The Tephra Layers Distributed around the Eastern Foot of the Zao Volcano Ages and Volumes of the Za-To + to. Tephras

Transcription

1 Letter Bull. Volcanol. Soc. Japan Vol. / (,**2) No. /, pp. +/+ +/1 The Tephra Layers Distributed around the Eastern Foot of the Zao Volcano Ages and Volumes of the ZaTo + to. Tephras Kotaro M, Masao B and Hiroshi Y IURA AN AGI (Received, February,3,,**2 ; Accepted, October 3,,**2) Ages of four scoriaceous volcanic sand layers (ZaTo + to.) distributed around the eastern foot of the Zao volcano are determined to be ca.,, *. 1,,1. +, and +,. 3 cal kyr BP, respectively, based on four newly obtained radiocarbon dates and loess chronometry. Besides, we detect ToH (ToHP) glasses from the loess below the ZaTo. tephra. The estimated age of the loess horizon is consistent with the previously reported one for ToH, (ToHP). Volumes of ZaTo + to. tephras are estimated to be ca. +*, +*, + +* and + +* DRE km, respectively, and the averaged discharge rate from ZaTo + to. is calculated to be ca. *.*, *.*km /ky. Key words : Zao volcano, tephra layers, radiocarbon dating, volcanic glass, discharge rate +. Introduction area. Imura ( +333) also estimated that the ages of The Zao volcano is a stratovolcano which is located ZaTo + to. tephras are older than ca.,* ka, based on in the central part of the volcanic front of the northeast a radiocarbon age (Itagaki et al., +32+ ), and the es Japan arc (Fig. + ). This volcano started its activity at timated AT (AiraTn tephra) horizon. AT is a wideabout + Ma (Takaoka, et al., +323 ), and has continued spread tephra ejected from the Aira caldera about,3 cal to the present day. Geologic and petrologic studies of kyr BP (Okuno,,**,), widely distributed in Japan. In the whole of the eruptive products were performed by the Zao, AT volcanic glasses which is the bubble wall various authors (e.g. Chiba, +30+ ; Oba and Konda, type (Machida and Arai,,**) are found in the upper +323 ; Sakayori, +33, ), and according to these studies, part of the loess below the ZaTo. tephra (Imura, the newest stage of the Zao volcano began at ca. * ka, +330). In this study, we determined the ages of ZaTo and numerous small to medium sized eruptions have + to. tephras layers based on newly obtained four continued for the past *ky. At about * ka, the horse radiocarbon dates, coupled with the loess chronometry shoeshaped Umanose caldera ( +. 1 km in diameter), (Hayakawa, +33/ ) and tephrochronology. Using this which is located in the central part of the Zao, was new age data and the magma discharge volume esformed by explosive eruptions (Sakayori, +33, ). The timated from the isopach maps, we have calculated the newest cone Goshikidake is situated in the inner part of magma discharge rates during the ZaTo + to. acthe Umanose caldera and the newest crater lake Okama tivities. is located in the western part of the Goshikidake. Tephrostratigraphy is very useful to establish the,. Stratigraphic description of the ZaTo + to. tephra detailed eruptive history of volcanoes (e.g. Thorarinsson, layers +32+ ). In the case of the Zao newest stage, the tephro ZaTo + to. tephra layers consist of scoriaceous stratigraphic studies were performed by various authors volcanic sand, and are well sorted and laminated. In (e.g., Imura, +330, +333 ; Itagaki et al., +32+ ; Saigusa an outcrop at Haraobi (Loc. 1 ; Fig. + ), all of the ZaTo and Shoji, +32. ). Imura ( +333) recognized ten volcanic + to. tehpra layers can be observed and they are sand layers (ZaTo + to +* ). ZaTo + to. tephras are intercalated in the brown colored loess (partly reddishmostly distributed around the eastern foot of the Zao dark brown colored paleosol), observed at the eastern volcano, while ZaTo / to +* are mainly near the summit foot of Zao. Division of Interactive Symbiosphere Sciences, Graduate Department of Information, Environmental and Food School of Science and Engineering, Yamagata Sciences, Faculty of Education, Art and Science, Yama University, +,, Kojirakawamachi, Yamagata, 33* gata University, +. +,, Kojirakawamachi, Yamagata, 2/0*, Japan. 33* 2/0*, Japan. Department of Earth and Environmental Sciences, Faculty of Science, Yamagata University, +. +,, Kojira Corresponding author: Masao Ban kawamachi, Yamagata, 33* 2/0*, Japan. ban@sci.kj.yamagatau.ac.jp

