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.
Highlights
Records of explosive basaltic eruptions of Plinian intensities are improving (e.g., Thordarson and Self 2003; Sable et al 2009; Costantini et al 2010; Marzoano et al, 2013) and some authors have proposed eruption mechanisms (e.g., Houghton et al 2004; Houghton and Gonnermann 2008; Sable et al 2009; Costantini et al 2010; Goepfert and Gardner 2010; Kennedy et al 2010), the mechanics remain debated
We propose that conduit wall heating and resulting deformation influences conduit margin outgassing and may enhance the intensity of such large basaltic eruptions
We describe the textures of (1) a rhyolitic breccia clast situated at 10 m from the basaltic dyke margin, taken to represent the original rhyolitic textures, unmodified by the 1886 eruption and (2) two oriented clasts of rhyolitic breccia at the margin of the AD 1886 basaltic pyroclastic dyke
Summary
Records of explosive basaltic eruptions of Plinian intensities are improving (e.g., Thordarson and Self 2003; Sable et al 2009; Costantini et al 2010; Marzoano et al, 2013) and some authors have proposed eruption mechanisms (e.g., Houghton et al 2004; Houghton and Gonnermann 2008; Sable et al 2009; Costantini et al 2010; Goepfert and Gardner 2010; Kennedy et al 2010), the mechanics remain debated. Explanations for the exceptional explosivity of basaltic magmas include magma-water interaction (e.g., Houghton et al 2004), high magma viscosities due to degassing-induced microlite crystallisation or due to cooling of the magma triggered by degassing (Houghton and Gonnermann 2008; Sable et al 2009; Goepfert and Gardner 2010), critical bubble volumes causing the magma to disrupt (Goepfert and Gardner 2010), high degrees of outgassing
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
