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Newly Found ..1 ka Pyroclastic Flow Deposits on the Northwestern
Foot of Tokachi-dake Volcano, Central Hokkaido, Japan andReexamination of the Eruptive Activity During Holocene
῍῍Shinya FUJIWARA῍,῍῍, Yoshihiro ISHIZUKA῍῍῍, Toshitsugu YAMAZAKI῍
and Mitsuhiro NAKAGAWA῍
Tokachi-dake volcano restarted eruptive activity at the Ground crater in middle Holocene after longdormancy of nearly +* ky. The activity at the crater (Stage +) was the most explosive and voluminous one in thevolcano during Holocene. We newly found a pyroclastic ﬂow deposit beneath already recognized pyroclastic ﬂow
deposit (Gﬂ-+ : Ground Crater pyroclastic ﬂow deposit +) of the Stage-I, intercalated with lahar deposits and thinsoils. The deposit is composed of gravel-sized blackish blocks, pumices, banded pumices and altered blocks withyellowish to reddish brown colored ﬁne sand matrix. Progressive thermal demagnetization experiments for theblackish blocks and the pumices show that the direction of the remnant magnetization of the deposits is close tothat of the geocentric axial dipole and did not change up to blocking temperature of about /2* , indicating thatthese blocks and pumices are essential magmatic materials. Thus, the deposit can be considered as pyroclastic
ﬂow one. However, this is characterized not only by the heterogeneous distribution of these essential materials
in the deposit but also by heterogeneous matrix. These features and abundant accidental altered blocks suggestthat generation of the pyroclastic ﬂow might be related with sector collapse of the volcanic ediﬁce. This isconsistent with the topographic feature of the Ground crater. The petrography and the whole-rock chemicalcompositions of these essential materials are similar to those of the Stage ῌ. This suggests that the newly foundpyroclastic ﬂow must be related to the initial eruptive activity of the Stage ῌ. New +.C ages for the deposit are
in ..1 ka, indicating that the activity of the Stage ῌ began much earlier than previously recognized age (-.- ka).
Finding of the pyroclstic ﬂow deposit, named as Gﬂ-*, suggests that the Stage ῌ had continued and been derived
from the same magma system for more than +.** years. However, considering a long interval between Gﬂ-* andGﬂ-+, the Stage ῌ might be divided into two sub-stages, Stage ῌ-* and Stage ῌ-+.
Key words : Tokachi-dake volcano, pyroclastic ﬂow, sector collapse, thermal demagnetization experiments+ῌ ῐ ῎ ῑ ῏QRS -STUVWSX/YZ[T\ ]^ _`abcdefQ -.;g NE-SW hiXjV+31+ WSg-klmWngopQ abcgqrst abdeugv;Xwxys defgop

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*0*ῌ*2+2  +*  2  !"#$%&'(/0Department of Earth and Planetary System Sciences,Graduate School of Science, Hokkaido University,
N+* W2 Kita-ku, Sapporo *0*ῌ*2+*, Japan.
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FG2HIJK
Present address, Kokusai Kogyo Co., ,ῌ,.ῌ+, Harumi-chou, Fuchu +2-ῌ**/1, Japan.
 -*/ῌ2/01 )*+, +  +ῌ+ )*+-. 11234567 89:;<Institute of Geology and Geoinformation, Geological
Survey of Japan, AIST, Central 1, Higashi +ῌ+ῌ+,
Tsukuba, Ibaraki -*/ῌ2/01, Japan.
Corresponding author : Mitsuhiro Nakagawa
e-mail : mnakagawa@mail.sci.hokudai.ac.jp