Quantifying Heterogenous Textures in Rafted Pumice from the 2019 Eruption of Volcano F, Tonga

Abstract

Large swaths of floating pumice (pumice rafts) tens to thousands of kilometers in area can be a striking and potentially hazardous product of submarine eruptions. Recent work has suggested that isolated porosity may be abundant in highly vesicular, rafted pumice, but it has rarely been measured. Because isolated porosity can influence raft flotation timescales, we focus on the presence of isolated porosity in pumice from a raft-producing volcanic eruption. In August 2019, “Volcano F,” a submarine volcano located in the Tonga-Kermadec Arc, produced a 195km2 raft of floating pumice. We analyze the textural and flotation characteristics of 45 pumice clasts sampled while floating and from the shores of Fiji. We measured connectivity and total porosity using a helium pycnometer, recorded flotation time with laboratory experiments, and identified textural classes by observing samples in hand sample and x-ray computed microtomography. We identified three textural classes: frothy (porosity > 85%, connectivity = 1), non-frothy (porosity < 85%, 0.15 > connectivity > 1), and transitional, which contain both frothy and non-frothy textures. We found that non-frothy clasts are collected further from the vent (up to 900 km) than frothy clasts, which are all collected proximally (< 200 km) while still floating. We also found abundant isolated porosity (connectivity < 0.6) in non-frothy and high total porosity clasts, which is a rarely measured textural feature of vesicular volcanic pumice. Considering the observation of abundant isolated porosity in the Volcano F clasts, we discuss potential formation mechanisms of isolated porosity, which deviate from traditional views of magma ascent.