桜島火山,安永噴火(1779-1782年)で生じた新島(安永諸島)の成因

抄録

I reconstructed the eruptive sequence of the An-ei eruption (1779-1982) of Sakurajima volcano based on documentary records. The important results are as follows; first, submarine eruption started earlier than the entrance of terrestrial lava flow into the sea; second, a larger volume of magma intruded into the sediments at the bottom of the sea, and generated a large submarine cryptodome. When magma was ejected, floated islets consisting of giant pumice were formed for a while, which were subsequently followed by violent phreatomagmatic eruptions, and finally massive lava appeared on the sea. However, when the sandy islets were uplifted, they were not accompanied by any eruptions. All the islets were formed nearly within a year, then followed by submarine explosions that generated tsunami. The explosions were produced by the interaction between seawater and the intruded magma of the An-ei cryptodome. Some tsunamis might be triggered by landslides on the slope of the submarine cryptodome. I found similar explosion vent structure at Yuwandake, an early Pleistocene intrusive body into wet sediments, to the northwest of the Aira caldera. Funnel-shaped vent breccia penetrated the central part of the jointed Yuwandake lava, suggesting that the submarine explosion occurred at the final stage of the consolidation of magma. Similar vent structures may exist in the An-ei submarine cryptodome.