Submarine Silicic Calderas on the Northern Shichito‐Iwojima Ridge, Izu‐Ogasawara (Bonin) Arc, Western Pacific

Eight submarine silicic calderas have been discovered on the northern Shichito-Iwojima Ridge, Izu-Ogasawara Arc. They have typical caldera structures with a circular to elliptical rim 2-10 km across with a flat or slightly concave floor 1-7 km across and a wall rising 0.1-1.1 km above the floor and gently inclining, mainly at an angle of 14-30°. They are exclusively developed on submarine stratovolcanoes with aspect ratios 1:20 and comparable in size to either Nigorikawa or Crater-Lake calderas. Gravity anomalies of the submarine calderas are not low, due to central lava domes or major mass of the stratovolcanoes underlying the calderas. Magnetic anomalies are low with narrow wavelength, presumably reflecting complex growth of stratovolcanoes. Acoustic reflection profiles show stratified layers on the flanks and within these calderas, and recent submersible surveys have disclosed a thick pumiceous sequence on the wall of Myojin Knoll caldera. Pumice deposits recovered from ODP sites 788-791 close to Minami-Sumisu and Sumisu calderas commonly contain not only pumice lapilli and blocks but also blocky or platy, variably vesicular glass shards, suggesting a submarine eruption by magma vesiculation and water-ingestion into the eruption plume. Catastrophic submarine silicic eruption and massive collapse of the eruption column by ingestion of the ambient water are likely to have resulted in thick accumulation of pumice deposits. Stratification of the deposits might represent recurrent eruption or recurrent emplacement from a suspension mass unsteadily loaded by a sustained eruption.