HOLOCENE CATASTROPHIC COASTAL EVOLUTION RELEVANT TO VOLCANIC ACTIVITIES IN SOUTHERN KYUSHU VOLCANIC FIELDS, SOUTH JAPAN

Volcanic activity in coastal region is one of the factors greatly responsible for catastrophic coastal change. Southern Kyushu coastal area presented here is most suitable for constructing a model for catastrophic change in coastal landforms, because some Quaternary gigantic calderas and intra-caldera volcanoes are characteristically developed in the southern Kyushu coastal area. They have resulted in various modes and magnitudes of eruptions in the Holocene. The volcanic ejecta and volcano-tectonic activities have catastrophically affected coastal evolution. Here I present a model for mid- and late Holocene catastrophic coastal evolution dominated by two modes of volcanic activities in this region: rapid deposition by volcanic ejecta and geomorphic deformation by volcano-tectonic activity. Tephrochronological and coastal studies show that four eruptions 6,400 - 8,100 cal BP caused most distinct coastal progradation in the Holocene, because contemporaneous sea level attained the culmination, which formed intricate coasts with shallow sea. Various modes and magnitudes of eruptions occurred in this period influenced different modes of coastal evolutions : base surge from Yonemaru maar (8,100 cal BP ,VEI 3), tephra fall from Sakurajima volcano (8,000 cal BP, VEI 4 ), gigantic pyroclastic flow and tephra fall from Kikai caldera (7,300 cal BP, VEI 7), and pyroclastic flow and tephra fall from Ikeda caldera (6,400 cal BP, VEI 5 ). In addition to these volcanic products, volcano-tectonic activity has characteristically caused coastal deformation in the Holocene as evidenced by distinctly raised mid- to late Holocene coastal remnants with a maximum elevation of c. 15 m a.s.l. in the Aira caldera volcanic field. Their occurrences indicate Holocene upwarping of Aira caldera. Its comparison with recent crustal movements of Aira caldera associated with recent volcanic activities of intra-caldera active volcano suggests that magma inflation of Aira caldera has been affected the Holocene upwarping. Coastal evolution studies in Quaternary volcanic fields provide fundamental data on assess future volcanic eruptions in this region.