Eruptive History of Ohachi Volcano, Kirishima Volcano Group, Southern Kyushu, Japan

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1 /, (,**1) + +,+,**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 that the 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-++) inthe 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 a pyroclastic 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 from AD +1+2 up to present (,3* years). This indicates that more than 2* of the total amount of magma was produced in the early stage, while the remaining,* 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 had grown mainly during the two eruptive stages of KzT and ThT, which are comparatively large-scale sub-plinian eruptions (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 + +,.,/ m ,,1, - DIA Consultant Co., Ltd., Sabo and Disaster Prevention Center,,,1, - Yoshino-cho, Kita-ku, Saitama , Japan. 2+. *+2* Department of Earth System Science, Faculty of Science, Fukuoka University, Nanakuma, Jonan-ku, +322 Fukuoka 2+. *+2*, Japan. 23* **0/ +-,+--/ Department of Earth and Environmental Sciences, Faculty of Science, Kagoshima University, +,+ -/ Korimoto, Kagoshima 23* **0/, Japan. Corresponding author: Masaaki Tsutsui m.tsutsui@diaconsult.co.jp

2 2 Fig. +. Map showing the location and topography of the southern part of Kirishima volcano group. Contour interval is +** meters. +,/1. m (Fig. +) 0.2 calkabp;,**, +322 /** m,** m ,- +,,**/ (+3,,) (+3/1) (+3.2) (+30/) (+300) (+303) (+322) Imura (+33,) Inoue (+33-) (+32/) +,+** 3** m +,+** 3** m +313 ; Kobayashi et al., +32+ ; +32. ; Kobayashi and Kagi-yama, +322

3 3 Table +. Comparison of tephra layers with previous studies.,, + +/ (AsT) (KzT) (MsT) (ThT) + ++(TgT-+ TgT-++) +/ Table + Fig.,a,b Fig. - Inoue (+33-) AsT, Oh+ Oh, Oh+ Oh, (1,-** cal BP) (Inoue +33-) / 0 +. C.,1**,,,** cal BP AsT TgT-++ Sm-KP ; +33+ (Figs.,a,,b) Sm-KP + m

4 4 Fig.,a. Stratigraphic columns showing the lateral correlations of representative tephra (Locality number +-+/). AsT: Araso tephra, KzT : Katazoe tephra, MsT : Miyasugi tephra, ThT: Takaharu tephra, TgT-+ TgT-++ : Takachihogawara tephra +-++, MiP : pumice of Miike and Sm-KP : Shinmoedake-Kyoho pumice (AD ). Fig.,b. Stratigraphic columns showing the lateral correlations of representative tephra (Locality number +0-,+). ThT: Takaharu tephra, TgT-+ TgT-++ : Takachihogawara tephra +-++, Sm-KP: Shinmoedake-Kyoho pumice (AD ). Locality number are referred to Fig.,a.

5 5 Fig... Isopleth map of Araso tephra (AsT). Values in centimeters. Dashed lines indicate the limit of distribution of tephra, and dotted lines indicate the isopleth. Fig. -. Idealized columnar section of tephra layers from Ohachi volcano (not to scale)., km (,**/), + + (AsT) (AsT) (Fig..) KzT, / cm Fig.,a + + cm *./ cm../ cm AsT AsT, +, (KzT) (KzT) (KzS) (KzA) (Fig. /) KzS Fig.,a / +3 cm *./ cm *./ cm + cm - - KzA Fig.,a +* KzS - cm KzA KzS KzA Inoue (+33-) (+331) KzA KzA KzA KzS KzA, + - (MsT) (MsT) (MsA)

6 6 Fig. /. Isopach maps of KzS (scoria) and KzA (ash) of Katazoe tephra (KzT). Values in centimeters. Dashed lines indicate the limit of distribution of tephra, and dotted lines indicate the isopach. Fig. 0. Isopach maps of MsA (ash) and MsS (scoria) of Miyasugi tephra (MsT). Values in centimeters. Dashed lines indicate the limit of distribution of tephra, and dotted lines indicate the isopach. MsA (MsS) (Fig. 0) MsS MsA Fig.,a 1 +0 cm MsS, Fig.,a 1 1 cm + cm, +. (ThT) (ThT) ThT-a ThT-b ThT-c - (Fig. 1) ThT-a ThT-b ThT-c Fig.,a +-,, cm / cm.. cm ThT-a + cm - ThT-b - cm +* cm ThT-c +./ cm ThT-a ThT-b ThT-b ThT-c ThT-a ThT-c - ThT,, 0

