Submarine volcanic activities around Miyakejima volcano, Izu-Bonin Arc based on bathymetric mapping and high-resolution acoustic reflection profiles

The Izu-Bonin Arc volcanic front is formed from a number of volcanic islands. In order to clarify the geological history of volcanic islands, mode of volcanic activity and magma plumbing system, it is very important to understand entire volcanic activity in both subaerial and submarine parts of the volcano.

We focused on Miyakejima volcano of the Izu-Bonin Arc, located about 180 km south of Tokyo, Japan. This volcano is extremely active, and repeats eruptions in A.D. 1983 and A.D. 2000 in recent years. Consequently, in Miyakejima, many surveys on land areas have been carried out actively, and not only summit crater but also the existence of many satellite cones is clarified. The volcanic products ejected from volcano reach sea area around the island, however, the survey of the submarine part of the volcano has hardly been conducted. In order to clarify the continuity of volcanic system from land to submarine area, we conducted high resolution acoustic reflection survey and bathymetric mapping survey.

In the reflection profiles obtained in this survey, it is possible to clearly understand the distribution and shape of the surface sedimentary layer in the northeastern area of the Miyakejima volcano. At the mounds of sea bottom, the surface sedimentary layer indicates lens-like shape. This lens-like layer is recognized as transparence layer with remarkable reflections on the bottom surface. The submarine topography and distribution of the lens-like layer suggest that this layer was formed by the eruptive products such as scoria and spatter from the submarine volcanos. The remarkable reflections on the bottom become uncontinuous in the thick part of the lens-like layer, and it is presumed that this uncontinuous are the volcanic vents or dikes. The submarine cones off northeastern coast of island observed by us appears to be continuation from several chains of cones aligned in the northeast- southwest direction on the land. In addition, in the northern coast of the Miyakejima, lava flow erupted in 1874 reached submarine slope, and formed submarine lava delta. Coarse sediments are distributed on and around the lava flow, and prominent ripples of up to 7-8 m high were observed. In the future, we plan to collect samples in submarine areas and estimate the volcanic activity of whole of Miyakejima including submarine volcanos.