ῒΐ  /,  (,**1)
 +  +ῌ,+ ῕ ῔῔ῑ῎ῒ῍῏ῌΐ῍ῐ   ,**0  .  ,/  ,**0  +,  0 Eruptive History of Ohachi Volcano, Kirishima Volcano Group, Southern Kyushu, Japan
Masaaki TSUTSUI῍, Mitsuru OKUNO῍῍ and Tetsuo KOBAYASHI῍῍῍The Kirishima Volcano Group is composed of many Quaternary volcanoes occupying an area about 0** km,.
Ohachi Volcano is a small, active stratovolcano situated at the southeastern part of the Kirishima Volcano Group.
It has a considerably large crater compared to the size of its volcanic ediﬁce, with thick agglutinates that crop out
on the wall of the crater.
We studied the eruptive history of Ohachi Volcano by tephrochronological method, and recognized +/ tephra
deposits around the volcano. Geological and archeological data and documentary records of eruption indicate thatthe oldest tephra from the Ohachi Volcano is the Araso tephra (AsT) erupted in ca. AD 1**. All other tephra
deposits were erupted in historic time such as Katazoe tephra (KzT) in AD 122, Miyasugi tephra (MsT) in ca.
AD +***, Takaharu tephra (ThT) in AD +,-/, and Takachihogawara tephra +-++ (TgT-+ to TgT-++) in the
period of AD +,/*-+1**. Thus, the Ohachi is a very young volcano at +-** years old.
We recognized two types of magmatic eruption : sub-plinian and vulcanian. Lava emission took place at least
four times in association with sub-plinian eruptions. Among these eruptions, ThT is the most voluminous, and apyroclastic ﬂow was also generated. The total volume of erupted magma (volcanic ediﬁce included) was calculated
to be about ,./+*2 m- (DRE). Eruption rate was not constant throughout the eruptive history of Ohachi
volcano, that is, the eruptive history is conveniently divided into three stages by the eruption rate : the early stage
from AD 1** to +,-/ (ca. /.* years), the middle stage from AD +,-0 to +1+1 (.2* years), and the late stage fromAD +1+2 up to present (,3* years). This indicates that more than 2*ῌ of the total amount of magma wasproduced in the early stage, while the remaining ,*ῌ of magma was erupted in the middle stage. Although there
are many recorded eruptions during the late stage, it is di$cult to ﬁnd any tephra deposits around the Ohachi
Volcano, which suggests that the scale of volcanic eruption is progressively diminishing through time.
The volcanic ediﬁce of Ohachi is composed mainly of succession of thick tephra layers, some of them changed
to agglutinates. Correlation of the proximal deposits to the distal tephra suggests that the volcanic ediﬁce hadgrown mainly during the two eruptive stages of KzT and ThT, which are comparatively large-scale sub-plinianeruptions (order of +*1 m- to +*0 m- ). Hence, the most important process in generating agglutinates is rapid
accumulation of voluminous tephra around the crater which are generated during sub-plinian eruptions.
Key words : Ohachi volcano, Kirishima volcano group, eruptive history, sub-plinian eruption, agglutinate

0123

=>

A+ῌ ῐ ῎ ῑ ῏/%456789:+;<2 "?@BC45 DE !" +,.,/ m  #$%&'()*+,-.

+322 7F:GHIJ.45KL.M "?@BC45N--+῍20-2 OP=3QRST U ,῍,1,῍VWXYZ[\]^_`a^bcddef^ag
DIA Consultant Co., Ltd., Sabo and Disaster Prevention Center, ,῍,1,῍- Yoshino-cho, Kita-ku, Saitama--+῍20-2, Japan.
N2+.῍*+2* hiQj8Skl 2῍+3῍+himnn:opqrnr
Department of Earth System Science, Faculty of Science, Fukuoka University, 2῍+3῍+ Nanakuma, Jonan-ku,Fukuoka 2+.῍*+2*, Japan.
N23*῍**0/ #$%Qst +-,+--/#$%mnn:opu*rnr
Department of Earth and Environmental Sciences, Faculty of Science, Kagoshima University, +῍,+῍-/ Korimoto, Kagoshima 23*῍**0/, Japan.

 

Corresponding author : Masaaki Tsutsui
e-mail : m.tsutsui@diaconsult.co.jp