We collected high-resolution sidescan sonar imagery over the central part of active back-arc spreading basin, Mariana Trough, at 17° North (“Seg-17”). Spreading center is situated in the eastern side of the Mariana Trough, which divides the trough into approximately 3:2 in the west and east of the axis. Multi-scale observations, which are shipboard multi-beam bathymetric survey using R/V Yokosuka, the deep-towed side-scan sonar survey using Wadatsumi, and visual observation using submersible Shinkai6500, show that two predominant trends in geological features (fault, fissure, and volcanic ridge) are developed on the axial valley floor in either scale of observations. These two trends of linear features are almost parallel to each trend of “V-shaped ridge” developed along centerline of the axial valley floor. One trend is N15° W subparallel to the trend of the axial valley itself. Another trend is almost NS. There are no obvious differences in age between these two trending features. Volcanic activity is dominant in the western-half of the axial valley floor, which is indicated by the backscatter intensity pattern and by visual observations.

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Formation mechanism of basaltic pyroclastic flows has not been sufficiently clarified yet because basaltic pyroclastic flows do not occur as frequently as felsic ones. We studied the Osawa pyroclastic flow 3 deposit (OsPfl-3), which took place on the western flank of the Fuji volcano between 2.9 and 3.0 ka. OsPfl-3 has two flow units and one cooling unit, which have a combined volume of 6.2 × 10⁶m³. The flow overlies another unit composed of two scoria fallout deposits (YokSfa-2a and 2b) which sandwich a pyroclastic flow deposit (OtPfl). OsPfl-3 mainly consists of welded blocks and dense blocks with composition and petrographical characteristics of basaltic andesite. Some of the dense blocks have cracks on their surfaces and look like “cauliflower-shaped bomb”. They have a flat surface on one side with concentration of vesicles near the surface. The matrix of OsPfl-3 has dense fragments that are thought to have originated from dense lava blocks and poorly vesiculated scoria. The emplacement temperature of the blocks is estimated to be higher than 580℃ from thermoremanent magnetization measurements. These observations indicate that the blocks in the OsPfl-3 originated from welded pyroclasts, lava flow or lava lake at the summit crater. The sequence of the eruptions that formed OsPfl-3 and underlying deposits are summarized as follows: Stage 1: Deposition of fallout tephras (YokSfa-2a and 2b) and an intercalated pyroclastic flow (OtPfl) which are composed of fairly vesiculated scoria; Stage 2: Formation of lava flow or lava lake at the summit crater, and deposition of pyroclastics on the lava; Stage 3: Occurrence of the pyroclastic flow (OsPfl-3) caused by collapse of lava and pyroclastics. OsPfl-3 is prominently distributed on the western flank. This observation implies that the westward flow from the source lava that filled the summit crater could cross the lower part of the crater rim.

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Semi-quantitative thermoluminescence (TL) age data are reported for 13 volcanic areas in Hokkaido and Tohoku. Most of TL age data have an error of 21-37% from sample characters and other factors. Even such semi-quantitative ages are useful for preliminary stratigraphical and geological interpretation. Ages of 58 samples for 13 areas range from 0.009 to 1.7 Ma. We give a new age data for Kanpu area. The age data of Tokachi, Shiribetudake, Hachimantai, Nanashigure and Adachi areas show large difference with previously reported ages. The TL ages of other seven areas coincide with previously reported ones except for data older than 0.7 Ma.

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For better estimation of the temperature of red-hot volcanic ejecta from their images taken by online digital cameras at night, we studied the applicability of a new pyrometer to the images. Our pyrometer applies the theory of black-body radiation to the color of red-hot object. We examined the color of red-hot volcanic ejecta, a heated basaltic rock with known temperature, and infra-red LEDs. Because of the sensitivity to infra-red ray, a digital camera Nikon D40 can visualize hot basalt specimen at temperatures much lower (ca. 370℃) than the naked eyes (500～550℃). As a side-effect of this capability, color data of the hot basalt discord from the isothermal color lines calculated from the black-body radiation and the CIE colorimetric system. Night photographs taken by the online digital camera aimed at the explosive ash eruptions of Asama volcano (2 a.m. 2 Sep., 2009) indicated that the color of red-hot volcanic ejecta was disturbed by the influence of infra-red ray in the same manner as observed on the heated basalt specimen in the laboratory. As a result, the temperature of the volcanic ejecta would be much lower than their appearance in digital images (e.g., 1000℃) but higher than the detection limit (370℃). Comparison of the color of red-hot volcanic ejecta and isothermal color lines revealed that some of the volcanic ejecta fell along the 600℃ isotherm line of the black-body radiation, which suggests that volcanic cloud reduces the influence of infra-red ray. The estimated temperature (600℃) is consistent with those deduced from petrological observation and thermodynamic computation (600〜700℃). We conclude that color analysis of the images of red-hot ejecta taken by online digital cameras are useful in temperature evaluation when images suffer less influence of infra-red ray.

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