Quaternary Volcanoes > Active Volcanoes > Hachijojima

4: Submarine volcanic activities around Hachijojima

4.1. Kenkenyama

   The Hachijo Basin to the west of the Hachijojima Volcano is a back-arc rift basin. These basins are confirmed to exist in the south of this area, i.e., to the west of Aogashima, Sumisujima, and Torishima ( Figure 1). These basins align from the north to south and developed as an extensional field owing to the balance of relative movement between the Philippine Sea Plate and the Pacific Plate that subducts underneath.. It is also known that volcanic activities occur along with the formation of these basins (e.g., Taylor, 1992; Ishizuka et al., 2002).

   Kenkenyama located to the west of Kojima is within this Hachijo Basin and is a submarine volcanic edifice with a height of 1100 m relative to the surrounding seafloor. It can be divided into the northern edifice with a domal shape and the southern edifice with a caldera. The northern edifice has a crater with a diameter of 200–400 m on the southwest slope and the summit. On the summit, there are several domes with relative heights of dozens of meters to 100 m, consisting of andesite–dacite lava. On the other hand, the southern edifice has two calderas with diameters of 2 and 3 km and a lava dome consisting of dacite and rhyolitic lava formed inside the caldera. On the caldera wall, deposits consisting of rhyolitic lava and stratified rhyolitic pumice are exposed, indicating that the southern edifice was developed by activity of rhyolitic magma.

   Furthermore, there are lava cones with basal diameters of approximately 1 km aligned roughly in a north-south direction on the slopes of the northern and southern edifices. The samples collected were all basalts, indicating that this volcanic chain formed because of the activity of basaltic magma. Based on this geomorphological relationship, this volcanic chain formed after the formation of the caldera in the southern edifice.

4.2. Kurose-Nishi Hole
   Kurose-Nishi Hole is a submarine caldera volcano with a long diameter of 7 to 8 km. As there are extremely fresh outcrops on the caldera walls, these walls continue to collapse, and the diameter of the caldera must have been expanding since the time of its formation (NT12-19 Natsushima Cruise researcher, 2012). Past sampling and observation of the caldera wall indicate that it was formed owing to multiple explosive eruptions that erupted a large amount of rhyolitic pumice. The seismic reflection profile also confirmed units that are equivalent to multiple deposits, and a basement unit formed prior to the formation of the caldera below these units ( Figure 4b). The thickness of the deposits in the caldera wall was estimated to be approximately 430 m based on this profile. Pumice deposits from the Kurose-Nishi Hole is widely distributed in the Hachijo Basin, and its thickness decreases toward the south away from the hole. Units equivalent to pumice deposits eroded the hemipelagic sediments below; thus, they are supposed to have been pyroclastic flows or turbidites instead of fall deposits. The active period of Kurose-Nishi Hole is unclear. However, as the latest large-scale eruption with the eruption of pumice overlaps with the active period of the submarine satellite cones of the Nishiyama Volcano (please see the section on satellite cones around Nishiyama Volcano), the hole must have been active after 10,000 yr BP.

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