Much remains unknown about submarine explosive eruptions. Their deposits are found to great depths in all the world's oceans, but eruptions are typically described by analogy to a subaerial nomenclature that ignores the substantial and inevitable influences of hydrostatic pressure and magma-water interaction at submerged edifices. Here we explore magmatic volatile exsolution and magma-water interaction for a pyroclastic cone-forming eruption at ∼1 km depth on Loihi Seamount, Hawaii. We examine vesicle textures in lapilli—the physical manifestation of degassing; dissolved volatiles in matrix glasses and olivine-hosted glass inclusions—the geochemical record of ascent and volatile exsolution; and fine ash morphology—the evidence for if and how external water assisted in fragmentation. This approach allows a submarine explosive eruption style to be defined: the magma achieved ∼40% vesicularity through almost perfectly closed-system volatile exsolution from ∼3 km below the vent, which accelerated and weakened the melt, allowing it to be fragmented by explosive magma-water interaction. We introduce the name “Poseidic” for this end-member style of submarine basalt explosivity. Poseidic eruptions are identifiable from measurable features in pyroclasts, and are possible at all subaqueous basaltic volcanoes.