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,**2 F 0 G +- HIJ ,**3 F + G +- HI*Tephrostratigraphy and Eruption Style of Mashu Volcano,During the Last +.,*** years, Eastern Hokkaido, JapanHiroshi KISHIMOTOῌ῍, Takeshi HASEGAWAῌ, Mitsuhiro NAKAGAWAῌ and Keiji WADAῌῌThe eruptive history of Mashu volcano, located at the southeastern rim of Kutcharo caldera in eastern
Hokkaido, Japan, has been divided into three stages : stratovolcano building, caldera-forming and central conebuilding. Tephrostratigraphy and eruption styles of the latter two stages were reinvestigated in conjunction withpetrological analysis. We found new evidence of several eruptions from the volcano. These data, combined withrecent +.C ages and the presence of wide-spread tephras, allow us to evaluate the temporal evolution of eruptive
activity and styles of Mashu volcano during the last +.,*** years.
After the formation of the stratovolcano, activity of the caldera forming stage started with plinian eruption(Ma-l) about +. cal ka. Approximately 1./ cal ka, climactic caldera-forming activity began with a phreatomagmaticeruption (Ma-j) followed by plinian falls (Ma-ig), and a catastrophic pyroclastic ﬂow (Ma-f) occurred resulting
to the formation of the summit caldera, 1.//./ km in diameter. Total volume of the climactic eruption deposit
is estimated to be +2.0 km-. Temporal variation of pumice/lithic fragment and white (silicic)/gray (maﬁc)pumice ratio with eruption sequence suggest withdrawal of a zoned magma chamber with more silicic magma
overlying more maﬁc one through the newly opened and enlarging vent. In the central cone building stage, atleast eight eruptions (Ma-e, Ma-e῎, Ma-d, Ma-c.-c+ and Ma-b, in ascending order) have occurred repeatedly
during the last 0,*** years. The latest eruption (Ma-b : ca. *.3 cal ka) was the largest one in this stage.
Although most of the tephra layers in this stage are composed of pyroclastic fall deposits, thin pyroclastic ﬂow
deposits can also be recognized from the two eruptions (Ma-e and -d).
Juvenile materials in the Mashu tephras are commonly pyroxene dacite (SiO,0.-1, wt.ῌ, K,O*./῍*.1 wt.
ῌ in whole-rock compositions). The pumice of the caldera-forming stage are nearly aphyric (+0 wt.ῌ),
whereas those of the central cone building stage are more porphyritic (+-,. wtῌ). They can be also
distinguished in SiO,-oxides diagrams.
Most of Mashu tephras consist of gray ﬁne ash layers including blocky lithic/pumice fragments andaccretionary lapilli, indicative of magma-water interactions. In the case of large eruptions, such as the climactic
caldera-forming eruption (Ma-jf) and Ma-b, eruptive styles changed from the wet to dry due to increase of themagma/water ratio. The long-term magma discharge rate was *.2 and *.- km- DRE/ky during the last +.,***
and 0,*** years, respectively. Mashu volcano can be interpreted as one of the most productive and active
volcanoes in Japan during Holocene.
Key words : Mashu volcano, tephrostratigraphy, zoned magma chamber, magma water interaction, active volcano
*0*῍*2+*   +* !" 2 #$%&'('()*(+,)-./0(12Department of Natural History Sciences, Graduate
School of Science, Hokkaido University, N+* W2,Kita-ku, Sapporo *0*῍*2+*, Japan *1*῍20,+ @  2A 3%&BC'(@ D=(+,EEarth Science Laboratory, Hokkaido University ofEducation at Asahikawa. Hokumon-cho 3, Asahikawa*1*῍20,+, Japan.

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Department of Disaster Prevention, Asia Air Survey
Company with Limited Liability, ,῍,, Manpukuji-+,Asao-ku, Kawasaki ,+/῍***., Japan
Corresponding author : Takeshi Hasegawa
e-mail : hasegawatakeshi@mail.sci.hokudai.ac.jp