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!"#$%&'*.()&*+,-./0,**/ ( - 1 -+ 2345 ,**/ ( 1 1 +. 2367On the Essential Ejecta of the September ,**. Eruptionsof the Asama Volcano, Central Japan
Yasuyuki MIYAKEῌ, Kou TAKAHASHIῌῌ, Tatsuro TSUGANEῌῌ, Kuniaki MAKINOῌ,Hisakazu KAKUZENῌ, Kuniaki NISHIKIῌῌ, Takashi FUKUIῌ
and Shinshu University Research Group for Asama *. Eruptionsῌῌῌ
Asama volcano in central Japan became active on September +st, ,**. with a vulcanian eruption which was
the biggest eruption among a series of small eruptive events from September to December in ,**.. The ejecta of
the Sept. +st eruption were mostly polygonal fresh andesitic lithic fragments. Most of them were derived from theandesite body which played the role of cap rock for the rising new magma and escaping volcanic gas. Less
amount of pumice fragments were also ejected and most of them are mantled by the black and dense andesitic
crust, which is broken and expanded to form breadcrust structure. These breadcrust pumices are concluded tohave been the fragments of the new magma. After they were broken into pieces by the explosion, the outermargin of them rapidly consolidated, and then degassing and inﬂation in the inner melt caused rupturing of theouter crust. The pumices are clearly discriminated from the andesitic lithic fragments by their whole rockchemistry. The next vulcanian eruption occurred on September ,-rd. Majority of the ejecta were again
polygonal lithic fragments of andesite, although their chemistry corresponds not to the lithic fragments, but to thepumices of the Sept. +st eruption. Small amount of scoria are found among the Sept. ,-rd ejecta. Their wholerock chemistry and the assemblage and chemical composition of phenocrysts are quite similar to those of the Sept.
+st pumice. The black appearance of the scoria is derived from the less crystalline, therefore less di#erentiatedgroundmass glass in the scoria than in the pumice.
From these petrologic evidences the following magmatic processes are deduced. Before Sept. +st, a columnof new magma had risen into the pre existing andesite body beneath the crater ﬂoor. On Sept. +st, the built up
gas pressure surpassed the tensile strength of the andesite body to result in a vulcanian eruption. This eruption
provided fragments of the pre-Sept. +st andesite as polygonal lithic fragments, along with smaller amounts of thebreadcrust pumices. In the course between Sept. +st and Sept. ,-rd, the column of new magma uplifted because
the cap rock had disappeared. The upper part of the column was cooler and more crystalline than the lower.
Some portion of the upper part of the magma column e#used and made a dome inside the crater. On Sept. ,-rd,
next vulcanian eruption took place. The ejecta consisted of the lithic fragments which had been upper part of theconsolidated magma column, and the scoria derived from the lower level of the same magma column.
Key words : Asama volcano, vulcanian eruption, essential ejecta, breadcrust pumice
8-3*ῌ20,+ 9:;< -ῌ+ῌ+!"#6#=>?@#@
Department of Geology, Faculty of Science, Shinshu
University, Asahi -ῌ+ῌ+, Matsumoto -3*ῌ20,+, Japan.
8-3*ῌ20,+ 9:;< -ῌ+ῌ+!"#"#AB#CDE@>FGHIJ@#KL
Division of Environmental System Science, Graduate

School of Science and Technology, Shinshu University, Asahi -ῌ+ῌ+, Matsumoto -3*ῌ20,+, Japan.
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Corresponding author : Yasuyuki Miyake
e-mail : ymiyake@gipac.shinshu-u.ac.jp