῍῎  /*  (,**/)
 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 of
Asama-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 molten
lava 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 the
groundmass 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.
 ,**.  !"#$% :@ABC D ++ E

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¶·¨©~¦ª~¸'¹«~¬ k`q ,**. eºBCW»¼

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Fig. +. Columnar sections showing the stratigraphyof pyroclastic fall deposits from the ++*2 eruption to the present on Asama-Maekake Volcano.
Numbers shown by “Sp.” show the stratigraphic
levels of the ash fall deposits observed in thisstudy. Sp. +ῌ+ : Layer - of As-B (++*2 A.D.),
Sp. +ῌ, : Layer 1 of As-B, Sp. , : “Red ash”
which is composed of numerous colorful ash falllayers. ,ῌ+ : yellowish ash layer and ,ῌ, : grayish blue ash layer, Sp. - : Layer 1 of As-B’(eruptive age is unknown), Sp. . : Layer +a of
As-A (+12- A.D.), Sp. / : ash-soil mixture which
deposited after the +12- eruption. Direction
and distance of each sample are in the following ; Sp. /ῌ+ : ENE ,.. km, Sp. /ῌ, : E ... km,
and Sp. /ῌ- : ENE 0.- km, and Sp. 0 : ash fall
deposits from the ,**. eruption. Dates of theeruption are as follows ; Sp. 0ῌ+ : +st Sept., Sp. 0ῌ
, : ,-rd Sept., Sp. 0ῌ- : +*th Oct. and Sp. 0ῌ. : +.th
Nov.

Fig. ,. (+) Location of the sites where samples
were taken. Open circle : ash fall deposit, solid
circle : volcanic block or bomb. Italic numbers
show sample number in this study. AVO :Asama Volcano Observatory, Univ. of Tokyo.
(,) Isopach map of “Red ash”. Plural ash fall
deposits consist of the “Red ash” at each locality. Thickness in cm.

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22  * ,.. km 94 /῍+:  ...

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789: 9 y +$ =
Table +.

505

Classiﬁcation of ash particles in this study.

Type A : particles with spray-like outline, Type B : particles with polygonal outline, Type C : free crystal, TypeD : fragment of altered volcanic rock and Type E : fragment of accidental material. + : Outline, SP :spray-like, SA : sub-angular. , : Presence of vesicles, V : vesicular, M : massive. - : Crystallinity of groundmass, G : glassy, C : crystalline (cryptocrystalline to hyalopilitic). . : Arrangement of minerals in groundmass, R : random, A : arrangement.



 (Fig. --(+,)) E

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EF , ^|9:}{_ mLnEF - ^B῎uh9vwxy'$((+30-) 45(Fig. --(+-)) &)*+#j ( k-.@l6 EF ,/ - mLn

6789:

;<+

=>?3>

CD> 6EF GHIJ (Fig. .) @-K
LMNOP,Q. RSTUVWX  YZ[\]# ^B _ `abc/ de# ^A _  + fgh>i  de#z 1 {_ pq~V B-/B-2 II +*ῌ   pq ^C _  +*ῌ gh`@-A+B> 6a/ EF . ^9+a {_, /*ῌ   6+B ^D _ Iῌ@- -*ῌ j/ EF 0῍+ ^,**.  3  +  9:}_ >EF , ^|
9:}{_  NO EF / ^9gu

506  ,**. 

Fig. -. Photomicrographs of typical grains of types A,B,C,D and E in this study. Grain size is + to , mm indiameter for each photo. Photos + to +- : open nicols. Photo +. : a grain of Type B-. rich in ﬁne-grained
crystal debris. Crossed nicols. For details of individual types, see text in the subsection -ῌ+.
(+) Type A-+ (Sp. 0ῌ,), (,) Type A-, (Sp. 0ῌ-), (-) Type B-+ (Sp. 0ῌ-), (.) Type B-, (Sp. 0ῌ-), (/) TypeB-- (Sp. 0ῌ-), (0) Type B-. (Sp. 0ῌ-), (1) Type B-/ (Sp. ,ῌ+), (2) Type B-0 (Sp. -), (3) Type B-1 (Sp. -),(+*) Type B-2 (Sp. -), (++) Type C (Sp. 0ῌ.), (+,) Type D (Sp. 0ῌ-), (+-) Type E (Sp. 0ῌ-) and (+.) Type
B-. (Sp. /ῌ,).

 

Fig. -.

Continued.

507

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Fig. /. Number of grains in percentage for six features for the thirteen ash fall deposits on Asama-Maekake
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Fig. 0. Relation between number of grains withcrystal arrangement and that with eutaxitic texture for the thirteen ash fall deposits on AsamaMaekake Volcano.

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Fig. 2. Representative photomicrographs showing textures of the ballistic ejecta of Vulcanian eruptions between+12- A.D. and +32- A.D. of Asama-Maekake Volcano, (+) Type + : ordinal lava with euhedral phenocrysts ofplagioclase and pyroxene and homogeneous groundmass. Open nicols. (,) Type , : an extreme example ofType , in which abundant broken crystals are contained. Open nicols. (-) Type , : lava which contains avein ﬁlled with ﬁne-grained crystal debris. Open nicols. (.) Type - : welded pyroclastic rock with remarkable eutaxitic texture. Open nicols. (/) Enlarged photo of an area shown by a circle in photo (-). Avein ﬁlled with ﬁne-grained crystal debris can be seen. Crossed nicols. (0) A sketch showing areas X and Y
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Fig. 3. Photomicrographs of representative textures observed in Type , of the ejecta of Vulcanian eruptions
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A micro-fault running through the ﬁeld of view is shown by arrows. Open nicols. (,) A pair of fragments
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Aramaki, S. (+30-) Geology of Asama volcano. Jour. Fac.
Sci. Univ. Tokyo, Sect. II, +., ,,3῍..-.
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Blenkinsop, T. (,***) Magmatic and sub-magmatic deformation. In Deformation Microstructures and Mechanisms

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Heiken, G. (+31,) Morphology and petrography of volcanic ashes. Geol. Soc. Amer. Bull., 2-, +30+῍+322.
Heiken, G. and Wohletz, K. (+32/) Vulcanian eruptions.
In Volcanic ash, .0῍0+. University of California press,
Berkeley, Los Angeles, London.
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Morrissey, M. and Mastin, L. (,***) Vulcanian eruptions.
In Encyclopedia of volcanoes, .0-῍.1/. Academic Press,
London.
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Wohletz, K. H. (+32-) Mechanism of hydrovolcanic
pyroclast formation : size, scanning electron microscopy,
and experimental results. J. Volcanol. Geotherm. Res., +1,-+῍0-.
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Yasui, M. and Koyaguchi, T. (,**.) Sequence and eruptive
style of the +12- eruption of Asama volcano, central
Japan : a case study of an andesitic explosive eruption
generating fountain-fed lava ﬂow, pumice fall, scoria
ﬂow and forming a cone. Bull. Volcanol., 00, ,.-῍,0,.
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