Bayonnaise Rocks represent part of the barely exposed rim of a largely submarine 8-9 km wide caldera. Formation of the caldera was followed by construction of a large (2.6 cu km) lava dome and/or lava flow complex on the caldera floor, originally located at a depth of 1000-1100 m. Most historical eruptions, recorded since the late-19th century, have occurred from Myojin-sho, a large post-caldera lava dome constructed on the NE rim of the caldera. Deposits from submarine pyroclastic flows associated with growth of the dacitic lava dome mantle the conical dome and extend into the NE part of the caldera and down its outer slopes. An explosive submarine eruption from Myojin-sho in 1952 destroyed a Japanese research vessel, killing all 31 on board. Submarine eruptions have also been observed from other points on the caldera rim and outside of the caldera.