Sink or float: microtextural controls on the fate of pumice deposition during the 2012 submarine Havre eruption

Abstract

Silicic submarine volcanic eruptions can produce large volumes of pumices that may rise buoyantly to the ocean surface and/or sink to the seafloor. For eruptions that release significant volumes of pumice into rafts, the proximal to medial submarine geologic record is thus depleted in large volumes of pumice that would have sedimented closer to source in any subaerial eruption. The 2012 eruption of Havre volcano, a submarine volcano in the Kermadec Arc, presents a unique opportunity to study the partitioning of well-constrained rafted and seafloor pumice. Macro- and microtextural analysis was performed on clasts from the Havre pumice raft and from coeval pumiceous seafloor units around the Havre caldera. The raft and seafloor clasts have indistinguishable macrotextures, componentry, and vesicularity ranges. Microtextural differences are apparent as raft pumices have higher vesicle number densities (109 cm−3 vs. 108 cm−3) and significantly lower pore space connectivity (0.3–0.95 vs. 0.9–1.0) than seafloor pumices. Porosity analysis shows that high vesicularity raft pumices required trapping of gas in the connected porosity to remain afloat, whereas lower vesicularity raft pumices could float just from gas within isolated porosity. Measurements of minimum vesicle throat openings further show that raft pumices have a larger proportion of small vesicle throats than seafloor pumices. Narrow throats increase gas trapping as a result of higher capillary pressures acting over gas–water interfaces between vesicles and lower capillary number inhibiting gas bubble escape. Differences in isolated porosity and pore throat distribution ultimately control whether pumices sink or float and thus whether pumice deposits are preserved or not on the seafloor.

Variety of the drift pumice clasts from the 2021 Fukutoku‐Oka‐no‐Ba eruption, Japan

“…However, the common occurrence of the black pumice coexisting with the gray type with clear boundary strongly indicates that the black pumice played certain roles in the explosive eruption of FOB in 2021. Mitchell et al (2021) suggested that the pumice clasts form a floating raft and those that suddenly sink to the seafloor have distinct micro‐textures (i.e., the floating pumice has a higher vesicle number density and lower pore space connectivity). This could bias the pumice clasts that were sampled.…”

Coastal ecological impacts from pumice rafts

“…3 ). Pumice has a large surface area with many vesicles 40 – 42 . When pumice stones are impacted onto solid surfaces such as by wave action, they may break apart, thus enlarging the surface area and leading to a dramatic increase in available bacterial habitat; pumice may continue to serve as an ecosystem mediator for a long period of time.…”

Petrogenesis of Havre Volcano in the Kermadec Arc: 2012 Eruption of a Chemically Homogeneous Rhyolite

“…Embedded rock fragments are virtually unique to the raft banded pumice (see also Mitchell et al, 2021). Readily observable angular rock fragments are lapilli-sized (4-10 mm), but some are <2 mm in size (Figure 2).…”

“…An effusive exit velocity of approximately 14 ms −1 was estimated to produce the pumice raft based on magma ascent models inputting seafloor pumice properties (Manga et al, 2018). However, more recent work has shown that when only utilizing seafloor pumice properties, eruption rates of the associated pumice rafts could be considerably underestimated (Knafelc et al, 2020;Mitchell et al, 2021;Knafelc et al, 2022). Most recently, the Havre 2012 eruption model has been expanded to include a shortlived, violent explosive eruption, capable of ejecting hot pyroclasts through the 900 m water column into the atmosphere, which then air-cooled to be deposited at the ocean surface, producing the pumice raft (Knafelc et al, 2022).…”

“…The Havre 2012 pumice raft contains an apparent diversity of material based on the color and texture of pumice clasts (Knafelc et al, 2020(Knafelc et al, , 2022Mitchell et al, 2021). As first reported by Knafelc et al (2020), five different pumice types have been identified in deposits from the pumice raft found across the SW Pacific (see Supplementary figure S1 in Knafelc et al, 2020) and can initially be distinguished on color and texture, ordered here in relative abundance: 1) white, 2) banded white-grey, 3) pink, 4) grey, and 5) brown.…”