7 7 Fig. 1. Isopach maps of ThT-a, ThT-b and ThT-c of Takaharu tephra (ThT). Values in centimeters. Dashed lines indicate the limit of distribution of tephra, and dotted lines indicate the isopach., + / + ++ (TgT-+ TgT-++) + ++ (TgT-+ TgT-++) ThT Sm-KP TgT-, +* Fig.,b +1 KzT MsT ThT (TgT) Fig. 2 TgT-+ TgT-++ TgT-/ TgT-1 TgT-3 TgT-++ TgT-+ Fig.,a, -0 cm MsA TgT-+ TgT-, cm TgT-, - Fig.,a, / cm *./ cm ThT-a ThT-c - TgT-- Fig.,a, +, cm TgT-+ TgT-- TgT-. - TgT-.a TgT-.b TgT-.c - TgT-.a TgT-.b TgT-.c Fig.,a +/ *./ cm +- cm. cm TgT-.a, cm TgT-.b *./ cm TgT-.c MsA TgT-+ TgT-/ TgT-0 TgT-1 +, cm

8 8 Fig. 2. Isopach maps of TgT-+, TgT-,, TgT--, TgT-.a, TgT-.b, TgT-.c and TgT-2, and isopleth map of tephra layers from TgT-3 TgT-++of Takachihogawara tephra (TgT). Values in centimeters. Dashed lines indicate the limit of distribution of tephra, and dotted lines indicate the isopach or isopleth.

9 º» ¼!" #$ %& ' TgT-2 %()*+,-* -./ 0 1*. R / (KjL) ge # ' g %RS & I VE i a I (CrL) ' F % ThT ª 5«g?@AB J #KLJM eohsf ; +322f. ' Table, =* 56 >?@ABC9DE & ' Fig.,a H / cm./ I (SnL) (JdL) (TgL) 23* ῌ :;< G, % 9 / ±² x" ³ µ n Fig. 3 N OPQ RS ' TgT-3 TgT-+* TU TgT-++ % V Wῌ4 56 RS ¹ µ E & ' 2 %!" #$ V WXY - %& TgT-++ Comparison of lava flows with previous studies. Z[ & \] ^ _ ` a b %cd. efig.,b TgT-2 Sm-KP Table,. ' TgT-3+TgT-++ h ija FG +1f' b g kl TgT-3+ 9m n g' TgT-3+TgT-++ % op 22qr s/t gu I J ov w RS ' &x IyE zvh +./ km F G %{m +* cm }W ',ῌ, ΐ ῌ ῑῐῒ ~ % RSa W Fig. 3. Distribution of lava flows and pyroclastlc flow deposit from Ohachi volcano. Their stratigraphic positions are also shown in the idealized tephra section. Base map is + : /*,***-scale topographic map of Kirishimayama published by the Geographical Survey Institute of Japan.

10 10 (+322) - Oh+ Oh,, + Fig. +* K, O-SiO, SiO, wt AsT /*-/+ wt SnL TgT-3 TgT-++ CrL /2-0* wt ThT-b 0, wt /+-/0 wt Fig. +*. Whole-rock K, O-SiO, diagram. /3-0+ wt (+3,,) (+3/1) (,**+) +/,/,*** +/+*,*** +32/ +/-*,*** +333 Fig.++ +/,/,***,, + (KjL) (KjL) 0 km / m KjL MsT KzT Fig. ++. Schematic diagrams showing geomorphic development of volcanic edifices around Ohachi volcano. Eb: Eboshidake, Yn: Yunotanidake, Ss: Shishikodake, Yd: Yatake, Ry : Ryuodake, On: Onamiike, Ot : Ohatayama, Oh: Ohachi, Nk: Nakadake, Tk: Takachihonomine, OTk: Old-Takachiho, Sm: Shinmoedake, Ft : Futagoishi.

11 11,,, (SnL) (SnL) /./ km 3/* m / m +* m,*,/ m SnL MsA SnL MsA SnL MsA SnL MsS MsS MsA SnL MsS,, - (JdL) (JdL)../ km /m JdL (OhSf) OhSf ThT-c,-,-0 JdL ThT,,. (TgL) (TgL),./ km, m + m TgL TgT-- TgL TgL / +* m ThT TgT-,. +, TgL TgT-,,, / (CrL) (CrL),**/ SiO, wt /3.1- K, Owt +.2* ; +300 CrL CrL,, 0 (OhSf) (OhSf) +,+** +,*/* m cm, m. m OhSf OhSf ThT-b ThT-c ThT-c OhSf / m +./ km 10* m OhSf, - m, km, - OhSf +./ km - km JdL (Fig. 3) JdL OhSf +320a JdL OhSf OhSf +,+0* m OhSf, m (Fig. 3) OhSf OhSf

12 12 Fig. +,. Sketches of the crater wall of Ohachi volcano. Ed-L (Lower unit), Ed-M (Middle unit) and Ed-U (Upper unit). Ed-M is subdivided into four units; from the bottom, Ed-M+ to Ed-M.. - (Ed-L) (Ed-M) (Ed-U) - (Fig. +,) - + (Ed-L) (Ed-L) +/,* m Ed-L Ed-L Ed-M -, (Ed-M) (Ed-M). Ed-M+ Ed-M. Ed-M+ Ed-M. +, , Ed-M+ +/,* m +* +/ m, m Ed-M, +/,* m -* cm Ed-M- +* +/ m Ed-M,, Ed-M. / m

13 (Ed-U) (Ed-U, Ed-U +/,* m Ed-M Ed-U Ed-U -. Ed-U Ed-M+ Ed-M.,. m + - m Ed-M Ed-M Ed-U (TgL; Fig.+,).. + Ed-L Ed-M Ed-U (KzT) (ThT) (TgT) (Fig. +-) Ed-L Ed-M Ed-U Ed-M. + + (Ed-M) Ed-M Ed-M+ Ed-M.. Fig. +-. Correlation of the proximal to distal tephra layers of Ohachi volcano. Ed-L (Lower unit) corresponds to KzT, Ed-M (Middle unit) to ThT, and Ed-U (Upper unit) to TgT, respectively. Ed-M+ to Ed-M. correspond to ThT-a to ThT-d.

14 14 Ed-M, ThT ThT-b ThT-b Ed-M, Ed-M+ ThT-a Ed-M- Ed-M. Ed-M. / m Ed-M- ThT-c Ed-M. ThT ThT-a ThT-c ThS-d. +, Ed-U). + + Ed-M ThT Ed-U TgT-+ TgT-++ Ed-U Ed-M (TgL) TgL Ed-U TgT-, Ed-U TgT-+ TgT-+ -* cm TgT-.a TgT-.b TgT-.a, - - TgT-, TgT-/ TgT-++ TgT-+ - / cm TgT-3 TgT Ed-L) Ed-L,* m Ed-L AsT KzS MsS - AsT: KzS : MsS, :.2 : 1. / Ed-L KzS AsT MsS TgT-, TgT-.b TgT-, TgT-.b / m AsT MsS., Fig. +. : : SiO, wt. + MsS SnL AsT MsT SiO, (/* /2 wt ) TgT-+ TgT-3 TgT-++ SiO, wt (/- 0* wt ) ThT ThT-a ThT-d SiO, wt (/, // wt ) SiO, Walker (+31-) KzS MsS ThT TgT-, TgT-.b, +3// +320b MsA TgT-+ TgT-- TgT-/ -, MsS TgT-. TgT-/ TgT-2 KzS ThT

15 15 Fig. +.. Variation of Whole-rock SiO, content of the erupted material on the crater wall of Ohachi volcano and distal area. Br: breccia. Date of CrL are taken from Shinno (+300)... (+32/) +. C +331 ; +33, ; +332 ; Okuno et al C Table - +. C INTCAL32 (Stuiver et al., +332) Oxcal ver. -.+* (Bronk Ramsey, +33/ ;,**+) (+331).. + AsT KzT ThT KzT ThT +. C ThT (+3/1) ; +33, ; +322 ; +321 ; +32/ ThT 3 +* +330 ; +331 Okuno et al. (+332) +. C KzT ThT +,-/ (+331) KjL JdL KjL JdL KzT ThT KjL +. C (Table -) Okuno et al.(+332) KzT ThT ,-/ AsT KzT MsT ThT

16 16 Table -. Radiocarbon dates for the erupted material of Ohachi volcano. AsT KzT /*,** AsT KzT +** 1**.., MsT MsT KzT 122 ThT +,-/ (,**,) Okuno et al. (+332) +. C MsT +* (+331) MsT SnL 2** +*** +. C MsT +*** +,** SnL 2** +*** MsT (Table -). - MsT MsT ;,**- MsT 30* +++, , ++2-, +++, , / 0 km (Fig. +) MsT

17 17 Table.. The estimated eruption age of Takachihogawara tephra (TgT-+ to TgT-++). 3** ++** +***.. - TgT-+ TgT-++ TgT-+ TgT-++ ThT +,-/ Sm-KP //.,-,* TgT-+ TgT TgT-+ TgT C TgT-, TgL +-/* TgT-+ TgT-++ Table. TgT-+ +. C (10* 3* BP ; Okuno et al, +332) +,/* +-/* TgT-, +. C (/-*.* BP) TgL +. C (/,*.* BP) +-/* TgT-- +. C (03*.* BP) +-/* +0/* TgT-. +. C (,,*.* BP) +0/* TgT-/ TgT-++ TgT-1 TgT-2 TgT-3 +. C +0/* +1**. /. Fierstein and Nathenson (+33,) (TgT-. a) *./ +* 0 m - (TgT-/ TgT-1 TgT-3 TgT-++) *./ +* 0 m - KzT ThT +/+*,*** +32/,/ m,/m DRE (dense rock equivalent) Table / +./ g/cm - *.2 g/cm - +.* g/cm - +., g/cm -,./ g/cm - +., +* 2 m - *.3 +* 2 m - *./ +* 2 m -,./ +* 2 m (AsT KzT ThT) (TgT-+ TgT-++) TgT-+ TgT-++ (TgT-+ TgT--) (TgT-, TgT-.) TgT- / TgT-/ TgT-1

18 18 Table /. Volume (DRE) of erupted material of each eruption of Ohachi volcano. Eruption age is also shown. TgT-3 TgT-++ 1** +,-**,.* +* 2 m - /ky (Fig. +/) +* 1 +* 0 m - (KzT ThT) +* 0 +* / m - (TgT-+ TgT-.) +* / m - (TgT-/ TgT-++) AsT ThT /.* TgT-+ 11* *.. +* 2 m - /ky..* +* 2 m - /ky +/+*,**/. 1 Fig. +0 +/ +*,*** +32/,/* m -* m 3 / KzS +/* m ThT 2* m TgT,* m KzS ThT, ThT-a ThT-b ThT-c +* 1 +* 0 m - MsS TgT-.b +* / m - TgT-, +* / m - TgT-

19 19 Fig. +/. Cumulative magma volume (DRE) versus calibrated age of each eruption of Ohachi volcano. Light: tepura, upper middle: volcanic edifice, lower middle: lava and left : total erupted material. Fig.+0. Cross section of the Ohachi volcano., (TgL) ; Bacon * 0 m - (+322) +,+** m ;TgL (Fig. +1) ThT-c TgL,**. KzS KjL

20 20 Fig. +1. Distribution of tephra and lava deposits around Ohachi crater. ThT JdL ThT-c / + AsT KzT MsT ThT TgT-+ TgT-++ +/ KjL SnL JdL TgL CrL / KjL KzS SnL MsS JdL ThT TgL TgT-,, +,-** AsT,./ +* 2 m - 2 AsT ThT /.* 3 TgT-+ 11* +/+* Sm-KP KzS ThT,, +* 1 +* 0 m - +* / m -. ThT ThT-a ThT-b ThT-c Maria Hannah Mirabueno / 0,/.*/22 *.0.*1+*

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