2 152 Kotaro M IURA, Masao BAN and Hiroshi YAGI Fig. +. Maps showing the location of the Zao volcano (a), location of the outcrop (b), correlative stratigraphic columns for representative sections (c) and isopach maps of ZaTo + to. tephra layers (d). Topographic contour lines in (b) are at +** m intervals. P and L+ to 0 in (c) indicate the horizons of the paleosol samples and the analyzed volcanic glasses, respectively (see text for detail). The estimated sedimentation rates are also shown in the columns of Loc. 0and 1 (see text for detail). Numbers in (d) indicate the measured thickness of the layers in cm.

3 The Tephra Layers Distributed around the Eastern Foot of the Zao Volcano 153 Table +. Radiocarbon data and calibration ages. *, Samples dated at the Paleo Labo Tokai branch. **, Samples dated at the Tono Geoscience Center. a) Calibration age based on the calibration program CALIB /.*., (Stuiver and Reimer, +33 ) using IntCal*. (Reimer et al.,,**.). b) Calibration age based on Fairbanks calibration program (Fairbanks et al.,,**/). At this outcrop, the ZaTo + tephra layer has the (Fig. + ). We sampled the paleosols from these horismallest thickness (ca.. cm), while the ZaTo, zons for radiocarbon dating. The material of samples tephra layer is thickest (ca..2cm) with the ZaTo and +1*,+1*, and +1*. passed through a *+. mm sieve. tephra layers having intermediate values (ca. 2 and was stirred by an ultrasonic cleaner, and purified by acid,/ cm). Thicknesses of the loess between ZaTo + and treatment. The pretreated material was oxidized by,,, and, and and. tephra layers are ca., + and heating at 2/* together with CuO. The produced +0* cm, respectively. ZaTo + tephra presents as lenses, CO, was reduced catalytically to graphite on Fepowder while ZaTo, to. tephras present as stratified layer. with hydrogen gas at 0/*. For these three samples, However, ZaTo tephra sometimes presents as discon we used an AMS system at the Paleo Labo Co., Ltd to tinuous layer. The ZaTo, tephra layer is divided into make radiocarbon measurements of graphite targets lower, middle, and upper parts. The thicknesses of these with NIST OXII as standerds. The material of sample are ca.., / and 3cm at Loc. 1, respectively. The upper +1 *+ was purified by acid and alkali treatments. The and lower parts are composed of scoriaceous volcanic pretreated material was oxidized by heating in a vacuum sands, while the middle layer is composed of red colored together with CuO. The produced CuO was reduced scoria only. Itagaki et al. ( +32+ ) named the red col to graphite using the method of Kitagawa et al. ( +33). ored scoria the Zao Kawasaki scoria (ZaKw). For this sample, we used a Tandetron AMS system at In addition, a gravel layer is sometimes observed the Tono Geoscience Center to make radiocarbon measbelow the ZaTo + tephra layer. The thickness of the urements of graphite targets with NIST OXII as standloess lying between them is ca..