十和田火山後カルデラ期新郷軽石噴火にみるサブプリニアン噴火の噴火推移

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The systematic change of volcanic pyroclasts from Ninokura stage to Nakanoumi stage is observed in the post caldera stage of Towada volcano, northeast Japan. The transition from scoria eruption at Ninokura stage to pumice eruption at Nakanoumi stage, from basaltic to felsic in composition, and from tholeiitic to calk-alkaline in rock series was recognized. That transition event is Shingo pumice eruption. The object of this study is to clear the sequence of Shingo pumice. Particularly we have focused on the bulk density and the size distribution of pyroclasts. The localities of investigated and sampled outcrops in this ctudy are in a range of from 5 to 40km distance and from NNE to SSE direction from the inferred eruptive source, Nakanoumi crater at latitude 40°N and longitude 141°E. The dispersal area of G is narrow and elongated to east. It is classified as sub plinian eruption. Shingo pumice consist of gray pumice, yellow pumice and banded pumice. It is divided into six sub units from G1 to G6 downward. G1 and G2 is upper unit. They consists of reddish yellow well vesiculated coarse pumice. G3 is middle unit. It mainly consists of poorly vesiculated grayish fine pumice. G4 and G5 is lower unit. They consists of reddish yellow pumice, grayish yellow pumice and a few banded pumice. G6 is base unit. It consists of reddish yellow pumice. Anorthite (An) content of plagioclase in unit G has bimodal distribution. They has high An content (75-90%) in the core region and low An content (58-70%) in the rim region. This normal zoning is a result from the over saturation for volatile elements by the decreasing of pressure. The low total wt. % of glass inclusion also supports existing of hydrous magma. The bulk density variation of pumices in unit G is divided into three groups; 0.2-0.7, 0.8-1.2×10^3g/M^3 and more than 1.2×10^3g/m^3. The lighter pumice is reddish-yellow and large in diameter, and the heavier pumice is grayish and small. Just unit G4 and G5 has banded pumices. Banded pumice has more than 1.2×10^3g/m^3 in bulk density and elongated bubble structure. The yellow pumice and the grayish pumice has difference on the bulk density and the fragmentation, on the other hands, they has same chemical characters. It suggested that the fragmentation and degassing process changed temporally at G3 unit.論文Article
東北大学の論文
2007-03-30

十和田火山後カルデラ期新郷軽石噴火にみるサブプリニアン噴火の噴火推移

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