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,**/  -  -+  !" ,**/  +*  +  #$Using Aggregated Particles to Estimate a Cloud Height Sedimentation Process of the September ,-, ,**.,
Pyroclastic Fall at the Asama Volcano Eruption
Marekazu OHNOῌ, Shuji YAMAKAWAῌ, Masayuki O’ISHIῌῌ, Kou TAKAHASHIῌῌῌ,Tatsuyuki UENOῌῌῌῌ and Takafumi IDAῌA cloud height generated by a volcanic eruption reﬂects the immensity and/or magnitude of the eruption ;thus a measuring of the height’s temporal variation during the event is very signiﬁcant in judging whether theactivity will become violent or decline. However, when a volcanic eruption occurs during bad weather, we musttake information about the cloud’s height by means of the pyroclastic deposits. In general, the total time taken
for pyroclastic materials to be ejected and deposited at a given distance from the source vent can be divided into
three parts as follows : the time for the eruption cloud to ascend and reach its neutral buoyancy level (T+) ; the time
for the pyroclastic materials to be transported laterally by the eruption cloud (T,) ; and the time for pyroclastic
materials to fall and be deposited on the ground (T-). Since T- can be calculated from the settling velocity of
pyroclastic materials, if the time that the pyroclastic materials fell at a given locality was observed and a given
value for T+ is assumed, the most suitable wind velocity to explain T, can be determined. Thus the height at which
pyroclastic materials separate from the eruption cloud can be determined by using the vertical proﬁle of windvelocity around the volcano. These ideas were applied to the eruption occurred at +3 : .. (JST) on September ,-,,**., at the Asama volcano, which produced a pyroclastic fall deposit with a minimum weight of 1.,%+*0 kg.
Because this eruption occurred in bad weather, the pyroclastic materials fell as mud raindrops that were aggregate
particles saturated by the rainwater. Based on the depositional mass, the number of impact marks of the mudraindrops in the unit area, and the apparent density and the equivalent diameter of these drops during their fall wasestimated to be ,.,ῌ-.+ mm, which is consistent with the grain-size distribution of pyroclastic materials. According to some experienced accounts, mud raindrops several millimeters in diameter fell at ,* : *&+/0ῌ2//* '()*+ -ῌ,/ῌ.*,-.#-/0123456-6
Department of Geosystem Sciences, College of Humanities and Sciences, Nihon University, -ῌ,/ῌ.*,Sakurajosui, Setagayaku, Tokyo +/0ῌ2//*, Japan.
&+3,ῌ*-31 789:;< +ῌ+=>?@--A#-BC60#6-DE
Department of Geography, Tokyo Metropolitan University, +ῌ+, Minamiosawa, Hachioji +3,ῌ*-31, Japan.
&-3*ῌ20*+ F,:G -ῌ+ῌ+HI--AJ-KBC601LM23456DE
Division of Environmental System Sciences, Graduate
School of Science and Technology, Shinshu University, -ῌ+ῌ+, Asahi, Matsumoto -3*ῌ20*+, Japan.

&0/1ῌ2/*+ NO:P)QRST +ῌ+NO--AUV6-BC601WX23456-DE
Department of Earth and Planetary System Sciences,
Graduate School of Science and Technology, Kobe
University, +ῌ+, Rokkodai-cho Nadaku, Kobe 0/1ῌ
2/*+, Japan.
YZ[ : &+/0ῌ2//* '()*+ -ῌ,/ῌ.*,-.#-/UV6-BCZ
Present : The Institute of Natural Sciences, Collegeof Humanities and Sciences, Nihon University, -ῌ,/ῌ.*, Sakurajosui, Setagayaku, Tokyo +/0ῌ2//*, Japan.
Corresponding author : Marekazu Ohnoe-mail : mare@chs.nihon-u.ac.jp)BµOsj¶·s)<¸ks¹ ºsBlksu[»¼536in the Kitakaruizawa area (about 3 km north-northeast from the source). Assuming ,ῌ/ minutes for T+ and++./ῌ+,.* m/s of average lateral wind velocity, the height at which the mud raindrops separated from theeruption cloud can be estimated at -,.-*ῌ-,20* m (-,0+* m on average). From this conclusion, thetransportation and depositional process of the pyroclastic materials generated on September ,-, ,**., at theAsama volcano can summarized as follows : the explosion occurred at +3 : .. and the eruption cloud rose to-,0+* m while blowing ,..3 km downwind from the source. The cloud moved laterally for ../+ km withgenerating raindrops. At +3 : /., mud raindrops separated from the cloud 1.* km north-northeast from thesource, then fell to the ground at ,* : *- after being blown ,.* km downwind by a lateral wind.
Key words : cloud height, pyroclastic fall deposit, mud raindrops, sedimentation process, the Asama volcano

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