* cm. The gravel layer ards. is composed of subangular to subrounded gravel less The results are shown in Table +. Radiocarbon ages than, m in diameter, and poorly sorted. It is consid + were corrected by d C and calculated using a Libby ered that the gravel layer can be correlated to sub halflife of /,/02 years. The well known radiocarbon angular to subrounded gravel layer which was described age calibration curve IntCal *. (Reimer et al.,,**.) by Itagaki et al. ( +32+ ). Itagaki et al. ( +32+ ) reported extends back to,0 cal kyr BP, however the calibration that the radiocarbon age of wood sample from sub curve spanning,0 to /* cal kyr BP is tentative (van der angular to subrounded gravel layer which was obtained Plicht et al.,,**.). The age of the sample +1 *. was at the other location is +,/**,, 0+* / +, 31* yr BP. calibrated by the calibration program CALIB /.*., Isopach maps of ZaTo + to. tephra layers are shown (Stuiver and Reimer, +33 ) using IntCal *. (Reimer et in Fig. +. They are distributed mainly on the east al.,,**.), but the other three samples were calibrated downwind side. Isopach maps show that these tephras by Fairbanks calibration program (Fairbanks et al., originated from the Zao volcano. The main axis of,**/) (Table + ), because these three ages are out of ZaTo + to. tephras extend toward the eastsoutheast, range of the IntCal *.. The calibrated calendar ages eastnortheast, east and eastnortheast, respectively. are used in the following discussion. It is considered. Radiocarbon age that the radiocarbon ages of the samples +1 *,, +1 * and +1 *. are equivalent to those of the nearby tephra The representative columnar sections are shown in layers. Because the radiocarbon dates paleosols under Fig. +. At Loc. 1, the reddishdark brown colored lying tephra layers, can represent the eruption age paleosol (, cm thickness paleosol parts) is observed in (Okuno,,**+). Thus, the ages of the ZaTo, to. four horizons, just above the top of the gravel layer and tephra layers are deduced to be ca. *. 1,,1. +, and +,. 3 just below the bottom of ZaTo,, and. tephra layers cal kyr BP, respectively. The radiocarbon age of the

4 154 Kotaro M IURA, Masao BAN and Hiroshi YAGI Table,. Representative compositions of volcanic glasses in L+ to 0. FeO*, total iron calculated as FeO. sample +1 *+ is in the range of the error bar of that of 0) from the upper part of the loess ( 0* cm in thickness) a wood sample from the gravel layer which was reported below the boundary with the ZaTo. tephra layer. by Itagaki et al. ( +32+ ). Samples L+ to 0 were collected from the loess at 0* /* The sedimentation rate of loess is considered to be to +* * cm from the boundary (Fig. + ). The samples relatively constant in a flat area (Hayakawa, +33/ ) such were washed by an ultrasonic cleaner, dried in an oven as Loc. 0and 1. Using the obtained ages of ZaTo, to at 0*, and sieved. Volcanic glasses obtained in the. and the thickness of the loess between them, the rates range of *.*22 *. +,/ mm were used for microscope during ZaTo, to, and ZaTo to. tephra layers are observation and chemical analysis. Knowing the chemical calculated to be similar value of ca. **1. mm/yr at Loc. composition of the volcanic glasses is very useful 0. At Loc. 1, those are calculated to be ca. *.*3 and when identifying the widespread tephras. The chemi *++. mm/yr, respectively. The rate during the sample cal compositions of the glasses were undertaken with a no. +1 *+ loess horizon to ZaTo, tephra layers is also JEOL 20** M electron microprobe at Yamagata Univercalculated to be **3. mm/yr. These values are in the sity using a wavelengthdispersive technique. Operatrange of the sedimentation rate of the loess in Japan ing conditions were +/ kv accelerating voltage, *.*+ ma (*.