Comparison of Bubble Shape Model Results With Textural Analysis: Implications for the Velocity Profile Across a Volcanic Conduit

Tube pumice, which is characterized by aligned elongated bubbles, is valuable for our understanding of the shear field in a volcanic conduit. Most previous studies interpreted that tube pumice forms through simple shear, assuming a parabolic velocity profile across a volcanic conduit. However, simple shear cannot account for the observation that tube pumice is rare in plinian fall deposits but frequent in ignimbrites at the Taupo volcano, if the pyroclastic flows that formed them erupted through a wider vent and had a lower average shear rate. Here, we analyze the dynamics of bubble deformation that occurred during the 232 CE Taupo eruption by combining a bubble deformation model with a conduit flow model. The results indicate that bubble deformation is largely controlled by the velocity profile across a conduit. If the conduit flow has a parabolic velocity profile, the amount of bubble elongation is unrealistic compared to the Taupo fall deposits. It is more likely that the conduit flow has a plug-like velocity profile due to viscous heating. We numerically confirmed that bubbles in the plug-like flow are subject mainly to pure shear and accumulate less strain for elongation. We also calculated a viscous heating flow of the ignimbrite phase. We modeled a wider conduit, which suppresses viscous heating around the conduit walls and causes a higher proportion of the conduit to experience parabolic flow, compared to the plinian phase. The increased proportion of a parabolic profile may explain the high frequency of elongated bubbles in the ignimbrite phase pumice.

Automated summary

The study found that bubble deformation during the Taupo volcanic eruption is mainly controlled by the velocity profile across the conduit, and a piston-like velocity profile reduces the amount of bubble elongation compared to a parabolic velocity profile. This conduit flow condition can explain the frequent occurrence of tube pumice in ignimbrite deposits.