Strombolian-style volcanic activity has persisted for six years at the NW Rota-1 submarine volcano in the southern Mariana Arc, allowing direct observation and sampling of gas-rich fluids produced by actively degassing lavas, and permitting study of the magma-hydrothermal transition zone. Fluids sampled centimeters above erupting lava and percolating through volcaniclastic sediments around an active vent have dissolved sulfite >100 mmol/kg, total dissolved sulfide <30 μmol/kg, pH as low as 1.05, and dissolved Al and Fe >1 mmol/kg. If NW Rota is representative of submarine arc eruptions, then volcanic vent fluids from seawater-lava interaction on submarine arcs have a significant impact on the global hydrothermal flux of sulfur and Al to the oceans, but a minimal impact on Mg removal. Gas ratios (SO2, CO2, H2, and He) are variable on small spatial and temporal scales, indicative of solubility fractionation and gas scrubbing. Elemental sulfur (Se) is abundant in solid and molten form, produced primarily by disproportionation of magmatic SO2 injected into seawater. Se accumulates within the porous rock surrounding the lava conduit connecting the magma source to the seafloor. Accumulated Se can be heated, melted, and pushed upward by rising magma to produce molten Se flows and lavas saturated with Se. Molten Se near the top of the lava conduit may be ejected up into the water column by escaping gases or boiling water. This mechanism of Se accumulation and refluxing may underlie the relatively widespread occurrence of Se deposits of many sizes found on submarine arc volcanoes.