NW Kyushu is also the junction of the southwest Japan arc and the Ryukyu arc and of the Japan Sea and the East China Sea. Mantle plumes are generally believed to come from a thermal boundary at the 660-km discontinuity or the core-mantle boundary. Tomographic images, however, suggest that subducted Pacific plate stagnates at the 660-km discontinuity beneath NE Asia (Fukao et al., 1992), which means that the 660-km discontinuity beneath NW Kyushu is cold. Normal faults in NW Kyushu (Figure 3) suggest that the stress field there is essentially tensional. Volcanism in NW Kyushu is characterized by the eruption of basaltic lava flows since 10 Ma. Prior to this basaltic volcanism, formation of inner arc sedimentary basins and their uplift occurred. Because of petrochemical similarities, Nakamura et al. (1985) suggested that NW Kyushu volcanism is caused by plumes from the deep mantle.