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

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

Pumice rafts are floating mobile accumulations of low-density pumice clasts generated by silicic volcanic eruptions. Pumice in rafts can drift for years, become waterlogged and sink, or become stranded on shorelines. Here we show that the pumice raft formed by the impressive, deep submarine eruption of the Havre caldera volcano (Southwest Pacific) in July 2012 can be mapped by satellite imagery augmented by sailing crew observations. Far from coastal interference, the eruption produced a single >400 km2 raft in 1 day, thus initiating a gigantic, high-precision, natural experiment relevant to both modern and prehistoric oceanic surface dispersal dynamics. Observed raft dispersal can be accurately reproduced by simulating drift and dispersal patterns using currents from an eddy-resolving ocean model hindcast. For future eruptions that produce potentially hazardous pumice rafts, our technique allows real-time forecasts of dispersal routes, in addition to inference of ash/pumice deposit distribution in the deep ocean.

IODP Expedition 340 successfully drilled a series of sites offshore Montserrat, Martinique and Dominica in the Lesser Antilles from March to April 2012. These are among the few drill sites gathered around volcanic islands, and the first scientific drilling of large and likely tsunamigenic volcanic island-arc landslide deposits. These cores provide evidence and tests of previous hypotheses for the composition and origin of those deposits. Sites U1394, U1399, and U1400 that penetrated landslide deposits recovered exclusively seafloor sediment, comprising mainly turbidites and hemipelagic deposits, and lacked debris avalanche deposits. This supports the concepts that i/ volcanic debris avalanches tend to stop at the slope break, and ii/ widespread and voluminous failures of preexisting low-gradient seafloor sediment can be triggered by initial emplacement of material from the volcano. Offshore Martinique (U1399 and 1400), the landslide deposits comprised blocks of parallel strata that were tilted or microfaulted, sometimes separated by intervals of homogenized sediment (intense shearing), while Site U1394 offshore Montserrat penetrated a flat-lying block of intact strata. The most likely mechanism for generating these large-scale seafloor sediment failures appears to be propagation of a decollement from proximal areas loaded and incised by a volcanic debris avalanche. These results have implications for the magnitude of tsunami generation. Under some conditions, volcanic island landslide deposits composed of mainly seafloor sediment will tend to form 420Geochemistry, Geophysics, Geosystems PUBLICATIONS smaller magnitude tsunamis than equivalent volumes of subaerial block-rich mass flows rapidly entering water. Expedition 340 also successfully drilled sites to access the undisturbed record of eruption fallout layers intercalated with marine sediment which provide an outstanding high-resolution data set to analyze eruption and landslides cycles, improve understanding of magmatic evolution as well as offshore sedimentation processes.