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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 km3 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.

Dating young lava flows is essential for understanding volcano's eruption frequency, yet challenging due to methodological limitations of commonly used dating techniques. Ruapehu (Aotearoa New Zealand) produced many lava flows during the Holocene, but constraints on the timing of these eruptions are scarce. Here, we use paleomagnetic dating to deliver new eruption ages of 18 lava flows with uncertainties ranging between 500 and 2,700 years (at the 95% confidence level). Comparison between lava flows' paleomagnetic directions and a local paleosecular variation record indicates that the large lava flow field located on the Whakapapa area was emplaced during at least three distinct eruptive episodes between 10600 and 7400 BP. Two of these episodes closely followed a large collapse event that affected Ruapehu's northern area and generated large volumes of lava between 10600 and 8800 BP, with the third episode producing less voluminous lava flows between 8100 and 7400 BP. Following a smaller collapse of the southeastern sector of the edifice at ca. 5300 BP, several low‐volume lava flows were emplaced during at least two distinct eruptive episodes prior to ca. 1000 BP, which supplied the Whangaehu valley with lava. The youngest age inferred from our data represents the youngest eruption age provided for a lava flow outside Ruapehu's summit region. This research provides greater detail to the Holocene effusive chronology at Ruapehu, shedding light on partial cone reconstructions after edifice collapses during the Holocene, and the time relationships between trends observed in its effusive and explosive activity. Plain Language Summary: Knowing when volcanoes have erupted is essential for hazard assessments, but the precise timing of geologically young lava flow eruptions is hard to determine with traditional methods. Earth's magnetic field is constantly changing in both direction and strength. Magnetic minerals contained in the lava flows record the direction of the field as the lava cools after eruption. Hence, solidified lavas can be dated by matching their magnetization directions to a reference record. In this study, we analyze the magnetization directions recorded in prehistoric lavas at Mt. Ruapehu (Aotearoa New Zealand) and use them to determine when they erupted. We show that a large flow field in the Whakapapa area (northwest Ruapehu) was formed during at least three distinct eruptive episodes between 10,600 and 7,350 years ago, following a catastrophic volcanic landslide at around 10,600 years ago. We also determine that the lavas present in the Whangaehu valley (east Ruapehu) erupted during at least two episodes between 4,600 and 1,000 years ago, the latter representing the youngest lavas known outside the summit area of Ruapehu. This research helps understand how Ruapehu was rebuilt after slope collapses, and the frequency of lava‐forming eruptions. Key Points: Paleomagnetism was used to date eruptive episodes in lava flow sequences at RuapehuPaleomagnetic dating yields 1300–1050 BP as the youngest eruption age known for a lava flow at the volcanoOur data provide age constraints on the timescales of vent migration and partial edifice rebuilding after collapse events [ABSTRACT FROM AUTHOR]

A long-standing conceptual model for deep submarine eruptions is that high hydrostatic pressure hinders degassing and acceleration, and suppresses magma fragmentation. The 2012 submarine rhyolite eruption of Havre volcano in the Kermadec arc provided constraints on critical parameters to quantitatively test these concepts. This eruption produced a >1 km3 raft of floating pumice and a 0.1 km3 field of giant (>1 m) pumice clasts distributed down-current from the vent. We address the mechanism of creating these clasts using a model for magma ascent in a conduit. We use water ingestion experiments to address why some clasts float and others sink. We show that at the eruption depth of 900 m, the melt retained enough dissolved water, and hence had a low enough viscosity, that strain-rates were too low to cause brittle fragmentation in the conduit, despite mass discharge rates similar to Plinian eruptions on land. There was still, however, enough exsolved vapor at the vent depth to make the magma buoyant relative to seawater. Buoyant magma was thus extruded into the ocean where it rose, quenched, and fragmented to produce clasts up to several meters in diameter. We show that these large clasts would have floated to the sea surface within minutes, where air could enter pore space, and the fate of clasts is then controlled by the ability to trap gas within their pore space. We show that clasts from the raft retain enough gas to remain afloat whereas fragments from giant pumice collected from the seafloor ingest more water and sink. The pumice raft and the giant pumice seafloor deposit were thus produced during a clast-generating effusive submarine eruption, where fragmentation occurred above the vent, and the subsequent fate of clasts was controlled by their ability to ingest water.

A Robust tectonic reconstruction of the Philippine Sea Plate around ∼52 Ma is a prerequisite in understanding the process of subduction initiation and establishment of the Izu‐Bonin‐Mariana arc. This study investigates origins and timing of basin formation in the still poorly understood oldest part of the northern Philippine Sea plate. We have established that andesitic magmatism in the form of the Northern Philippine Sea volcanics is widely distributed across this area. It is founded on both a Mesozoic arc terrane (Daito Ridge Group) and an intervening basin (Kita‐Daito Basin). Their narrow Eocene age range (45‐41 Ma) and lack of systematic spatial variation in geochemistry implies that this magmatism was not associated with on‐going subduction, but related to the rifting/spreading event forming the Kita‐Daito Basin. The arc‐like geochemistry of the volcanics seems to indicate melting of lithospheric mantle which had been previously metasomatized by Mesozoic subduction of a plate with Pacific‐MORB isotopic characteristics. Late Eocene basaltic magmatism also found in the Kita‐Daito Basin does not have arc‐like characteristics, and could have formed from low‐degree melts of asthenospheric mantle associated with the final stage of Kita‐Daito Basin spreading. As onset of activity of the Northern Philippine Sea volcanics is essentially synchronous with the magmatism associated with the Oki‐Daito mantle plume, it is possible that both magmatism and rifting of the Kita‐Daito Basin were triggered by the arrival of the Oki‐Daito mantle plume in this region. These results demonstrate that the Kita‐Daito Basin postdates subduction initiation of the Pacific Plate along the Izu‐Bonin‐Mariana arc. Plain Language Summary: This study investigates origin and age of ocean basins in the still poorly understood northern Philippine Sea plate. This area will help us appreciate what happens during the initiation of a new destructive plate margin. We recovered rock samples from the Northern Philippine Sea Seamounts and Kita‐Daito Basin, and determined their age and composition. These volcanoes were active between 45 and 41 Ma and are found widely distributed across the older crust of the northernmost Philippine Sea Plate. Despite the geographical spread of these volcanoes, they have a similar composition across the region. This indicates they were produced during stretching of the older crust, which reactivated the older underlying upper mantle. A possible reason for this stretching may have been the impact of a mantle plume beneath this region. Key Points: Widely spread Eocene andesitic magmatism in northern Philippine Sea marks the onset of rifting in the Mesozoic arc terraneLate Eocene basalts in the Kita‐Daito Basin could represent the last stage of spreading of the Kita‐Daito BasinRifting and spreading of the Mesozoic arc terrane in northern Philippine Sea postdate subduction initiation along the Izu‐Bonin arc [ABSTRACT FROM AUTHOR]