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Evaluation of Total Ejected Ash in Volcanic Clouds Using Video Records :Application to the Eruption of Asama Volcano, Japan, on February 0, ,**Akihiko TERADA῍, Taketo SHIMANO῍῍, Sei IIJIMA῍῍῍ and Jun OIKAWA῍῍
From February to April ,**-, four small eruptions occurred in Asama volcano, central Japan. During these
eruptions, movements of volcanic clouds were automatically recorded by video cameras. Based on these camera
records, we studied the features of ascent processes of volcanic clouds. The volcanic cloud on 0 February ,**had mostly isolated symmetric shape and expanded linearly with height as it ascended. The squares of heights fromvirtual origin are proportional to time. These features are consistent with the characters of a thermal, which iswell known from dimension analyses and experiments.
To evaluate the masses of ejected ash we developed a simple model for volcanic clouds based on thermal
assumption, and described the ascent velocity as a function of mean temperature and mass of ash in a volcanic
cloud. In this model, a volcanic cloud is composed of ash and ideal gas, driven only by buoyancy. Any fall-out
of particles was neglected. We compared the model with the video records of the volcanic cloud on 0 February,**-, and deduced that the volcanic cloud had less than about .** ton of ash.
We found new vents at the bottom of the main crater. Volume of the most prominent vent is comparablewith the ash volume derived from our model. Chemical compositions of the products resemble those of Maekake
stage, the youngest stage of Asama volcano, suggesting that the ash were derived from the shallow part of the
volcanic ediﬁce. It is interpreted that the small explosion occurred at the shallow part beneath the main crater.
We propose that high temperature of ejecta was responsible for the formation of unusual volcanic cloud.
Key words : Asama volcano, thermal, volcanic cloud, camera recordings, mass of ash+ῌ ῐ ῎ ῑ ῏J-!.7/12h#WHI^ijklm-!./01

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_`a^bc.deK5);2 ῍?@ Ufg ,**- a31)!"-ῌ.&5 M'Q'(*0*ῌ*2+* nopnn +*  2 ¡nopW¢W¢£¢¤¥¦§¨QR!"¤¥de©ªlInstitute of Seismology and Volcanology, Graduate
School of Science, Hokkaido University, North +*West 2, Kita-ku, Sapporo *0*ῌ*2+*, Japan.
++-ῌ**-, «¬­®¬¯0 +ῌ+ῌ+«¬W¢QR¤¥°
Earthquake Research Institute, University of Tokyo,+ῌ+ῌ+ Yayoi, Bunkyo-ku, Tokyo ++-ῌ**-,, Japan.
-23ῌ*++/ ± ²n³´µ¶·¸¹º» ++/+ῌ,Ufg¶·¸e¼°
Karuizawa Weather Station, Japan Meteorological
Agency, ++/+ῌ, Oiwake, Karuizawa, Kitasaku, Nagano-23ῌ*++/, Japan.
Correspnding author : Akihiko Terada
e-mail : terada@eos.hokudai.ac.jp

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184

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Fig. +. Map of the area around Asama Volcano
showing locations of the cameras with azimuthof ﬁelds, sampling site of ejected ash andstations of upper air observations. The arrow
indicates the site from which the photograph
was taken by TSB (TV. Shinshu BroadcastingCo., LTD) in Fig. , (f).

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Table +.

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Parameters of camera at OWK station.

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185

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Table ,. The time sequence of noticeable activities ofAsama volcano. The data are in part from Japan
Meteorological Agency (,***; ,**+; ,**,; ,**- a).
Close circles in “Fumaroic Activity” denote thatthe monthly maximum heights were over /** mabove the top of the summit. Tri-angles denotesmall eruptions reported by Japan Meteorological
Agency.

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186

Fig. ,. Photographs of the volcanic cloud on 0 Feb., ,**-. (a)-(d) A sequence of photographs taken from OWK.
The times shown in the photographs are of - seconds slow relative to the Japan Standard Time. (e) The
photograph taken from ASE. The time shown in the photograph is of + second fast relative to JST. (f) The
photograph taken by TSB from Miyota town in Fig. +.

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Fig. -. (a)-(g) A sequence of photographs magniﬁed around the volcanic cloud on 0 Feb., ,**-. Numbers pointout the remarkable parts of volcanic cloud. (h) Schematic of convection pattern at surface of cloud.

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 Fig. .. Whole rock and mineral composition of the +12- and the ,**- ejecta of Asama Volcano. (a) Whole rockcomposition. Composition ranges of the products in Maekake, Hotokeiwa and Kurofu stages are also shown(Aramaki and Takahashi, +33, ; Takahashi et al., ,**-). (b) Anorthite content of plagioclase compared withthose of the +12- products (lower, middle, upper airfall pumice, lava ﬂow, and blast in ascending order). (c)Pyroxene compositions on the quadrilateral diagram of Lindsley and Anderson (+32-). Thick broken, solid,
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 =&: :E/" (Fig. 0)$ Fig. 0. Details of ascent process of volcanic clouds
on 0 Feb., ,**-. (a) Correlation between top
heights and maximum radius. (b) Top ofheight as a function of time. (c) Examples ofEq.(0) ﬁts to the observation data withvariances about m`+, -/., +/, and +/.. Z is
the top heights of volcanic cloud from virtual
origin. See section 0῍+ for detail. (d) Z, as afunction of time.

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191

>
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192

Fig. 1. Variations of parameters as a function of temperature T and mass of ash Mt with radius of 1*m and heightof ,3** m a.s.l.. (a) Density ratio (ra-r)/ra. in Eq. (,). (b) Velocity w in Eq. (,) with C of +.*. The
stippled area is enclosed by uncertain of observation data. (c) Velocity w in Eq. (,) with C of *.23 and +./.

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Aramaki S. and Takahashi M. (+33,) Field Workshop at
Asama and Kusatsu-Shirane Volcanoes, Japan. IAVCEI
Commission on Explosive Volcanism, Workshop Guide
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Roy. Soc. London, A+2/, ,+-ῌ,./.
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.yTt TT.pQ (,**-) 5 ,**- " , : 0 #  .2 .13ῌ.2..
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tTT<T_GH`T T<Zb T [ T b T   T 
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¥¦TZ¡r (+32+) §¨©lXª$«Y<C0  ,0 ++1ῌ++2.
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 .+ +.+ῌ+.-.
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? ¡T ¢£rT²_¤T.pQT ¥T¦GRqT§_ T³¨q (+31/) +31- "5@ >?]^D©A /* ++/ῌ+/+.
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194

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