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.

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Chemical analysis for major and minor elements was performed on the pumice fall deposit, essential blocks from the two pyroclastic flow deposits and the lava flow, erupted in succession during the 1783 (Temmei) volcanic activity on Mt. Asama. The pumice samples representing a vertical column exhibit no remarkable trend of chemical variation with time. The analytical results for the lava flow show small spatial variation in K, Mg, Sr and some other elements. Further inspection of the data for all the samples indicates that Fe, Na, Ti, Sr, Cu, Co and Ni tend to increase with time throughout the whole eruptive sequence. A plot of Sr/(SiO₂ + K₂O) vs. (Fe₂O^*₃+K₂O)/(SiO₂+K₂O) illustrates that the erupting magma became progressively more mafic and more enriched in Sr during the activity. This type of plot, combined with the spatial distribution of certain elements superimposed on the distribution pattern of the lava flow, reveals that, during the lava eruption, the composition of erupting magma still shifted to slightly mafic.

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A phreatomagmatic explosion is a volcanic eruption caused by the generation of high pressurized steam from interactions between magma and any body of water. Various manners of modes of phreatomagmatic explosions have been known. The author classified the phreatomagmatic explosion modes into two types based on the intensity of eruption; weak phreatomagmatic explosion and strong one. The weak phreatomagmatic explosion is characterized by almost silent tephra finger jets, associated with minor surge activity, reaching several hundred meters high. The strong phreatomagmatic explosion is characterized by a large eruption column reaching 10-20 km high, accompanied by a shock wave and major surge activity. High-pressurized steam generating the phreatomagmatic explosion is caused by the rapid evaporation due to the mechanical mixing of magma and water. This mixing process determines the boundary condition of heat transfer from magma to water. When a lava flow enters the ocean, a frozen glassy crust effectively prevents the hot fluid interior from self-sustained mixing with water. Namely, the initial mechanical mixing of magma and water is the most important factor for triggering the phreatomagmatic explosion. The initial mechanical mixing easily occurs when magma ascends through the vent filled with wet clastic materials, owing to the fluid instability. If pressure pulse produced by the initial mixing is great enough to propagate a shock wave, it promotes the thermal detonation causing further hydrodynamic mixing and subsequent much more energy release. The strong phreatomagmatic explosion takes place in this fashion. On the other hand, the weak phreatomagmatic explosion is accompanied by no shock wave; the pressure pulse is not sufficient for propagating a shock wave. The strong phreatomagmatic explosion is triggered at greater depth of the vent than the weak one. This is mainly because a stronger pressure pulse is required at a higher confining pressure.

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