Abrasion platforms off basalt sea cliff, Keya-no-Ohto, Fukuoka Prefecture, Japan

Abstract

A multibeam bathymetric survey and SCUBA diving observations were conducted off a basalt sea cliff with vertical columnar joints at Keya, Fukuoka, Japan. Two platforms were observed at depths of 12 m and 17 m. The –17 m platform is considered to be a relict landform of the abrasion platform that formed during the last interglacial period, owing to the gradual subduction trend of the study area. Based on the width of the present platform and the duration of the erosion, the average retreat rate of the basalt sea cliff was estimated to be 1.25–1.3 cm/year.