*. *. +/ mm/yr) reported by Hayakawa and Yui beam current and a +* mm defocused beam. Analyses ( +323 ). Using the obtained rates, the age of ZaTo + were corrected using the oxide ZAF method. tephra layer at Loc. 0and Loc. 1are calculated to be ca. Under the microscope, most of the volcanic glass and, ka, respectively. The age of the ZaTo + shards shape in L+ to 0 are the pumice type (Machida tephra layer would be some time between ca. and, and Arai,,**), while a small number of volcanic glass cal kyr BP. shards in L to 0 are the bubble wall type (Machida and Arai,,**). Representative compositions of volcanic.. Volcanic glasses in the loess glass shards in L+ to 0 are listed in Table,. All We reexamined the volcanic glass shards in the upper discussion here refers to analyses that have been part of the loess below the ZaTo. tephra where volcan normalized to +** volatile free with total iron (FeO*) ic glasses were reported by Imura ( +330). calculated as FeO. Representative major elements vs. At Loc. 1, we collected six samples (L +,,,,., / and SiO diagrams are shown in Fig.,. Data from the AT,

5 The Tephra Layers Distributed around the Eastern Foot of the Zao Volcano 155 and ToH (ToHP) (TowadaHachinohe tephra) by Machida and Arai (,**) are also shown in Fig., for comparative purposes, because these two are the potenposited in Zao and the surrounding area during ca., tial candidates among widespread tephras to have deka. ToH (ToHP) was discharged from the Towada +, caldera, with an estimated age of ca../ +1 ka. It is widely distributed mainly around the northern part of NE Japan. The shape of volcanic glasses is the pumice type (Machida and Arai,,**). The silica contents of the volcanic glass shards (Lto 0) ranged from 1/ to 2* wt.. In the K, O vs. SiO, diagram, the volcanic glass composition is split into two groups. One group has low K, O ( wt. ) content with a range of 1/ 2* wt. SiO,, and the other has a high K, O (,. /. wt. ) content of 11 2* wt. SiO,. All of the bubble wall type glasses, which were found in Lto 0, belong to the latter group, and the plotted area of this type of glass is similar to that of AT glasses in the K, O vs. SiO, diagram as well as in the other SiO, variation diagrams (Fig.,). The lowk, O glasses can be divided further into lower ( 1/ 12 wt. ) and higher ( 12./ 2* wt. ) SiO, groups. The glasses of former group were from L, to 0, and the compositional range of this group is in the ToH (ToHP) field in the K, O vs. SiO, diagram as well as in the other SiO, variation diagrams. Volcanic glasses of the highk, O pumice type and lowk, O higher SiO, group have di# erent compositions from either of the AT and ToH (ToHP) glasses. It is probable that they were derived from the volcanoes in and around the Zao area. Consequently, AT and ToH (ToHP) volcanic glasses were found in L to 0 and L, to 0, respectively. The age of L, is calculated to be ca ka, using the sedimentation rate of the loess which was obtained in the previous section. The value is comparable to the age of ToH (ToHP) by Machida and Arai (,**). Therefore, it is considered that the AT and ToH (ToHP) volcanic glasses found in the L to 0 horizons have a reworked origin and the ToH (ToHP) volcanic glasses were deposited as near primary tephra in the L, horizon and show the age of the loess. Fig.,. SiO, variation diagrams for volcanic glasses. FeO* total iron calculated as FeO. Solid and broken envelopes show fields of the AT and ToH volcanic glasses, respectively (Machida and Arai,,**). /. Discharge rate Discharge volumes of ZaTo + to. tephra layers were estimated using an empirical formula proposed by Hayakawa ( +32/ ). The formula is V +,., TS where V is the discharge volume, T is a certain thickness, and Sis the area enclosed within the isopach of thickness T., Volumes of ZaTo + to. tephra layers are ca. +*, *, + +* and + +* DRE km, respectively (Table ). A magmadischarge stepdiagram during the ZaTo + to. activities, using the volumes and the ages of the tephras obtained in this study, is shown in Fig.. The

