Conduit margin heating and deformation during the AD 1886 basaltic Plinian eruption at Tarawera volcano, New Zealand

Abstract

During explosive eruptions, a suspension of gas and pyroclasts rises rapidly within a conduit. Here, we have analysed textures preserved in the walls of a pyroclastic feeder dyke of the AD 1886 Tarawera basaltic Plinian fissure eruption. The samples examined consist of basaltic ash and scoria plastered onto a conduit wall of a coherent rhyolite dome and a welded rhyolitic dome breccia. We examine the textural evidence for the response of the wall material, built of ∼75 vol.% glass and ∼25 vol.% crystals (pore-free equivalent), to mass movement in the adjacent conduit. In the rhyolitic wall material, we quantify the orientation and aspect ratio of biotite crystals as strain markers of simple shear deformation, and interpret juxtaposed regions of vesiculation and vesicle collapse as evidence of conduit wall heating. Systematic changes occur close to the margin: (1) porosity is highly variable, with areas locally vesiculated or densified, (2) biotite crystals are oriented with their long axis parallel to the margin, (3) the biotites have greater aspect ratios close to the margin and (4) the biotite crystals are fractured. We interpret the biotite phenocryst deformation to result from crystal fracture, rotation and cleavage-parallel bookcase translation. These textural observations are inferred to indicate mechanical coupling between the hot gas-ash jet and the conduit wall and reheating of wall rock rhyolite. We couple these observations with a simple 1D conductive heating model to show what minimum temperature the conduit wall needs to reach in order to achieve a temperature above the glass transition throughout the texturally-defined deformed zone. We propose that conduit wall heating and resulting deformation influences conduit margin outgassing and may enhance the intensity of such large basaltic eruptions.Electronic supplementary materialThe online version of this article (doi:10.1007/s00445-016-1006-7) contains supplementary material, which is available to authorized users.

Permeability Evolution in Variably Glassy Basaltic Andesites Measured Under Magmatic Conditions

“…Conduit reheating and sealing is a physically feasible process that has been identified at rhyolite‐basalt interfaces (Kennedy et al, ; Schauroth et al, ). While we conclude that a significant increase in temperature is unlikely to result in a pronounced modification to the permeability of a devitrified volcanic dome between eruption cycles, remobilization of any residual glass phase both in the edifice and at conduit margins remains a compelling process for drastic and efficient permeability reduction.…”

“…Indeed, at Merapi, magma storage and pre‐eruptive recharge temperatures were between 925 and 1,000°C prior to the 2010 eruption (Erdmann et al, ); such temperatures are high enough to remobilize the glass phase of the juvenile samples used in this study, which may have resulted in thermally induced sealing of the permeable network. This process of reheating and relaxation remains a potential mechanism for sealing at conduit margins (Heap et al, ; Kennedy et al, ; Schauroth et al, ), inhibiting volatile escape to the surrounding edifice and contributing to gas overpressures that may tax the volcanic plumbing at depth. However, we emphasize that this process will undoubtedly be accompanied by other processes that significantly alter permeability including deformation (Farquharson et al, ; Heap et al, ; Kushnir et al, ; Laumonier et al, ; Lavallée et al, ; Okumura et al, , ; Violay et al, ) and the deposition of vapor transported phases (Baxter et al, ; de Hoog et al, ; Wright et al, ).…”

“…Volatiles often make their way to conduit margins where strain is localized (e.g., Gaunt et al, ; Rust et al, ; Thomas & Neuberg, ; Tuffen & Dingwell, ) and permeable networks can either siphon volatiles through the conduit walls (Heap et al, ; Jaupart, ) or direct them up along the conduit margins (Gaunt et al, ; Kushnir et al, ; Lavallée et al, ; Rust et al, ) and to the surface via the overlying edifice structure (Collinson & Neuberg, ; Collombet, ; Jaupart, ). The ability of the surrounding conduit wall rock to dissipate accumulated volatiles is dependent on the rock's structure, which may experience continued microstructural modification when exposed to temperatures that carry the amorphous phase of the rock above its glass transition (Kennedy et al, ; Schauroth et al, ). Indeed, fluctuations in temperature at conduit margins can lead to shifting zones of rock compaction and viscous deformation that operate to limit permeability (Heap et al, ).…”

Upscaling permeability in anisotropic volcanic systems

“…Okumura et al, 2013;Farquharson et al, 2016). Further, volcano modelling (for example by Hurwitz et al, 2003;Lillis et al, 2015;Schauroth et al, 2016;5…”

“…Since the work of Eichelberger et al (1986), numerous studies have examined the permeability of natural and synthetic volcanic materials representing a wide variety of volcanic systems (e.g. Westrich and Eichelberger, 1994;Klug and Cashman, 1996;Mueller et al, 2005;Degruyter et al, 2010;Kolzenburg et al, 2012;Ashwell et al, 2015;Heap et al, 2015;Farquharson et al, 2015Farquharson et al, , 2016Wadsworth et al, 2016;Kushnir et al, 2016Kushnir et al, , 2017a, amongst many others).…”