"Poseidic" explosive eruptions at Loihi Seamount, Hawaii

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 approximately 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 approximately 40% vesicularity through almost perfectly closed-system volatile exsolution from approximately 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.