and synchrotron-based X-ray computed microtomography as well as neutron computed tomography offers the ability to image and quantify rock textures directly in 3D in unprecedented detail and references therein, Lavallée et al 2013, Arzilli et al 2016. Through 3D textural investigations we are able to view structures that are the result of strain localization [Wright and Wimberg 2009, Shields et al 2014, deformation [Okumura et al 2010;Caricchi et al 2011, Ashwell et al 2015, crystal aggregation and crystal fragmentation [Pamukcu et al 2012], convection , Carey et al 2013] and development of permeability [Bai et al 2010, Degruyter et al 2010b, Bai et al 2011, Kendrick et al 2013, Ashwell et al 2015 in magma, which can be related to experiments, and which in the near future could feed into experimental and numerical modeling. Finally, information on fragmentation mechanisms (the terms "phreatic", "hydrothermal", and "hydromagmatic" are cause of great amount of discussion and debate about their meaning and use) can, in part, be assessed from particle morphology [Dellino et al 2012, Jordan et al 2014, Liu et al 2015b, while TGSD provide information on fragmentation efficiency [Kueppers et al 2006, Rust and Cashman 2011, Costa et al 2016.…”