Various types of phreatomagmatic explosive deposits are observed in Hime-shima Island. These deposits, Hime-shima Volcanic Rocks, are overlaid on the middle Pleistocene sediment, Hime-shima Formation, about 0.3 million years ago. Hime-shima Formation, composed of deltaic deposits and neritic sediments, was upheaved by the emplacement of a magma body beneath the island. The Hime-shima Volcanic Group is composed of seven volcanoes; Oomi, Yahazudake, Kane, Ukisu, Shiroyama, Darumayama and Inazumi Volcano. Structure of each volcanic edifice is identified as lava domes, hydroclastic ring-shaped cones (tuff rings and tuff cones), base surge deposits and/or pyroclastic now deposits. Mode of emotions are variable not only during the formation of single volcano but also during the formation of single crater. The former example is observed at Darumayama Volcano. An early-formed crater (East Crater) was related to the formation of tuff cone, whereas a later-formed one (Oikubo Crater) was related to the production of base surges and pyroclastic flows. Change of the volcanic activity at this monogenetic volcano might be caused by decrease of the influence of external water during the development of volcanic edifice. The latter example is observed at Inazumi Volcano. A tuff cone of Inazumi Volcano is composed of alternating two phases, one rich in finer fragments and another rich in coarser fragments. Such facies change during the formation of single tuff cone with single crater would be caused by a variation of mass ratio between the magma and external water.
