Norie Fujibayashi

Vesiculation and Eruption Processes of Submarine Effusive and Explosive Rocks from the Middle Miocene Ogi Basalt, Sado Island, Japan

The Middle Miocene Ogi Basalt, formed in a back-arc basin, contains products of a variety of eruption processes, from extrusion of pillow lava (Sawasaki pillow lava) associated with formation of hyaloclastite to explosiveformation of fluidal spatter and scoriaceous pyroclasts (Sawasaki pyroclastic rocks). The vesiculation textures of juvenile samples were examined to determine eruption style. The Sawasaki pillow lava, hyaloclastite, and pyroclastic rocks are characterized by their high vesicularities: 22-44% for the pillow lava and hyaloclastite, and 19-64% for the pyroclastic rocks. The Sawasaki pyroclastic rocks are characterized by their high contents of small vesicles (<0.2mm across) that show an abrupt increase in vesicle size-frequency patterns. Large vesicles, up to 1 cm in diameter, are minor in the spatter clasts. Population densities calculated for the small vesicles are similar to those of Kilauea lava fountain products. Vesiculated magma that rose slowly erupted effusively to make pillow lava and then hyaloclastite. Subsequently, accelerated magma, rich in small bubbles, must have erupted in lava fountain style through the water column and produced spatter clasts. Those were hydroclastically disintegrated into scoria lapillis, and formed thick scoriaceous pyroclastic rocks, instead of a formation of clastogenic lava on land.