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 0  /*+ῌ/+1 ῐ ῏   ,**.    ,**.  !"#$%&,**/  . ' +/ ()* ,**/  ++ ' ++ (+,Textural Variety in the Eruptive Products of Vulcanian Eruptions between ++*2 A.D.
and ,**. A.D. on Asama-Maekake Volcano
Maya YASUIῌ, Masaki TAKAHASHIῌ, Masayuki SAKAGAMIῌ and Research Groupon the Asama ,**. Eruption at Nihon UniversityῌῌAsh fall deposits composed mainly of lithic fragments are often recognized in the eruptive products ofAsama-Maekake Volcano. A comparative study was made on microscopic textures of coarse-grained particles of+ to , mm in diameter from thirteen ash-fall deposits. Average composition of the ash particles from Vulcanianeruptions after the +12- eruption was also investigated using the cumulative ash-soil mixtures. More than 1*
percent of the particles are sub-angular and blocky in shape. Crystalline, grayish, and non-vesicular grains arealso abundant. These features suggest that most particles have been produced by brittle fracture of solidiﬁed
magma. Individual ash-fall deposit has a particular composition. Some ash falls, which were generated after thelarge-scale eruption in ++*2 A.D., have fragments of welded pyroclastic rock as well as those of altered lava. It
suggests that they might have been generated from the speciﬁc condition inside the vent after the formation andcollapse of proximal pyroclastic cone. A series of ash falls through the ,**. eruption show obvious temporalvariation in their textures and compositions. Vitric and vesicular grains increased after the appearance of moltenlava in the crater-bottom on +0th September and decreased thereafter. Instead, crystalline, grayish, andsub-angular particles increased with time. Oxidized particles have been recognized in the later stage. These
temporal variations might correspond to the processes of the appearance and cooling of newly supplied magma.
Textures of thirty ballistics including bread-crust bombs from Vulcanian eruptions between +12- A.D. and+32- A.D. were observed. Half of them have unbroken, euhedral phenocrysts in the homogeneous groundmass
suggesting that they have been originated from magma column of the ordinal coherent lava (Type +). Forty
percent of them contain the veins ﬁlled with ﬁne-grained crystal debris in the matrix which is similar to Type +(Type ,). Ten percent of them show remarkable eutaxitic texture indicating that they are fragments of welded
pyroclastic rocks (Type -). These textures of the ballisitics could account for the variety of ash particles and also
imply the processes of fragmentation of magma in the conduit. Micro-faults are sometimes observed in thegroundmass of Type ,. It indicates that the outer part of solidiﬁed magma in the conduit experienced thepulverization probably due to shock wave throughout Vulcanian explosion. Zigzag cracks running through asingle phenocryst are also found around the veins. Characteristic vein ﬁlled with ﬁne-grained crystal debris in
Type , might be the evidence of the ejection of pulverized materials which occurs immediately after the explosion.
Key words : Asama-Maekake volcano, Vulcanian eruption, lithic fragment, texture, fragmentation-+/0ῌ2//* ./0123456 -ῌ,/ῌ.*7+89:;<=>??
Department of Geosystem Sciences, Nihon University, -ῌ,/ῌ.*, Sakura-josui, Setagaya-ku, Tokyo +/0ῌ
2//*, Japan.
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Research Group on the Asama ,**. Eruption at
Nihon University : Kunihiko Endo and others
Corresponding author : Maya Yasui
e-mail : yasui@chs.nihon-u.ac.jp