COASTAL EROSION OF NEW VOLCANIC ISLAND NISHINOSHIMA

Abstract

A new volcanic island named Nishinoshima Shinto was formed soon after the submarine volcanic eruption occurred about 800km south of Honshu in April, 1973. The Hydrographic Department of Maritime Safety Agency has continuously observed from aircraft the phenomena of the submarine volcanic eruption as well as its growth into a new volcanic island and the process of subsequent topographical changes. The result showed that large-scale coastal erosion occurred along the south coast of the island. The coastal process of the new volcanic island is very much interesting and important for geomorphological study. The Hydrographic Department carried out aerial photogrammetric surveys of the island ten times during the period from December, 1973 to January, 1978. These surveys revealed that the new volcanic island continued to grow according to active volcanic eruptions till August, 1974. The tendency thereafter was towards the reduction in size of the island due to the tranquillization of volcanic activities and the effect of active coastal erosion. Comparing the coastline of the maximum growth in August, 1974 with the latest coastline in May, 1977, it was found that the coastline retreated maximum 170 metres along the southern coast for a distance of 900 metres. To the contrary, the coastline along the northern bay formed by the old and the new islands advanced maximum 80 metres. In the process of the retreat of the coastline, there were two remarkable erosional stages with a calm stage in-between. The first erosional stage was from August to October, 1974 when the coastline retreated maximum 120 metres. The second erosional stage was from November, 1975 to August, 1976, and the coastline retreated more than 50 metres at this stage. The coastline in the calm stage was stable and the change was negligible. The process of the coastal erosion was closely related to storms attacking this locality. Namely, five typhoons passed over this area in the first erosional stage and five others in the second stage, while there were no typhoons visited there during the calm stage. In the first erosional stage, it was the main cause of the remarkable coastal erosion that Typhoon No.16 in 1974 passed northward on the west of Nishinoshima Shinto, as it caused strong southerly wind blowing to the south coast of the island. In the second erosional stage, Typhoon No. 20 caused similar coastal erosion in 1975. It was the effect of submarine topography that the eroded area was confined to the southern coast of the new island. Nishinoshima Shinto is a central cone of Nishinoshima submarine volcano. Nishinoshima old island westward and the several reefs northward, located several hundred metres apart from the new island, are parts of a somma of Nishinoshima submarine volcano. On the other hand, there is no somma to the south of the new island, where a flat sea floor deeper than 20 metres is widely extented. Accordingly, the north and west sides of the new island are protected by the comma from wave attacker, while the south side is exposed to wave erosion. The stable coastline in the calm stage was held by the resistance of lava stuffed in craters and those flown from such craters. In the erosional stages, lava stuffed in craters remained from wave erosion and stood as stacks above the sea surface. A new abrasion plafform with a width of more than 100 metres appeared along the south coast after the coastal erosion. The terminal depth of this platform was about 15 metres, which indicated the depth of the wave base of vigorous abrasion.