The majority of Earth's volcanic eruptions occur beneath the sea, but few direct observations and samples limit our understanding of these unseen events. Subaerial eruptions lend some insights, but direct extrapolation from subaerial to deep-sea is precluded by the great differences in pressure, thermal conditions, density, rheology, and the interplay among them. Here we present laboratory fragmentation experiments that mimic deep-sea explosive eruptions and compare our laboratory observations with those from the kilometre-deep submarine eruption of Havre volcano, Kermadec arc, New Zealand in 2012. We find that the Havre eruption involved explosive fragmentation of magma by a pressure-insensitive interaction between cool water and

Meter-scale vesicular blocks, termed "giant pumice," are characteristic primary products of many subaqueous silicic eruptions. The size of giant pumices allows us to describe meter-scale variations in textures and geochemistry with implications for shearing processes, ascent dynamics, and thermal histories within submarine conduits prior to eruption. The submarine eruption of Havre volcano, Kermadec Arc, in 2012, produced at least 0.1 km 3 of rhyolitic giant pumice from a single 900-m-deep vent, with blocks up to 10 m in size transported to at least 6 km from source. We sampled and analyzed 29 giant pumices from the 2012 Havre eruption. Geochemical analyses of whole rock and matrix glass show no evidence for geochemical heterogeneities in parental magma; any textural variations can be attributed to crystallization of phenocrysts and microlites, and degassing. Extensive growth of microlites occurred near conduit walls where magma was then mingled with ascending microlite-poor, low viscosity rhyolite. Meter-to micron-scale textural analyses of giant pumices identify diversity throughout an individual block and between the exteriors of individual blocks. We identify evidence for post-disruption vesicle growth during pumice ascent in the water column above the submarine vent. A 2D cumulative strain model with a flared, shallow conduit may explain observed vesicularity contrasts (elongate tube vesicles vs spherical vesicles). Low vesicle number densities in these pumices from this high-intensity silicic eruption demonstrate the effect of hydrostatic pressure above a deep submarine vent in suppressing rapid late-stage bubble nucleation and inhibiting explosive fragmentation in the shallow conduit.

Field studies of tephra-fall deposits traditionally use the density of juvenile pyroclasts to determine vesicularity of the host magma at the point of fragmentation. A range of pyroclast sizes between 16 and 32 mm has commonly been chosen for this purpose. Larger pyroclasts outside this range may undergo postfragmentation vesiculation due to slow cooling of the interior of the clasts, while smaller pyroclasts may be too small to represent accurately the distribution of the largest vesicles. The assumption of this method, of course, is that the 16–32 mm size range is representative of the fragmented magma. We explore, in detail, variations in density over a size range of 4–128 mm from Unit 2 pyroclasts of the 1.8 ka Taupo eruption and make inferences about the roles of postfragmentation vesiculation and secondary breakage of pyroclasts. We find (1) there is a clear threshold for onset of postfragmentation vesiculation at >32 mm, and (2) there are broken small pieces of the largest pyroclasts in the sample that artificially skew the density distribution for smaller size fractions. We constrain uncertainty associated with vesicularity measurements and offer best-practice recommendations in the hope of improving consistency of field sampling and laboratory processing of pyroclast populations for vesicularity studies.

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