HR: 17:30h
AN: V24B-07    [Abstracts]
TI: Plume and Pyroclast Dynamics Observed During a Submarine Explosive Eruption at NW Rota-1, Mariana arc
AU: * Deardorff, N
EM: ndeardor@uoregon.edu
AF: University of Oregon, 1272 University of Oregon, Eugene, OR 97403, United States
AU: Cashman, K V
EM: cashman@uoregon.edu
AF: University of Oregon, 1272 University of Oregon, Eugene, OR 97403, United States
AU: Chadwick, W W
EM: William.W.Chadwick@noaa.gov
AF: CIMRS, Oregon State University, 2115 SE OSU Drive, Newport, OR 97365, United States
AU: Embley, R W
EM: robert.w.embley@noaa.gov
AF: NOAA, Pacific Marine Environmental Laboratory, Newport, OR 97365, United States
AB: Strombolian submarine eruptions at 550-560 m water depth were observed in April, 2006 at NW Rota-1 volcano, Mariana arc. During six dives with the Jason II remotely operated vehicle observations made at close range documented a diverse and increasingly energetic range of activity. The initial dives observed lava extrusion followed by small, explosive bursts. Activity steadily increased to produce gas thrust jets, discrete thermals and eventually a sustained plume. Eruption video allowed analysis of submarine plume dynamics and depositional characteristics. Sustained plumes were white, billowy and coherent, measuring ~0.5-0.75m wide at their base and quickly spreading to >2m in diameter within ~2-3m above vent due to rapid seawater entrainment. Sustained, coherent plumes were observed rising >20-30m above the seafloor; the top of the plume was observed at ~490m b.s.l giving a total plume height of ~60-70m above the active vent. The initial ascent (<3-4 m) of plumes generated from explosive bursts was analyzed for ejection velocities (<4m/s), clast settling velocities (~0.38-0.72m/s), and changes in plume height and width. Gas thrust jets were determined to transition from momentum-driven plume rise to buoyancy-driven plumes, both visually and using rise velocities, at ~ 0.5-1 m above the vent. These data contrast with the dynamics of plumes generated in subaerial Strombolian eruptions, which maintain momentum-driven rise to ~ 100 meters (Patrick, 2007) above the vent, and illustrate the strong dampening effect of the overlying seawater. Ash and lapilli were observed falling out of the plume at heights >3-4m after being transported by the convecting plume and are assumed to have wider range of travel, vertically and laterally, and deposition. Most bomb-sized ejecta were carried vertically with the plume for 1-3m before falling out around the vent, indicating that the dense (~1700-2350 kg/m3) clasts were transported primarily within the momentum-driven part of the plume. These bomb-sized ejecta were deposited within ~1-2m from the vent with numerous clasts falling back into the vent. The average maximum bomb size increased over time from <13cm blocks during early phases of the dive sequence to ~30-70cm during the later, most energetic eruptions. The positive correlation of bomb size with mass eruption rate is opposite to that seen for highly explosive (plinian) eruptions and suggests that mass eruption rate at NW Rota-1 is determined primarily by gas flux (that is, the ability of the streaming gas phase to transport pyroclasts).
DE: 3070 Submarine landslides
DE: 8404 Volcanoclastic deposits
DE: 8419 Volcano monitoring (7280)
DE: 8427 Subaqueous volcanism
DE: 8428 Explosive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting