Eruptive shearing of tube pumice: pure and simple

Abstract

Abstract. Understanding the physicochemical conditions extant and mechanisms operative during explosive volcanism is essential for reliable forecasting and mitigation of volcanic events. Rhyolitic pumices reflect highly vesiculated magma whose bubbles can serve as a strain indicator for inferring the state of stress operative immediately prior to eruptive fragmentation. Obtaining the full kinematic picture reflected in bubble population geometry has been extremely difficult, involving dissection of a small number of delicate samples. The advent of reliable high-resolution tomography has changed this situation radically. Here we demonstrate via the use of tomography how a statistically powerful picture of the shapes and connectivity of thousands of individual bubbles within a single sample of tube pumice emerges. The strain record of tube pumice is modelled using empirical models of bubble geometry and liquid rheology, reliant on a constraint of magmatic water concentration. FTIR analysis reveals an imbalance in water speciation, suggesting post-eruption hydration, further supported by hydrogen and oxygen isotope measurements. Our work demonstrates that the strain recorded in the tube pumice dominated by simple shear (not pure shear) in the late deformational history of vesicular magma before eruption. This constraint in turn implies that magma ascent is conditioned by a velocity gradient (across the conduit) at the point of origin of tube pumice. Magma ascent accompanied by simple shear should enhance high eruption rates inferred independently for these highly viscous systems.

Accounting for the species-dependence of the 3500 cm−1H2Otinfrared molar absorptivity coefficient: Implications for hydrated volcanic glasses

“…Hydration of obsidian in particular has a long history of study, not only by the geological, but also the archaeological community, since diffusion modeling of hydration profiles at glass margins could offer a way to date obsidian flows or tools (e.g., Long 1976, 1984;Anovitz et al 1999;Riciputi et al 2002). Recent studies have also demonstrated that secondary hydration is widespread and has altered the glass water contents of many erupted pyroclasts, with the effect most pronounced for samples with greater surface area exposed to outside water, such as vesicular glasses (e.g., Giachetti and Gonnermann 2013;Dingwell et al 2015). Determining the original eruptive H 2 O t content of hydrated glasses is therefore critical to volatile studies of erupted pyroclasts.…”

Permeability of polydisperse magma foam

“…In our approach, we have made the simplifying assumption that a polydisperse bubbly magma can be approximated as a system of overlapping spheres. While this model approach is used widely (e.g., Blower, 2001;Vasseur and Wadsworth, 2017), the geometry imposed neglects the effects of bubble-bubble flattening and deformation prior to coalescence (Gonnermann et al, 2017) and the development of tube pumice (Dingwell et al, 2015), which have been observed and modeled. We note that textures that seem to record the capillary resistance of magma bubbles to coalescence may be one reason for the high percolation threshold invoked to explain magma bubbly permeability (Colombier et al, 2017;Giachetti et al, 2019).…”

Submarine giant pumice: a window into the shallow conduit dynamics of a recent silicic eruption

“…We use equations from Canedo et al (1993) and Rust and Manga (2002) for Ca and AR calculations. In this instance, vesicles-and subsequent AR calculations-are assumed to undergo simple shear due to the velocity profile as opposed to pure shear prior to conduit fragmentation (Dingwell et al 2016). Details of the model are explained in Appendix 2.…”

“…A conduit with a 25-m radius would not be able to produce the nonsheared vesicle textures observed in regular and banded giant pumices. Instead, textures would be dominated by elongate tube vesicles (Dingwell et al 2016).…”

“…1e and 3c). Giant tube pumices are rare in subaerial explosive eruptions as strain rates in regions of high vesicle shear usually fragment magma efficiently with tube vesicles and elongate permeable pathways (Dingwell et al 2016;Wright et al 2006). Greater length scales of observations of magma heterogeneity in the shallow conduit at Havre could, thus, be an analogue for the deeper conduit dynamics in subaerial silicic systems at equivalent pressures.…”