6 156 Kotaro M IURA, Masao BAN and Hiroshi YAGI Table. Discharge volumes of ZaTo + to. tephras. Densities were determined using weight of a +** ml beaker filled with the tephra which was dried in an oven at +*/., *, +*, + +* and + +* DRE km, respectively and the averaged discharge rate during ZaTo + to. is calculated to be *.*, *.*km /ky. Acknowledgements Many constructive comments and suggestions from Dr. M. Okuno of Fukuoka University, Dr. T. Oikawa of Geological survey of Japan, and the editor Dr. D. Miura of Central Research Institute of Electric Power Industry greatly helped to revise the manuscript. We are grateful to Dr. R. W. Jordan for correcting the English in this paper. This work was financially supported in part by GrantiAid for Scientifice Research (no. +1/.*./+ ) from the Japan Society for the Promotion of Science to M. Ban. References Chiba, T. ( +30+ ) Petrology of Zao volcano. J. Japan. Assoc. Mineral. Petrol. Econ. Geol,.0, (in Japanese with English abstract). Fairbanks, R.G., Mortlock, R.A., Chiu, T.C., Cao, L., Kaplan, A., Guilderson, T.P., Fairbanks, T.W., Bloom, A.L., Grootes, P.M. and Nadeau, M.J. (,**/) Radiocarbon calibration curve spanning * to /*,*** years BP,*,.,2 based on paired Th/ U/ U and C dates on pristine corals. Quarter. Sci. Rev.,,., Hayakawa, Y. ( +32/ ) Pyroclastic geology of Towada vol cano. Bull. Earthq. Res. Inst., 0*, /*1 /3,. Hayakawa, Y. ( +33/ ) Characteristics of Japanese loess, and its eolian origin. Bull. Volcanol. Soc. Japan.,.*, * (in Japanese with English abstract). Hayakawa, Y. and Yui, M. ( +323) Eruptive history of the Kusatsu Shirane volcano. Quarter. Res.,,2, ++1 (in Japanese with English abstract). Imura, R. ( +330) Products of ZaoGoshikidake. In Inventory of Quaternary OutcropsTephras in Japan (Japan Association for Quaternary Research ed), +0*, Japan Association for Quaternary Research, Tokyo, /, p (in Japanese). Imura, R. ( +333) Zao volcano. In Field Guide of Volcanoes in Japan,., Volcanoes in Tohoku districts (Takahashi, M. and Kobayashi, T. eds), 1* 22, Tsukiji Press, Tokyo, +03 p (in Japanese). Itagaki, N., Toyoshima, M. and Terado. T. ( +32+ ) The scoria bed of late Pleistocene in Sendai and its environs. Tohoku Geography, +,.2 / (in Japanese). Kitagawa, H., Masuzawa, T. and Matsumoto, E. ( +33) A batch preparation method for graphite targets with low background for AMS C measurements. Radiocarbon, /,,3/ **. Machida, H. and Arai, F. (,**) Atlas of Tephra in and Around Japan. Univ. Tokyo Press, 0p (in Japanese). Oba, Y. and Konda, T. ( +323) Geology and petrology of central Zao volcano, Yamagata Prefecture. Bull. Yamagata Univ., +,, +33,+* (in Japanese with English abstract). Fig.. A magmadischarge stepdiagram during ZaTo + to. tephras. intervals of ZaTo + to,(ca.,. +. ky) and ZaTo, to (ca.. 0ky) are much shorter than the interval of ZaTo to. (ca.., ky), thus ZaTo + to are regarded as a series of activities. While the average discharge rate during ZaTo + to. is calculated to be ca. **,. **. km /ky, that of the series of activities is calculated to be larger, ca. **1. **3. km /ky. These dis charge rates are comparable to those of the newest stages of Adatara and Azuma volcanoes, which are the two nearest active volcanoes to Zao, in the volcanic front. Tephrostratigraphic studies have been performed on the newest stages of these two volcanoes (Yamamoto and Sakaguchi,,***; Yamamoto,,**/), and the discharge rates are estimated to be ca. *.**2 and *.*1 km / ky, respectively. 0. Conclusions ( + ) The ages of the ZaTo + to. tephra layers in the newest stage were determined to be ca.,, *. 1,,1. + and +,. 3 ka, respectively. (,) The AT and ToH (ToHP) volcanic glasses were found in the upper part of the loess below the ZaTo. tephra. While the AT glasses have a reworked origin, the ToH (ToHP) glasses were deposited as near primary tephra. () The estimated volumes of the ZaTo + to. are ca.

