A fluid dynamic theory of pyroclastic flow is presented. This is based on the theory of simple inclined thermals and on the theory of powder snow avalanches. The theory of pyroclastic flow presented herein is composed of the mass conservation equations of air and pyroclastic materials, the momentum and the kinetic energy equations. In addition, the equation of temperature is included. The constitutive relations such as the air entrainment coefficient and the pyroclastic material entrainment coefficient, and the relations of shear stresses and the level of turbulence are assumed by the knowledge of powder snow avalanches. This theory is applied to the pyroclastic flow occuring at Mt. Fugen, Unzen volcano. The simulation of the dynamic behavior of pyroclastic flows is strongly dependent on the diameter of materials. If we choose the diameter as 0.075-0.1 mm, we can simulate the pyroclastic flow occurring at Mt. Fugen, Unzen Volcano, June 3, 1991. Also it becomes clear that the effect of the temperature is very important in the simulation for pyroclastic flows.

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The eruptive history of the Higashi Izu monogenetic volcano field for the past 32,000 years is revealed by tephrochronology and loess-chronometry. Morphology, color, and size of basaltic tephra grains are widely variable depending on the mode of the eruption; e.g., spinose red scoria are fallout from a strombolian eruption column when a scoria cone is established around the crater; yellowish green lapilli and hard tuff (kata) are products from phreatomagmatic explosions, the former being more magmatic than the latter. In the field, these discriminations are useful not only for identification of each tephra bed, but also for understanding the transition of eruptive styles during one eruptive event. Because vents are closed or sealed at the end of an eruption, an absolutely quiescence occurs between eruptive events in a monogenetic volcano field. This proves the validity of loess-chronometry. Some of the NW-SE or NE-SW trending alignments of volcanoes proved to be created by eruptive fissures; i.e., they are erupted simultaneously. Among them, the 11 km-long Iwanoyama-Iyuzan volcanic chain is the most conspicuous, which was active about 2,000 years ago. Eruptive events more than 10⁹ kg of magma discharge are recognized 13 times during the past 32,000 years, so that the average frequency of eruption in this field is calculated one every 2,500 years. The last is the Iwanoyama-Iyuzan eruption. The discharge rate of magma is 100×l0⁹ kg/ky for the past 32,000 years or 330×10⁹ kg/ky for the past 5,000 years. The rate seems to be accelerated recently, however, it is still an order of magnitude lower than that of a polygenetic volcano such as Izu Oshima. The Kawagodaira eruption of 3,000 years ago is remarkable for two reasons: the largest with 765×10⁹ kg of magma and the first appearance of rhyolite in the field.

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We propose a model of the boninite magma chamber beneath Chichijima, Bonin Islands on the basis of texture and mineral chemistry of a porphyritic boninite. The boninite-series volcanic rocks differentiate from boninite through andesite and dacite up to quartz-phyric rhyolite. Whole-rock compositions and mineralogical evidence indicate that both boninite and andesite are formed by mixing of magmas, while andesite, dacite and rhyolite are products of crystallization differentiation. This view is supported by the fact that both boninite and andesite lavas are intercalated with each other but that dacite and rhyolite lavas are underlain by boninite and andesite, and are superposed one after another in ascending order. Density and viscosity contrasts, which largely affect the ability of mixing of two magmas, are small between boninite and andesite magmas, and enabled mixing of these two. Detail examination of texture and zoning of orthopyroxene phenccrysts in CH501, which is a representalive of pophyritic boninites, has shown that 80% of the orthopyroxene phenocrysts (Type I) have cores with uniform compositions with magnesian rims. The homogeneous core compositions differ from grain to grain and range in Mg# from 90 to 78, but have the highest frequency at Mg#86. Another 20% of phenocrysts (Type II) have cores with multiply repeated resorption and reverse zoning in Mg, Fe and Ca, which are formed by repeated injection of hot, primitive magmas. In order to explain the population, texture and chemistry of these phenocrysts,a flat, sheet-like magma chamber is proposed. Magma in the chamber is cooled from above and crystallizes against the roof of the chamber. The flat roof prohibits segregation of residual melt from crystals and thus forms mushy layers growing downward. New input of hot, primitive magmas form fountains rising up to the roof of the chamber and destruct the mushy layers and incorporate cumulus crystals into hybrid magmas. Some parts of the mushy layers resisted against the flush of the fountains are resorbed and reequilibrated with the hybrid magmas, and are overgrown with new crystals and melts. Type II phenocrysts are such long-lived cumulus crystals which experienced repeated magma input. Type I phenocrysts are derived from cumulus crystals in the newly formed part of the mushy layers.

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