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38 9 : ;,**3 . - / 3 012 ,**3 . 0 / ,. 01!Eruption History of Oakan Volcano, Eastern Hokkaido, Japan
Jun-ichi TAMADAῌ,ῌῌ and Mitsuhiro NAKAGAWAῌ
Oakan volcano is one of the post-caldera volcanoes of Akan caldera, and its eruptive history has not yet beenclariﬁed well. In order to reveal the structure of volcanic ediﬁce and eruptive history with possible age data, wecarry out not only geological survey but also tephrochronological study around the volcano. We identify +*tephra units, nine of which are wide-spread tephras from other volcanoes in Hokkaido and Baitoushan volcano.
Only one tehpra unit (Oafa) from Oakan volcano has been recognized, but the other nine tephras can be used as
good time markers for understanding the activity of the Oakan volcano.
The volcanic activity can be divided into two main stages : the early stage (E stage) and late one (L stage).
Although most of the ediﬁce of the E stage is covered by eruptive materials of the L stage, several lava ﬂows aredistributed on the southern ﬂank. After the formation of the ediﬁce, sector collapse of its southern part occurredand formed a debris avalanche deposit on the ﬂank. The ediﬁce and the debris avalanche deposit of the E stage
is covered by Meakan tephra (NaPS : ca. +.- ka), which indicates that the activity of the E stage had terminatedbefore +.- ka. After a certain period of dormancy, the activity of the L stage started, which can be divided into
two sub-stages : L-+ and L-,. During the L-+ stage, lava e#used from four crater areas, and the Futatsudake cone
was formed at one of these areas. Based on petrological features of these lavas, the L-+ stage could be divided
into L-+-+ and L-+-, groups. Oafa tephra layer recognized at the ﬂank was derived from the cone (L-+-,) judging
from its isopach and isopleth maps, and also from its petrological features. The tephra layer is sandwitchedbetween Ma-f (ca. 0.0 ka) and Ta-c, (ca. ,./ ka) tephras. Considering thickness of soils between Oafa and these
two layers, we estimate that eruption age of Oafa is about / ka. Although Oafa can be correlated with the activityof L-+-, group, it seems that the activity of L-stage began around / ka, because there existed no obvious timeinterval between L-+-+ and L-+-, groups. Thus, the dormancy period between the early and late stages can beestimated to be about 2*** years. In the L-, stage, eruption centers moved northeast to construct the summit(Oakandake) pyroclastic cone, in which four craters were formed. At the same time, lava ﬂows repeatedlye#used from the cone to widely cover the north to southeast ﬂank. Based on the location of eruption centers andthe time sequence of these lava ﬂows, the activity of L-, stage can be divided into four groups : L-,-+ to L-,-..
The youngest crater at the summit cone of L-, stage was formed before + ka, because it is covered by Ma-b tephra(ca. +.* ka). This suggests that the latest magmatic activity of L-, stage occurred before + ka. Although noeruptive activity was recorded, weak fumaloe activity at the north crater on the mid ﬂank was reported. Our
study reveals frequent eruptions of Oakan volcano during Holocene, and suggests that the volcano must be
considered as an active volcano.
Key words : Oakan volcano, Akan caldera, tephrochronology, eruption history, active volcano

 +*  2 *0*ῌ*2+* ! "#$%&'Department of Natural History Sciences, Graduate
School of Science, Hokkaido University, N+* W2Kitaku, Sapporo *0*ῌ*2+*, Japan.
()* //*ῌ**+- + ,- +  --ῌ,*Mont Bell Co. Ltd., Shinmachi +, Nishiku, Osaka//*ῌ**+-, Japan.
Corresponding author : Mitsuhiro Nakagawa
e-mail : mnakagawa@mail.sci.hokudai.ac.jp