TEPHRA DISPERSAL FROM MYOJINSHO, JAPAN, DURING ITS SHALLOW SUBMARINE ERUPTION OF 1952-1953

A new and derailed bathymetric map of the Myojinsho shallow submarine volcano provides a framework to interpret the physical volcanology of its 1952-1953 eruption, especially how the silicic pyroclasts, both pr imary and reworked, enlarged the volcano and were dispersed into the s urrounding marine environment. Myojinsho, 420 km south of Tokyo along the Izu-Ogasawara are, was the site of approximately 1000 phreatomagma tic explosions during the 12.5-month eruption. These explosions shatte red growing dacite domes, producing dense clasts that immediately sank into the sea: minor amounts of pumice floated on the sea surface afte r some of these events. The Myojinsho cone has slopes of almost precis ely 21 degrees in the depth range 300-700 m. We interpret this to be t he result of angle-of-repose deposition of submarine pyroclastic gravi ty flows that traveled downslope in all directions. Many of these grav ity flows resulted from explosions and associated dome collapse, but o thers were likely triggered by the remobilization of debris temporaril y deposited on the summit and steep upper slopes of the cone, Tephra w as repeatedly carried into air in subaerial eruption columns and fell into the sea within 1-2 km of the volcano's summit, entering water as deep as 400 m. Because the fall velocity of single particles decreased by a factor of similar to 30 in passing from air into the sea, we exp ect that the upper part of the water column was repeatedly choked with hyperconcentrations of fallout tephra. Gravitational instabilities wi thin these tephra-choked regions could have formed vertical density cu rrents that descended at velocities greater than those of the individu al particles they contained. Upon reaching the sea floor. many of thes e currents probably continued to move downslope along Myojinsho's subm arine slopes. Fine tephra was elutriated from the rubbly summit of the volcano by upwelling plumes of heated seawater that persisted for the entire duration of the eruption. Ocean currents carried this tephra t o distal areas, where it presumably forms a pyroclastic component of d eep-sea sediment.