Degassing, porosity and hydration age characteristics of a giant submarine lava dome: Implications for post-caldera volcanism of the Kikai caldera, Japan

The mostly submarine Kikai caldera in SW Japan underwent a caldera-forming eruption at 7.3 ka. Its subsequent post-caldera volcanic activity has included subaerial eruptions whose onland deposits have been well studied, and a historical island-forming eruption from a submarine vent. Marine surveys have also documented a giant, 32 km³ submarine dome that has been emplaced on the caldera floor during the post-caldera period, but the timing and style of its eruption is not known. Here we investigate this giant submarine dome using seafloor observations and rock samples collected during dredging surveys. The dome is covered by large blocks; based on porosity, matrix glass volatile contents and thermal demagnetisation data we infer that these are giant pumice that floated hot at the sea surface prior to sinking. Without suitable radiocarbon material we instead use the extent of secondary hydration of giant pumice matrix glasses to estimate eruption age, yielding an age of 13th Century CE or younger. We infer that the dome grew both endogenously under a thin sediment covering and also via exogenous seafloor lava flow, and that the giant pumice formed from a pumiceous carapace on this lava. We also find preliminary textural evidence that this shallow submarine dome was formed by cryptic fragmentation and pyroclast sintering as is proposed for subaerial silicic lavas. Together our findings suggest that the Kikai caldera has a high average eruption rate in its current post-caldera volcanic stage, and that eruptions, potentially explosive, from submarine vents comprise a significant portion of this activity.