Grain-size and Componentry Analysis of Multiple Vents in 2010 from Submarine Volcano NW Rota-1, Mariana Arc

In 2010 a research cruise aboard the R/V Kilo Moana visited the submarine volcano NW Rota-1, Mariana arc. This visit occurred shortly after a large landslide that removed a large cone that was observed the previous year. The collapse of the south face of the volcano disturbed the upper conduit system and subsequent eruptive activity was scattered among five active vents, each displaying intermittent activity. The observed activity ranged from effusive with degassing events, to submarine strombolian explosive eruptions. This range in activity produced tephra ranging from coarse ash to lapilli. Tephra morphologies range from fluidal and angular, providing evidence of magmatic fragmentation, to blocky morphologies suggesting quench granulation due to rapid cooling.

In this study we describe the morphology, grain size, and componentry variabilities between the three most active vents (Brimstone, Phantom and Styx) at NW Rota-1 in 2010. Grain sizes for all three vents have a normal distribution with a mode at 1-2mm. Brimstone vent exhibited primarily effusive activity, with periods of mild degassing which remobilized tephra with primarily blocky, equant morphologies, suggesting quench granulation of the extruded lava. Phantom vent sporadically produced more vigorous degassing events and produced dominantly blocky, lithic tephra, with a minor component of juvenile tephra with more fluidal and glassy textures. Activity at Styx vent varied from mild degassing to moderate strombolian-style bursts and produced tephra dominated by juvenile and glassy textures, with 30% lithic, blocky tephra.

Componentry of tephra collected from the three primary vents indicates pyroclast morphologies clearly represent the range of eruptive styles observed. Blocky, equant clasts were more prevalent at vents exhibiting effusive behavior and more fluidal and glassy clasts were more commonly associated with submarine strombolian-style burst. Interestingly, the grain size distribution is remarkably similar for each vent, even though the mechanisms (quench granulation vs magmatic fragmentation) differ.