The dominantly Pleistocene Abu volcano group, located along northern coast near the SW end of Honshu, consists of basaltic-to-dacitic lava flows, small shield volcanoes (some with associated cinder cones), and lava domes. More than 40 monogenetic volcanoes are located in an area of 400 km2. Iraoyama forms the high point of this group of edifices, some of which form offshore islands or submarine vents. Volcanism here is considered to be related to subduction of the Philippine Sea Plate. An earlier phase of activity during the late-Pliocene to early Pleistocene was followed by renewed activity about 800,000 years ago that continued into the Holocene. The latest known activity was determined by thermoluminescence to be about 8,800 years ago, when the Kasayama scoria cone was formed.