Eruption interval between Hb- and Px-types pyroclastic-flows from the Ohachidaira caldera of Taisetsu volcano, central Hokkaido, Japan, was estimated from the paleomagnetic directions. Px-type pyroclastic-flow deposit rests on Hb-type one, and gravel beds are intercalated between them. Oriented 138 samples were collected from 13 sites for paleomagnetic analysis. The paleomagnetic direction of Hb-type pyroclastic-flow deposit shows a normal polarity with a westerly declination(overall mean is N=7, D=-27.1°, I=66.3°, α95=2.7°, k=511.2), while that of Px-type pyroclastic-flow deposit shows a normal polarity with an easterly declination (overall mean is N=6, D=19.8°, I=67.5°, α95=4.6°, k=213.8). The two paleomagnetic directions are significantly different, and the time interval between the two pyroclastic eruptions is estimated to be more than about 100±40 years based on the geomagnetic secular variation in China, Russia, Europe, North America, and Japan.

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Previously known lahar deposits along the Sukawa River, western slope of Adatara volcano, consist of at least 7 muddy (cohesive) units and were formed during the last 2500 years. This paper reports stratigraphy, chronology and depositional processes of the newly found lahar units that were deposited older than 2500 years ago. ¹⁴C dating for wood fragments within lahar deposits and intercalated paleosol layers indicates that at least 18 lahar events occurred in the area during the last 14000 years. The newly found lahar units comprise 10 muddy matrix-rich units and one bouldery gravel unit. The muddy matrix-rich units are very poorly sorted with pebble to cobble sized andesite clasts and hydrothermally altered clasts. Bouldery gravel unit is filled with sandy matrix and composed of clasts with oxidized rind locally. Bulk chemistry of these lahar units indicates that high sulfur content in the muddy units whereas Fe₂O₃ content is higher in the bouldery gravel units. The muddy units are derived from syn- or post-eruptive lahars in relations with phreatic eruptions or degradation of hydrothermally altered source rocks nearby the Numanotaira crater at the summit. The bouldery unit distributed farther downstream may have resulted from a non-cohesive debris flow or an outburst flood from the crater or a volcanically dammed upstream area. The presence of oxidized clasts indicates that during traveling, the watery flow entrained clasts which were exposed on a gravelly riverbed in an inter-eruptive period. The two types of lahar deposits show flows with different characteristics (cohesive and non-cohesive) that originated from Adatara volcano. Proximal eruptive units in the summit area sometimes can be degraded easily and overprinted by other eruptive units that result in a paucity and difficulty in understanding the precise eruption history of a volcano. Therefore, approaching from analysis of distally deposited lahar units will give a hint to assess not only a lahar trigger but also the types, frequency and volume of volcanic eruptions.

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The Yatsugatake volcano located on the volcanic front in central Japan, has been active since the Middle Pleistocene, with many N-S trending eruptive centers. Little work has been previously performed on the age determination for the Late Pleistocene and/or Holocene age. Therefore, it is necessary to reconstruct the activity during that time duration. In this paper, I report a new AMS dating result of the Yt-Pm4 tephra, which is the youngest pumice fall from the Yatsugatake volcano. The pumice fall is actually from the Yokodake, one of the volcano located on the northernmost part of Yatsugatake. A ¹⁴C age of 30667±130 yr BP (32951-32333 cal BP: 2σ) was obtained from the charred wood collected at the uppermost part of the silty soil which is covered directly by the Yt-Pm4 tephra, suggesting that the eruption of the Yt-Pm4 tephra occurred at ca. 32 cal ka BP. The Yokodake consists of nine lava flows (Y₁ to Y₉, from oldest to newest). The stratigraphic position of Yt-Pm4 tephra is between Y₅ and Y₆. In addition, Okuno and Kobayashi (2010) estimated the eruptive age of Y₉ lava flow to be 2.35-2.15 cal ka BP. This means that the younger four lava flows (Y₆-Y₉) erupted in 29.85-29.65 ky. The magma discharge rate at Yokodake, including the volume of Yt-Pm4 tephra, is calculated to be ca. 0.0102 km^3/ky. It is emphasized, based on the age determination, that small-scale eruptive activities have been continuous in the northern part of Yatsugatake volcano even during Late Pleistocene and Holocene.

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During the 1783 Asama eruption, volcanic threads were thrown and fell as well as a large quantity of pumice and volcanic ash over the Kanto and the southern part of Tohoku area. We observed and analyzed two samples of the volcanic thread, which were collected and stored by a pharmacopoeia-seller. One is translucent white to pale yellow, and the other is black to dark brown. Our studies with an optical microscope, SEM-EDX and XRD equipment revealed that these volcanic threads are not made of silicate glass as produced in basaltic eruptions, but are made of amorphous sulfur.

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