7 The Tephra Layers Distributed around the Eastern Foot of the Zao Volcano 157 Okuno, M. (,**+) Tephrochronology and radiocarbon KAr dating of lavas from Zao volcano, northeastern chronology in Japan. Quarter. Res.,.*,.0+.1* (in Japan. J. Geol. Soc. Japan, 3/, +/1 +1* (in Japanese with Japanese with English abstract). English abstract). Okuno, M. (,**,) Chronology of tephra layers in southern Thorarinsson, S. ( +32+ ) Tephra studies and tephrochrono Kyushu, SW Japan, for the last *,*** years. Quarter. logy: a historical review with special reference to Ice Res.,.+,,,/,0 (in Japanese with English abstract). land. In Tephra studies (Self, S. and Sparks, R.S. J. eds), Reimer, P. J., Billie, M.G.L., Bard, E., Bayliss, A., Beck, J. + +,. D. Reidel Publ. Co., Dordrecht, Holland,.2+ p. W., Bertrand, C. J.H, Blackwell, P.G., Buck, C.E., Burr, Van der Plicht, J., Beck, J.W., Bard, E., Billie, M.G.L., G.S., Cutler, K.B., Damon, P.E., Edwards, R.L., Blackwell, P.G., Buck, C.E., Friedrich, M., Guilderson, Fairbanks, R.G., Friedrich, M., Guilderson, T.P., Hogg, T.P., Hughen, K.A., Kromer, B., McCormac, F.G., A.G., Hughen, K.A., Kromer, B., McCormac, F.G., Ramsey, C. Bronk, Reimer, P. J., Reimer, R.W., Remmele, Manning, S., Ramsey, C.B., Reimer, R.W., Remmele, S., S., Richards, D.A., Southon, J.R., Stuiver, M., and Southon, J.R., Stuiver, M., Talamo, S., Taylor, F.W., Weyhenmeyer, C.E. (,**.) NotCal*. comparison/ van der Plicht, J. and Weyhenmeyer, C.E. (,**.) IntCal calibration C records,0 /* cal kyr BP. Radiocarbon, *. terrestrial radiocarbon age calibration, *,0 cal kyr.0, +,,/ +,2. BP. Radiocarbon,.0, +*,3 +*/2. Yamamoto, T. (,**/) Eruptive history of Azuma volcano, Saigusa, M. and Shoji, S. ( +32. ) Distribution and pro NE Japan, during last 1,*** years : stratigraphy and perties of Zao tephras, Pedologist,2,,/ (in Japanese magma pluming system of the Azuma Jododaira prodwith English abstract). ucts. J. Geol. Soc. Japan, +++, 3. ++* (in Japanese with Sakayori, A. ( +33, ) Geology and petrology of Zao volcano. English abstract). J. Mineral. Petrol. Econ. Geol, 21,.... (in Japanese Yamamoto, T. and Sakaguchi, K. (,***) Eruptive history with English abstract). of Adatara volcano, NE Japan, during last,/*,*** years Stuiver, M. and Reimer, P. J. ( +33) Extended C data based on tephrostratigraphy. J. Geol. Soc. Japan, +*0, base and revised CALIB.* C age calibration program. 20/ 22, (in Japanese with English abstract). Radiocarbon, /,,+/,*. (Editiorial hundling Daisuke Miura) Takaoka, N., Konno, K., Oba, Y. and Konda, T. ( +323) ZaTo ZaTo ZaTo,. *. 1 ka,,1. + ka, +,. 3 ka ZaTo +, ka ZaTo. ToH, ToHP, ZaTo +. +* +* + +* + +* DRE km ZaTo +. **,**.. km ky, ZaTo + **1**3.. km ky Jizo : Kumanodake : Goshikidake : Kattadake : Eboshidake : Sugigamine: Byobudake: Haraobi: