// (,*+*), 23 +*1,**3,1,*+*, Eruption Age and Sequence of Ogurayama Lava Dome at Towada Volcano, Northeast Japan Arc Takashi K UDO Towada volcano is an active volcano located in the northern part of the Northeast Japan arc. Ogurayama Lava Dome (OLD), which is a dacitic lava dome located near the center of Towada volcano, has been regarded as a product of the latest eruptive episode A. In this paper, the author reports that the OLD is older than previously thought and that it was formed at the end of eruptive episode D. The OLD overlies pyroclastic deposits of the eruptive episode E and is overlain by pyroclastic deposits of the eruptive episode C. These stratigraphic relations restrict the eruption age of the OLD to 3,. 0,. cal kyr BP. Within this time interval, two eruptive episodes (D and D) are recognized as tephra fall deposits in the distal area. The distribution of Herai Ash from the eruptive episode D shows that the source vent is located in the vicinity of the OLD. Furthermore, the petrological features of the OLD closely resemble those of the Herai Ash. These observations indicate that the OLD is the product of the eruptive episode D ( 1/. cal kyr BP). The probable eruption sequence of the eruptive episode D is as follows. Intermittent phreatomagmatic eruptions occurred in the earliest stage. These eruptions produced the lower part of the Herai Ash. Subsequent lava eruptions formed the OLD and accompanied intermittent vulcanian eruptions produced the main part of the Herai Ash. The source vent of the eruptive episode A is not the Ogurayama, because the Ogurayama was formed before this episode. Since the only crater topography currently recognized in the Towada volcano is the Nakanoumi crater (NC), the source vent of the eruptive episode A is considered to be the NC. Since the NC has been the main crater throughout the postcaldera stage, future eruptions will probably occur in the NC. There is the current NC at the bottom of the lake of,* m in depth. A detailed examination of probable eruption style in the future will be required for predicting volcanic hazard of Towada volcano. Key words : Towada volcano, Ogurayama Lava Dome, stratigraphy, eruption age, eruption sequence, eruptive episode D, eruptive episode A + ++ km A, B, C + a, b (Hayakawa, +32/ ) Hayakawa ( +32/ ) ( +320 ) (Hayakawa, +32/ ) ( +320) + a, c // ka (// +/./ ka) +/./ ka Hayakawa, +32/ ; +30, ; */ 2/01 + + + 1 Tsukuba, Ibaraki */ 2/01, Japan. AIST, Institute of Geology and Geoinformation, Geo Corresponding author: Takashi Kudo logical Survey of Japan, Central 1, Higashi + + +, email: kudotaka@aist.go.jp
90 Fig. +. (a) Topographical map of Towada volcano showing the locality of outcrops. (b) Location map of Towada volcano. (c) Topographical map of Ogurayama Lava Dome showing a survey route and the locality of outcrops. A part of + :,/,*** topographic map Towadakotoubu published by the Geographical Survey Institute is used. + (a) (b) (c), / +
91 Hayakawa, +32/ VEI. / Hayakawa, +32/ ;,**2 ;,**1 + +33 ; Kuno et al., +30. ; +320 ; +31* ; +320 ; +310 ; +32. ; +31, ( +310) Haya kawa ( +32/ ) 3+/ A +331 ;,** ; +333 ( +3,3 ) ( +31, ) +32. ; Fig.,. Eruptive history during the postcaldera stage of Towada volcano. Ages : *Machida et al. ( +32+ ) and Hayakawa and Koyama ( +332) ; **Kudo and Sasaki (,**1) ; ***Kudo et al. (,**) ; ****Kudo (,**2) ; *****Horiuchi et al. (,**1)., : * ( +32+ ) ( +332); ** (,**1) ; *** (,**) ; **** (,**2) ; *****Horiuchi et al. (,**1).,, cal kyr BP +/./ cal kyr BP (Horiuchi et al.,,**1) L (Hayakawa, +32/ ) (Hayakawa, +32/ ) Hayakawa, +32/ ; Hunter and Blake, +33/ ;,**,**. ++. 1 cal kyr BP 2**,**2 ++. 1 cal kyr BP Hunter and Blake, +33/ ; +333 ;,**1
92 Table +. Modal compositions of eruptive rocks from the eruptive episodes A to G in Towada volcano. +. AG (Hayakawa, +32/ ) + c ++ cal kyr BP,**2 G 3+/ +332 ;,2 km,, km +32+ A 2 + a (Hayakawa, +32/ ) E C, km G + a, c Hayakawa, (vol. ) +32/ ; +320 ; +310 (Hayakawa, +32/ ) + ***.*** A B +* * g + +
93 Table,. Representative wholerock major element compositions for eruptive rocks from the eruptive episodes A to G in Towada volcano.,. AG */. mm */. *//. mm + 2** +/** G 3. Lipman ( +301) X PHILIPS PW +.*. + : +* +, A B +** wt., (, s), JB+ a +* B D + vol.
94 F 0 F, m +/ m E,* cm / m F E D * cm /* cm +* m E C. m D / cm B 3 m C +* cm.* cm A +* m cm m A B.+, B C D Fig.. A stratigraphic columnar section at the crater + wall of Nakanoumi (loc. + ). Its location is shown in Fig. + c. + E (Hayakawa, + c +32/ ).. TiO MnO.+..++ + + + c BD, + c 2m A +,
95 Fig... Wholerock TiO, SiO, and MnOSiO, diagrams for eruptive rocks from the eruptive episodes C, E and G. The other eruptive rocks during the postcaldera stage of Towada volcano are plotted in the outside of these frames.. C, E G TiO, SiO, MnOSiO,,, E 2*. LG. 1 cm K,1cm BD E I ; / mm +* cm E H, cm F / K I F (Hayakawa, +32/ ) H L, J, G, E.,.,+ +, +2 / F ah 2 / / AL +, / L J G E a d f h 0* cm. cm,* cm, cm, 1/ cm / a E.* +* cm / h G L + m / b
96 h G E + cm D 0/ cm C / C + cm B m A /,* cm.,, E L F A 0 C, D F + 0 0 A C 0 1 1 Fig. /. A stratigraphic columnar section at the southeast of Ogurayama (loc. ). Fig. + c. Its location is shown in /.. + c A c e g + cm C. m C h +32
97 Fig. 0. Modal compositions of eruptive rocks from the eruptive episodes A to G in Towada volcano. 0. AG.. + c + c. + c..* cm 2 C 2. C 2. 2 2 cm.. +
98 Fig. 1. Wholerock TiO SiO and MnOSiO diagrams for the Ogurayama Lava Dome and related rocks.,,, 1. TiO SiO MnOSiO,,, / D D 2., cal kyr BP,**1 D 1./ cal kyr BP,**1 D (Hayakawa, +32/ ) Fig. 2. A sketch of a fallen tree near the summit of D Ogurayama (loc..). Its location is shown in Fig. + c. D D 2.. + c D D E 3 +* D D ++ C C. D C (Hayakawa, +32/ ) Hayakawa ( +32/ ) 3., cal kyr BP,**2 ++ E 0. + cal kyr BP,** ;,**1 C */. mm, /+ D
99 Fig. 3. Correlative stratigraphic columns for representative sections. Locations of each section are shown in Fig. + a. 3. + a
100 Fig. +*. Stratigraphic columns for Nakanosawa Ash and Herai Ash at each type locality. Locations of each section are shown in Fig. + a. +*. + a +* 3 (Hayakawa, +32/ ) ++ *.. km DRE (Dense Rock Equivalent) **2. km (Hayakawa, +32/ ) +, (,**1) ++ + a +, +* Houghton et al.,,*** ; Self and Sparks, +312 ; Walker, +32+ / mm ++ / mm ++
101 Fig. ++. Isopach maps for tephras from the eruptive episode D (upper) and D (lower). Numerals show the thickness of tephra in centimeters. A part of thickness data for Oguni Pumice (marked by *) is quoted from Hayakawa ( +32/ ). ++. D D (cm) * Hayakawa ( +32/ )
102 Fig. +,. Photomicrographs of grains in tephra samples. (a) The middle unit of Nakanosawa Ash. It contains blocky and flaky glass fragments, spongy pumices and fresh dacite blocks. (b) Thin section of the middle unit of Nakanosawa Ash (open nicol). (c) Herai Ash. Most grains are fresh dacite blocks. It contains small amount of spongy pumices. (d) Thin section of Herai Ash (open nicol). All grains are fresh dacite blocks. +,. (a) (b) (c) (d) Fig. +. Relation between the area enclosed by isopachs (S) and the thickness (T), for Nakanosawa Ash and Herai Ash. Broken lines are isovolume lines assuming a relation V+,., TS presented by Hayakawa ( +32/ ) (see text in detail). +. (S) (T) Hayakawa ( +32/ ) V+,., TS
103 Hayakawa ( +32/ ) V +33, ; Houghton et al.,,*** ; +,., TS V : T : S: Self and Sparks, +312 ; Walker, +32+ + Haya +, kawa ( +32/ ), *. + km +33/ ; +320 ;,** ; Morrissey and Mastin,,*** +32+ +.. gcm +1. gcm +.. gcm +/. gcm,/. gcm DRE **2. km ++ /, D ++ Hayakawa ( +32/ ) D + Hayakawa ( +32/ ) +/ cm Hayakawa ( +32/ ) + +* cm / cm *+0. km, +. km +, + a +32+ +/. gcm +0. gcm +0. gcm +*,./ gcm DRE *+. km +/., cm 0 D + +* ++ / cm, cm +. G / mm D +. */. wt. SiO +* +. +, / mm + +* vol. 3 2vol. A C., vol. (armored lapilli) 0 D,
