A submersible study of the products of a large submarine eruption demonstrates the influence of the ocean on eruption dynamics.

collapse events; and (6) estimating the long-term eruption rates of composite volcanoes. 2. Geologic setting 2.1. Regional geology Volcanism in central North Island, New Zealand, is associated with westward subduction of oceanic crust of the Pacific Plate at ~45 mm yr-1 beneath the Australian Plate along the Hikurangi Trench system (Fig. 1; Cole and Lewis, 1981; Reyners et al., 2006; DeMets et al., 2010). Most arc-related volcanism is manifested in the Taupo Volcanic Zone (TVZ), a rifting arc active since 2 Ma, which comprises northern and southern segments dominated by andesite-dacite composite cones and a rhyolite-dominated central segment (Wilson et al., 1995). The southern TVZ segment comprises the prominent Tongariro and Ruapehu composite volcanoes as well as several smaller inactive volcanic edifices, which collectively form the Tongariro Volcanic Centre (Fig. 1; Cole, 1978). Rifting and extension in the southern TVZ occurs at a direction of ~115° and a rate of 2.3 ± 1.2 mm yr-1 (Villamor and Berryman, 2006a). This motion is manifested by the 40 km-wide Mount Ruapehu graben, which is bounded by the Rangipo and Raurimu faults to the east and west, respectively, and by the NE-striking Karioi and the ENE-striking Ohakune fault sets to the south (Fig. 1; Villamor and Berryman, 2006b). Ruapehu volcano sits on late Tertiary sediments and Mesozoic basement rocks ('greywacke'). The latter are generally inferred to be part of the Kaweka Terrane, a Jurassic greywacke-argillite sequence of felsic composition that outcrops in the ranges east of the Rangipo Fault (Adams et al., 2009; Lee et al., 2011; Price et al., 2015). An index map of Ruapehu with geographical features referred to throughout the text is provided in Fig. 2. 2.2. Volcanological overview Ruapehu is New Zealand's largest active andesite volcano, with a ~150 km 3 edifice surrounded by a volcaniclastic ring plain of similar volume (Hackett and Houghton, 1989; Gamble et al., 2003). The flanks of the edifice are composed of lava flows and autobreccias

25 26A long-standing conceptual model for deep submarine eruptions is that high hydrostatic pressure 27 hinders degassing and acceleration, and suppresses magma fragmentation. The 2012 submarine 28 rhyolite eruption of Havre volcano in the Kermadec arc provided constraints on critical 29 parameters to quantitatively test these concepts. This eruption produced a > 1 km 3 raft of floating 30 pumice and a 0.1 km 3 field of giant (>1 m) pumice clasts distributed down-current from the vent. 31We address the mechanism of creating these clasts using a model for magma ascent in a conduit. 32We use water ingestion experiments to address why some clasts float and others sink. We show 33 that at the eruption depth of 900 m, the melt retained enough dissolved water, and hence had a 34 low enough viscosity, that strain-rates were too low to cause brittle fragmentation in the conduit, 35 despite mass discharge rates similar to Plinian eruptions on land. There was still, however, 36 enough exsolved vapor at the vent depth to make the magma buoyant relative to seawater. 37Buoyant magma was thus extruded into the ocean where it rose, quenched, and fragmented to 38 produce clasts up to several meters in diameter. We show that these large clasts would have 39 floated to the sea surface within minutes, where air could enter pore space, and the fate of clasts 40 is then controlled by the ability to trap gas within their pore space. We show that clasts from the 41 raft retain enough gas to remain afloat whereas fragments from giant pumice collected from the 42 seafloor ingest more water and sink. The pumice raft and the giant pumice seafloor deposit were 43 thus produced during a clast-generating effusive submarine eruption, where fragmentation 44 occurred above the vent, and the subsequent fate of clasts was controlled by their ability to ingest 45 water. 46 3 47

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Ice exerts a first-order control over the distribution and preservation of eruptive products on glaciated volcanoes. Defining the temporal and spatial distributions of ice-marginal lava flows provides valuable constraints on past glacial extents and is crucial for understanding the eruptive histories of such settings. Ice-marginal lava flows are well displayed on Ruapehu, a glaciated andesite-dacite composite cone in the southern Taupo Volcanic Zone, New Zealand. Flow morphology, fracture characteristics and 40 Ar/ 39 Ar geochronological data indicate that lavas erupted between~51 and 15 ka interacted with large valley glaciers on Ruapehu. Icemarginal lava flows exhibit grossly overthickened margins adjacent to glaciated valleys, are intercalated with glacial deposits, display fine-scale fracture networks indicative of chilling against ice, and are commonly ridge-capping due to their exclusion from valleys by glaciers. New and existing 40 Ar/ 39 Ar eruption ages for ice-marginal lava flows indicate that glaciers descending to 1300 m above sea level were present on Ruapehu between~51-41 and~27-15 ka. Younger lava flows located within valleys are characterised by blocky flow morphologies and fracture networks indicative of only localised and minor interaction with ice and/or snow, mainly in their upper reaches at elevations of~2600-2400 m. An 40 Ar/ 39 Ar eruption age of 9±3 ka (2σ error) determined for a valley-filling flow on the northern flank of Ruapehu indicates that glaciers had retreated to near-historical extents by the time of emplacement for this lava flow. The applicability of 40 Ar/ 39 Ar dating to ice-marginal flows on glaciated andesitedacite composite volcanoes makes this technique an additional proxy for paleoclimate reconstructions.