In order to study quantitatively color changes of basaltic scoria erupted at Takatsukayama, North-east Izu, Japan, the scoria colors were measured by spectro-colorimetry and described m the international standard color space(CIE L^* a^* b^*)The color values [a^* (red) and b^* (yellow) values]of Takatsukayama scoria cone increase from margin to center, indicating their color change from black to red. The bulk chemical compositions of scoria, including total Fe content, remain almost unchanged despite the color variations. 0n the other hand, the FeO(Fe²⁺) contents determined by the phenanthrolme method have a good linear relation with a^* values (red) of scoria. Under the optical microscope, the red parts are not found m the black scoria, whereas the red scoria have red parts m the groundmass and the olivine phenocryst. For dull red scoria, the red carts are observed only m the groundmass These indicate that red parts appear first m the groundmass and then in the olivine phenocryst. Visible and Raman microspectroscopy indicated the presence of hematite m these red Darts A series of heating experiments of the black scoria under an atmospheric condition at 500-1150℃ were conducted to simulate the color changes of Takatsukayama scoria. The heated scoria showed the increases in a^* and b^* values (i e. red coloring). The FeO(Fe²⁺) contents of the heated scoria had similar linear correlation with a^* values (red) as for natural ones. The presence of hematite was also found by visible and Raman microspectroscopy. These results imply that the increases in a^* and b^* values (red coloring) of Takatsukayama scoria can be explained by the high temperature oxidation of Fe²⁺, resulting m the formation of hematite first in the groundmass and then m the olivine phenocryst. Since the direction of the red coloring is oblique to the depositional sequences of the cone, the high temperature oxidation process might have occurred after the scoria deposition from the center of a heat source to the outer parts.

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Akita-Komagatake volcano is situated at about 30 km west of volcanic front in the northeastern Japan arc. Development history of the volcano is divided into 3 stages, main strato cone. syn-caldera formation, and post-caldera emotion stages, in descending order. In order to reveal the comprehensive emotion history m the post-caldera stage of the volcano, correlation between eruptives constituting the volcanic edifices and dispersed air-fall tephra are investigated, along with re-examining geology of the volcanic edifices formed m the concurring stage. Petrological affinities such as whole rock major-element chemistry and mineral assemblages including modal abundances of phenocrystic chases are utilized m identifying the contemporaneous eruptives of various modes of emplacement. To avoid the effect of phenocryst-matrix separation, coarse-grained tephra samples were selected to the whole rock chemical analysis. The revealed outline of eruption history is as follows: Tholentic magmatism has been dominant through the stage, with episodic calc-alkahc magmatism at the final activitiy m the northern region (ca. 4,000-3,100 y.B.P.). Relatively large eruptions occurred at the beginning of the stage (ca. 13,000 to 10,000 y.B.P.), resulting m caldera and probably large horse-shoe shaped crater as well, rather than forming volcanic edifices. Formation of the northern volcanic edifice began ca. 10,000 y.B.P. Five pyroclastic cones were built up successively, and subordinate lava flows or pyroclastic flows accompanied with them. Composition of magma was abruptly changed from andesitic to basaltic around 8,000 y.B.P., and the basaltic volcanism lasted to 7,000 y.B.P. After ca. 3,000 years of dormancy, phreatic explosions occurred successively, probably giving rise to horse-shoe shaped crater at the center of the northern area. Calc-alkali andesitic magma effused to build up a small pyroclastic cone immediately after the major explosion event. After ceasing the magmatism m the northern area, volcanism in the southern area was initiated by phreato-magmatic explosion m the floor of the south caldera about 2,500 y.B.P., followed by vulcanian eruptions, lasting until ca. 1,600 y.B.P About 1,500 y.B.P., eruption center was localized to two major vents at northern part of the caldera. Repeated eruptions from the vents have built up Medake and Kodake cones and their associated lava flows. The latest emotion occurred at the summit of Medake in 1970 AD

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We determine the three-dimensional P- and S-waves velocity structures around the Hakusan volcano m the Ryohaku mountains area, where there are some Quaternary volcanoes, using the travel-time tomography method. W use 104,764 P- and 97,232 S-waves arrival time data observed at 521 seismic stations from 3,930 earthquakes in and around this region. The initial model, hypocenters and one-dimensional velocity structure, for the three-dimensional inversion are simultaneously determined by the Joint Hypocenter Determination method. Since dumping parameters and iteration number affect the absolute value of velocity perturbation, we determine these values quantitatively by applying “Cross Validation Technique”. A checkerboard resolution test is applied to the dataset to examine the resolution scale of this dataset. We find a velocity anomaly zone with low-V_p, low-V_s and high-V_p/V_s beneath the Hakusan volcano at the depth of 10 to 14 km, while there is no such a region beneath other volcanoes m the Ryohaku mountains area. Seismicity is shallower beneath the Hakusan volcano than beneath the surrounding areas and no earthquake occurs within the velocity anomaly zone beneath the Hakusan volcano. These features support that the velocity anomaly is a partially melting rock matrix with high temperature. The Hakusan volcano ls one and only active volcano in recent 0.2 Ma in the Ryohaku mountain area. This geological feature is consistent with our tomographic result. The existence of magma body, which can be shown as low-velocity body, may indicate that Mt. Hakusan is still noteworthy active volcano.

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The activity of Towada Volcano started about 200,000 years ago and more than 20 eruptive episodes have been geologically recognized. The Ninokura scoria group, investigated in this study, 1s a series of pyroclastic fall deposits (NK-a to NK-k) that erupted just after the latest caldera-forming Hachinohe pyroclastic now (13,000 y. bp.). In order to understand both the deep and shallow magma processes of the Ninokura stage, we have particularly examined the chemistry of glass inclusions enclosed m phenocrysts, in addition to whole-rock and mineral chemistry. Glass inclusions are useful because they are considered to represent magmatic melt present at the time of crystal growth. The bulk rock compositions of the Ninokura scoria belong to tholeiitic series. On the other hand, the compositions of glass inclusions range from tholeiitic series to calk-alkalic series, suggesting the mixing of both series magma. The compositional change and supposed magma processes during the Ninokura stage are summarized as follows. In the earlier stage (NK-k to NK-i), each scoria has large heterogeneity m the composition of phenocrysts and glass inclusions, and the chemical compositions of magmas tend to be more mafic with time. The compositions of glass inclusions are on a mixing line formed by the Hachinohe magma and a basaltic magma. We interpret the compositional variation as a result of mixing of the residual felsic magma (Hachinohe) with the newly injected mafic magma. In the middle (NK-h to NK-e) and later (NK-d to NK-a) stages, the compositions tend to become SiO₂-rich. The heterogeneity is minimum at the NK-h stage. The interpretation is that the residual magma was replaced by the mafic magma at the time of the NK-h stage. Magma of NK-g to NK-e can be explained by fractional crystallization. In the last stage (NK-b) remarkably SiO₂-nch glass appears. It is probably crustal melt generated by the heat of repeatedly injected basaltic magma.

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