104 Fig. +.. Wholerock TiO, SiO, and MnOSiO, diagrams for the eruptive rocks from the postcaldera stage of Towada volcano. +.. TiO SiO MnOSiO,,, 0 +. TiO MnO +. ++, +. +, D D D +/ 1 D * /* m
105 D Fig. +/. Schematic diagrams showing the eruption se quence of the eruptive episode D. (A) Initial A phreatomagmatic eruptions and sedimentation of the lowermost and lower parts of the Herai Ash. ( +32+ ) ( +33/ ) ( +332) (B) Initiation of lava e# usion. (C) Growth of the A + Ogurayama Lava Dome and intermittent vulcanian A eruptions that produced the main part of the Herai Ash. (D) After the eruption. A +/. D (A) (B) (C) A (D) D D +/ A,* m + E +320 ; +310 +/ A C (Hayakawa, +32/ ) B A +/ B +/ C / + +/ D *,. km DRE *. +3 km (Hayakawa, +32/ ) D *3. km DRE *,3. km F B 2 3+/ A ( +310 ) Hayakawa ( +32/ ) ( +320) ( +320) A
106 A X A +2,**1 Kano et al. ( +330) ( +33, ) no.,/ 032. 3 ( +32),2,0,1. + E Hayakawa, Y. ( +32/ ) Pyroclastic geology of Towada volcano. Bull. Earthq. Res. Inst. 0*, /*1 /3,. C ( +331), E C ( 3,. 0+. cal kyr /20*. BP) ( +332 ) +*, D..*.*1. Horiuchi, K., Sonoda, S., Matsuzaki, H. and Ohyama, M. D (,**1) Radiocarbon analysis of tree rings from a +/./cal kyr BP pyroclastically buried forest : a pilot study. Radiocarbon,.3, ++, ++,. 1/. cal kyr BP D Houghton, B.F., Wilson, C. J.N., Smith, R.T. and Gilbert, J.S. (,***) Phreatoplinian eruptions. In Encyclopedia of D Volcanoes (Sigurdsson, H. ed.), /+ /,/, Academic Press. Hunter, A.G. and Blake, S. ( +33/ ) Petrogenetic evolution of a transitional tholeiitic calcalkaline series : Towada volcano, Japan. J. Petrol., 0, +/130*/. ( +30, ) +, p. ( +33/ ).* ++3 +,.. D Kano, K., Yamamoto, T. and Ono, K. ( +330) Subaqueous A eruption and emplacement of the Shinjima Pumice, Shinjima A (Moeshima) Island, Kagoshima Bay, SW Japan. J. Volcanol. Geotherm. Res., 1+, +21,*0. ( +33),,,,.,3. / ( +320),* m no. + + +,. (,**2) E G / +3 +33. (,**1) ++0 0/00. (,**) 0*** +*3 +/+ +0/. Kuno, H., Ishikawa, T., Katsui, Y., Yagi, K., Yamasaki,
107 M. and Taneda, S. ( +30. ) Sorting of pumice and lithic Morrissey, M.M. and Mastin, L.G. (,***) Vulcanian erupfragments as a key to eruptive and emplacement mecha tions. In Encyclopedia of Volcanoes (Sigurdsson, H. ed.), nism. Japan J. Geol. Geogr., /,,,,2. Academic Press,.0.1/. ( +333 ) ( +320) /* 0331+*..2 p. (,**) ( +31, ).2,.3,/2. 1+2. (,**.) ( +310).3 012+. no., 0/ 1. (,**1) ( +32. ) no. ++ +/3 +1,. 2 1/ 2. Lipman, P.W. ( +301 ) Mineral and chemical variations ( +32+ ) within an ashflow sheet from Aso caldera, Southwestern,0 ++1 ++2. Japan. Contrib. Mineral. Petrol., +0, **,1. Self, S. and Sparks, R.S. J. ( +312) Characteristics of wide ( +33/ ) spread pyroclastic deposits formed by the interaction of 1 silicic magma and water. Bull. Volcanol.,.+, +30,+,. ++. ++2. ( +333) ( +332),*. p.. ( +32+ ) 1,,. /+ /0, /03. ( +31, ) 01 (,**) +,2 +2., ( +3,3),+. no.. +,/. ( +320) Walker, G.P.L. ( +32+ ) Characteristics of two phreatoplinian no.. + 0.. ashes, and their waterflushed origin. J. Volcanol. Geotherm. ( +31* ) Res., 3, 3/.*1. +31*.+ p.