On May 24^th 1926, the eruption of Tokachidake volcano, in central Hokkaido, efficiently melted the snow pack on the hill slope, triggering the Taisho lahar which killed 144 people in the towns of Kamifurano and Biei. A geological survey and paleomagnetic and granumetric studies were conducted on the northwestern slope of Tokachidake volcano to reconstruct the sequence of the 1926 eruption and decipher the triggering mechanism for the Taisho lahar. The Taisho lahar deposits in the proximal area of the volcano are divided into five distinct units (unit L1, L2, and A through C, from oldest to youngest). Unit L1 is an older lahar deposit that underlies the 1926 deposits. The 1926 sequence consists of debris avalanche deposits (unit A and C), a laminated sandy debris flow deposit (unit B), and a lahar deposit including scoria clasts (unit L2). Each unit contains hydrothermally altered rocks and clay material with more than 5 wt.% fragments smaller than 2mm in diameter. The progressive thermal demagnetization experiments show that the natural remanent magnetization (NRM) of all samples in unit A, B and C have a stable single or multi-component magnetization. The emplacement temperatures are estimated to be normal temperatures to 620℃ for unit A, 300 to 450℃ for unit B, and normal temperature to 500℃ for unit C. On the basis of geological and paleomagnetic data and old documents, a sequence for the eruption and the mechanism of formation and emplacement of the Taisho lahar can be reconstructed. The first eruption at 12: 11 May 24th triggered a small lahar (unit L2). Collapse of central crater at 16:17 May 24th 1926 then resulted in a debris avalanche containing highly altered hydrothermal rocks with hot temperatures ranging from 300 to 620℃ (unit A). The debris avalanche flowed down the slope of the volcano, bulldozing and trapping snow. Immediately following the collapse, a hot (approximately 400℃) hydrothermal surge (unit B) melted snow and transformed into a lahar causing significant damage and deaths in the towns downstream. Just after the generation of the lahar, another collapse occurred at the crater causing another debris avalanche (unit C).

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Towada volcano is an active volcano located at the northern part of the Northeast Japan arc. I carried out AMS radiocarbon dating of two soil samples taken from directly below the tephra layers derived from the eruptive episodes E and G at Towada volcano. The obtained ¹⁴C ages were 8110±30BP (PLD-8913) for the eruptive episode E (Nambu Pumice) and 9970±35BP (PLD-8914) for the eruptive episode G (Shingo Pumice). On the basis of the calibrated ¹⁴C ages and the stratigraphic relations, the most suitable ages of eruptive episodes E, F, G and Ninokura Scoria were estimated to be approximately 9.2, 10.2, 11 and 11.7-15.5cal kyr BP, respectively. These results enabled more detailed and reliable chronology of eruptive history during the post-caldera stage of Towada volcano.

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A small ash deposit was recognized on snowpack around the Nakadake crater, Aso Volcano, SW Japan, on 18 February 2008. The ash deposit was distributed around the first crater of Nakadake with the NE- and SE-trending axes. Based on the isopleth map, the total weight of ash was estimated at about 2 tons. The ash deposit was composed of fine-grained (＜0.5mm) crystallized or altered glass shards, altered lithics, crystals and apparently clear glass shards. The clear glass shards were transparent to pale-brown in color, and the proportion of these clear glass shards in the deposit was about 19%. Some clear glass shards presented dark-brown rims and/or were characterized by typical blocky textures with cracked surfaces. The surface morphology of clear glass shards strongly indicates that they resulted from brittle behavior by interaction of magma and water under wet condition. We believe that the clear glass shards included in the February 2008 ash were related to newly ascending magma and that they were partially hydrated or altered due to hydrothermal water after fragmentation of magma, and erupted due to an increased gas flux probably in the evening of 17 February 2008.

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