South Hachijyo Volcano -Initial Stage of Submarine Saldera Sctivity-

South Hachijyo Volcano (SHV), a part of Northern Izu-Bonin Arc, is located between Hachijyojima Is. and Aogashima Is. SHV is composed of four blocks (somma) surrounding a bathymetric low. Each block is flat-topped and has a scarp with a gentle slope. The bathymetric low, encircled by the -400m contour line, extends vertically 160m to the top of the SHV. At the bottom of the bathymetric low lies a small depressional structure (caldera floor), which is approximately 2 km in diameter. At the middle of this structure lies a N-S trending central cone that is 1.3*0.9km in diameter and 100m high. We collected many dredge samples of pumice, basaltic fragments, and Q-diorite. Based on observations from a submersible, the sediments on the caldera floor consist of sorted pumice, unsorted pumice and coarse sand. Huge pumice blocks were observed near the slope of the central cone. Angular M-type pumice block and rhyolitic breccias were observed on the top of the central cone. Several rhyolitic volcanic cones (knolls) were observed around the central cone in the caldera floor. Therefore these rhyolitic fragments are associated with dome activities. The result of total geomagnetic analysis indicates that non-magnetic layer (about 600-900m thick) exists at the Hachijyo insular shelf (HIS). Non-magnetic layer in SHV covers magnetic basement, and the south part of it is shallower than the north part. Single-channel seismic records indicate that well-stratified layers (with a total thickness of 700m) exist at the HIS. These layers are corresponds to non-magnetic layer, and the thickness of these layers (interpreted as volcanic products from Hachijyojima Is.) decreases from Hachijyojima toward SHV. Therefore, the production of volcanic material from SHV has been small. Some crypt dome structures are observed under each block (somma). The seismic profiles show knolls atop the stratified somma and the caldera floor. From the geological and geophysical data, we interpret the following regarding the evolution of the SHV. Despite the fact that this volcanic structure is young, we observe morphology that suggests the existence of a caldera that is characterized by gentle slopes that are not well stratified. We interpret the relief between the caldera floor and the somma tops as having been caused by crypt dome intrusion that elevates the somma, rather than by depression of caldera floor after huge eruption. This interpretation is supported by the lack of voluminous volcanic product around the somma. In conclusion, we consider SHV to be in an initial stage of submarine caldera volcanism that is characterized by dike (knoll) complex activity without large volcanic eruptions.