“Experimental petrology” is one of the useful ways of approach to the genetic study on pyroclastic flows. The temperatures of pyroclastic flows directly measured at Komagatake Volcano range from 385° to 510℃, which are probably much lower than the actual maximum temperature. The density of the pumice ranges from 0.2 to 0.8, the average being 0.51, but increases as the grain size becomes finer. However, the total pressure, say at the bottom of 10m thick flows, will not probably exceed 1.5atm. Systematic variation of bulk density and porosity is observed within a flow unit of Shikotsu pumice flows. Chemical composition of pyroclastic flows from various parts of the world is plotted in Q-Or-Ab diagram. The Japanese rocks lie closer to Q-Ab line than the others, and consequently the temperature of beginning of melting of these rocks is expected to be slightly higher than that in other rocks. Diagram to estimate the chemical composition from the indices of refraction of melted glass of the volcanic rocks is given. Thermal experiments on the pumice, obsidian, and related rocks under atmospheric pressure and under small or moderate hydrostatic pressures have been carried out. The results give clue to the estimation of the temperatures of welding in the pyroclastic flows or related subjects. Thermal effects on phenocrystic minerals in the pyroclastic flows are also discussed.

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The Aso-IIIA pyroclastic flow deposit presents distinct variation of lithologic characters. Fig. 1 shows variation of lithologic and petrographic characters and the intensity of natural remanent magnitization (NRM) depending on the height of an outcrop of one cooling unit from the non-welded base to the zone of dense welding. Some magnetic characters are summarized as follows. [table] From the results following a few points are discussed. 1. The intensity of TRM of the rock specimens and its variation with the height is very similar to that of NRM. It shows that NRM of this column is due to TRM and it corresponds to the fact that the outcrop belongs to one cooling unit. 2. The variation of NRM does not correspond to the variation of abundance of magnetite phenocryst. It is thought that the increase of the intensity of NRM from the B part to the C part is due to formation of submicroscopic magnetite by devitrification. 3. The Curie temperature of the ferromagnetic mineral separated from the B part is lower than them from the A and the C parts. It seems that the Curie temperature from the B part is near to the original one and the higher Curie temperature is caused by oxidation just after the emplacement of the pyroclastic flow.

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The Cenozoic volcanic activity in Hokuriku district is divided into four stages. The age and the products of each stage are as follows: Stage I - Early Miocene - consists of lava flows and pyroclastic fall deposits of pyroxene andesite, and dacite welded tuff occurs in the lower part. Stage II - Middle Miocene - is mainly composed of deposits of pyroclastic flow and fall of rhyolite and dacite, associated with small amount of lava flow of rhyolite～dacite and basalt. Stage III - Late Miocene～Early Pliocene - lava flow and pyroclastic fall deposits of pyroxene andesite are common; pyroclastic flow deposits also exist. Stage IV - Latest Pliocene～Quaternary - several volcanic cones of pyroxene-hornblende andesite were formed (Volcano Tateyama, Hakusan, etc.) and pyroclastic flows also erupted in this stage. Present surface distribution of above volcanic products is shown in Fig. 1. Volcanism of Stage I, II and III took place in the Neogene sedimentary basin, and the distribution areas of their volcanic products became narrower and more localized from Stage I to later. Plio-Pleistocene (Stage IV) volcanism erupted on the uplifted Hida mountainland. Estimated total volumes of erupted materials in each stage are given in Table 1. The proportion of three types of volcanic products are also described. Eruption of pyroclastic flow played significant role in every stage, especially in Stage II, and ranges from pumice flow, some of which are strongly welded, to nuee ardantes (s. s.) or pelean type. Pyroclastic deposits consisting of ill-sorted, no welded essential ejecta, which is characteristic of pyroclastic flow deposits, are often inserted in the clastic sediments. The nature of these deposits, probably a type of subaqueous pyroclastic flow deposits, should be studied in future. The study of these pyroclastic strata is of prime importance to the Cenozoic stratigraphy as well as the volcanology in Japan.

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