Numerical simulation of bubble deformation in various velocity profiles across a conduit

Tube pumice is characterized by aligned highly elongated bubbles and is a common product of explosive silicic eruptions. The relative abundance of tube pumice and non-tube pumice in the stratigraphy has been interpreted as resulting from temporal and spatial variations in a conduit flow. Therefore, understanding the formation mechanism of tube pumice is valuable, but still debated. Most previous studies interpret tube pumice forming from simple shear deformation, assuming a parabolic velocity profile across a conduit. However, simple shear cannot explain the observation that tube pumice is rare in plinian falls but frequent in ignimbrites (interpreted to have wider vents).

In this study, we combine a bubble deformation model with a quasi-two-dimensional steady conduit flow model. A bubble is deformed by the velocity gradient while moving within the conduit flow. The conduit flow model is calculated for the 1.8 ka Taupo plinian eruption, which produced a high proportion of tube pumice in the ignimbrite phase. In this abstract, we explain results from two rheological models showing distinct velocity profiles. In the Newtonian isothermal fluid, the velocity profile across the conduit becomes parabolic. In a fluid that allows viscous heating, the temperature near the conduit wall rises up sharply, leading to a strong reduction in viscosity, and the velocity profile changes from a parabolic shape to a plug-like shape. The parabolic velocity profile produces highly elongated bubbles mainly by simple shear, while the plug-like velocity profile is dominated by pure shear and accumulates less strain to elongate bubbles. The bubble shape at the fragmentation surface depends significantly on the velocity profile and its change along the conduit.

We also conduct a quantitative and statistical bubble shape analysis of pumice erupted at Taupo volcano. It shows that the plinian pumices have a single peak in the bubble shape distribution, while the ignimbrite pumices have a broad distribution and contain highly elongated bubbles. The comparison of the distribution of pumice textures with the simulation results suggests that the velocity profile of the plinian phase is close to a plug-like shape. We also calculate bubble deformation for the Taupo ignimbrite eruption, using the viscous-heating model. We model a　wider conduit for the ignimbrite phase which leads to lower shear rate around the conduit walls and a higher proportion of the conduit experiencing parabolic flow compared to the plinian phase. This increased proportion of parabolic velocity profile in the conduit can explain a large number of tube pumice in the Taupo ignimbrite.