The Fate of Pumice from Large Submarine Eruptions: Microtextural Differences and Their Control on Clast Transportation and Deposition Processes

Submarine volcanic eruptions can produce large volumes of pumice that may rise buoyantly to the ocean surface or sink to the seafloor. If pumice remains afloat on the ocean surface, currents can disperse clasts thousands of kilometers away from their eruptive source in pumice rafts. The submarine geologic record is thus depleted of large volumes of information that may record key constraints such as total eruptive volume and mass eruption rate. The 2012 submarine Havre eruption presents a unique opportunity to study well-constrained rafted pumice (>1.2 km³) and seafloor pumice within fresh, detailed stratigraphy (>0.15 km³). We conducted macrotextural analysis (density, componentry, and clast morphology) and microtextural analysis (vesicle porometry, bubble number densities [BNDs], and bubble size distributions) on fragments from i) the pumice raft, ii) giant pumice blocks from the seafloor, and iii) a lapilli- and block-rich unit surrounding the main 2012 eruptive vent.

All three units share a common geochemical signature; have similar ranges of density, and indistinguishable macrotextural characteristics and componentry. Key differences are apparent in the connectivity of the vesicle structure and BNDs where raft pumices have up to an order of magnitude higher BNDs than pumice from the seafloor, and significantly lower vesicle connectivity despite similar bulk density. Pumices from the unit surrounding the main vent [iii] potentially mark a transition in eruptive behaviour due to the presence of macro- and micro-textural characteristics of both raft pumice and seafloor giant pumice. Very subtle differences in vesicle connectivity ultimately control whether pumices sink or float, and thus whether pumices are preserved in proximal stratigraphy. Our study also highlights the caution that should be considered when analyzing and interpreting the relative depositional timing of units in submarine volcaniclastic stratigraphy.