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Mizumoto-cho , Muroran, Hokkaido , Japan. College of Environmental Technology, Graduate School
Vol. ( ) No. , pp.
(Received March , ; Accepted September , )
Mt. Tenchozan, located on the Shiretoko Peninsula, eastern Hokkaido, Japan, is a Quaternary andesitic volcano with crater chains on its summit. This paper reports on a fallout tephra (the Ten-a tephra) extruded from the volcano. The tephra extends from the summit area of the volcano to the eastern shore of the peninsula, and is composed of pyroclastic lithic fragments, minor juvenile pumice and ash. The tephra increases in thickness and grain size toward the summit of the volcano. Radiocarbon dating of a buried soil located immediately beneath the tephra yields an age of years BP, and a calibrated calendar age of cal BP ( , probability). The distribution, components and radiocarbon age of the tephra suggest that phreato-magmatic eruptions took place at the summit of the volcano at . years BP, resulting in the formation of the crater chains.
: fallout tephra, radiocarbon age, crater chain, Tenchozan Volcano, Shiretoko Peninsula
Corresponding author : Yoshihiko Goto of Engineering, Muroran Institute of Technology, e-mail : [email protected]
In this paper, the tephra is referred to as the Tenchozan-chain trends southwest northeast, extends for m, Mt. Tenchozan is a Quaternary andesitic volcano and consists of explosion craters. Each crater is located on the Shiretoko Peninsula, eastern Hokkaido, circular to elliptical in plan view, m in diame-Japan (Fig. ). The volcano has two crater chains on ter, and m deep. Some craters are connected to its summit (Fig. , Moriya, ; Katsui ., ). each other. Larger craters are filled with water (Fig. The craters display well-preserved, primary morpholog- A and B). The southern chain trends southwest ical features, suggesting a series of eruptions occurred in northeast to west east, extends for m, and consists recent geological time. However, the timing of the of four explosion craters. Each crater is elliptical in eruptions remains unknown. This paper reports on the plan view, m in diameter, and m deep. distribution, components and radiocarbon age of a fall- At present, the craters in both chains contain no active out tephra extruded from the volcano, and discusses the fumaroles. Historical records make no mention of vol-timing of the crater-forming eruptions. canic eruptions from these craters.
Tenchozan Volcano is located . km southwest of
Rausu Volcano (Fig. A). It reaches an elevation of The newly identified fallout tephra extends from the m above sea level and has a base diameter of . Tenchozan summit to the eastern shore of the Shiretoko km (Fig. A). The volcanic edifice is flat-topped Peninsula. Outcrop locations are shown in Figure . with steeply sloping sides and is composed of andesitic
lavas (Tenchozan Lavas, Fig. ). The lavas retain a tephra (Ten-a tephra) (Goto ., ; Nakamura their primary morphological features, including curved ., ; Goto, ). The type locality is in a flow fronts, and consist mainly of hypersthene augite gully located m east of Shiretoko Pass (Loc. in andesite. Table lists the whole-rock major-element Fig. , latitude N, longitude E), chemical composition of the andesite (sample numbers where the tephra is located approximately cm be-Ten- , - , - , - , - , - ), which contains neath the surface (Fig. ).
wt. SiO .
There are two chains of explosion craters on the At the type locality, the Ten-a tephra is a cm thick, summit (Fig. B, Katsui ., ). The northern pale brown, massive (non-stratified) pyroclastic deposit,
Yoshihiko G
A Fallout Tephra from Tenchozan Volcano,
Shiretoko Peninsula, Hokkaido, Japan
ca et al et al et al et al Key words . Introduction
. Tenchozan Volcano . Fallout Tephra extruded from Tenchozan Volcano
Nomenclature Description
A
Arrttiic
clle
e
, ,1 + */* 2/2/ /0 ,*++ . / +-1 +./ . ,*++ 1 ,*++ +3-* .* +30* +2+* , 3/ +3** +2** +* +** ,/* + -* 1* , +32. +32/ - -1** 1* +** ,* -* . / , +*.0 , / . , , , ,**/ ,**2 ,**3 +** + + , .. *- +1 +./ *0 ,. -* +- +. +/ +0 +2 ,* /1 0+ . /* , +32/ + , -- + - , s῏ ῍ ῐ ῎ ῏ ῏ ῌ ῎ ῍ ῍ ῌ ῍ ῌ ῍ ῍ ῍ ῍ ῌ ῌ ῍ ῍ ῌ ῌ OTO
Fig. . Location of Tenchozan Volcano on the Shiretoko
138 Yoshihiko G
Peninsula, Hokkaido, Japan.
Formation comprises greenish, intensely altered, tu acious Figure shows the grain-size distribution of the Ten-a tephra sampled at the type locality. Because the matrix
tures (phenocryst assemblage and size) to the Tenchozan
Arai, ). X-ray di raction (XRD) analysis of the m fraction (separated by hydraulic elutriation) revealed the presence of opal-CT and minor cristobalite. XRD analysis with ethylene glycol treatment suggests the absence of clay minerals.
is partly cohesive, sieving was performed in a water bath. The Ten-a tephra has a high proportion of grains ranging from / to mm in diameter ( to ). Median diameter (Md ) of the sieved sample is . , and standard deviation ( , Inman, ) is . .
Representative stratigraphic logs of the Ten-a tephra are shown in Figure . At location (the type locali-ty), the geological section consists of (from lower to upper) : the Miocene Rausugawa Formation (Doi
., ; thickness m), a debris-flow deposit ( . m), a volcanic ash ( cm), the Rausudake pyroclastic flow deposit (Miyaji ., ; m), a reworked pyroclastic deposit ( m), the Ten-a tephra ( cm), and the surface soil ( cm). The lowermost Rausugawa breccia, consisting of angular andesite clasts, up to cm across, embedded in a fine-grained matrix. The formation yields a fission-track age of . . Ma (Koshimizu and Kim, ). The overlying debris-flow deposit is pale brown and composed of subangular andesite cobbles, cm across, in a sandy matrix. consisting of lithic fragments, pumice and ash (Fig. ). The volcanic ash is pale gray, fine grained, non-The lithic fragments make up vol. of the tephra laminated, and consists of fresh volcanic glass and crys-and are up to cm across. They consist of subangular tals of plagioclase, hypersthene, augite, and opaque to subrounded andesites of various compositions. The minerals. The volcanic ash is inferred to be a fallout andesites are gray to reddish brown, and vary in state of tephra derived from Rausu Volcano, because the ash is oxidation (non-oxidized to intensely oxidized) and al- positioned just beneath the Rausudake pyroclastic flow teration (fresh to intensely altered). The fresh andes- deposit and is identical in mineral assemblage to the ite lithic fragments are identical in petrographical fea- deposit.
The Rausudake pyroclastic flow deposit is gray and
Lavas. composed of several flow units (Goto, ). Each
The pumice accounts for vol. of the Ten-a unit is . m thick and consists of angular andesitic tephra and shows an upward increase in proportion lithic clasts ( cm across) and subrounded andesitic within the tephra layer. The pumice is pale gray, sub- pumice clasts ( cm across) in a matrix of andesitic rounded, up to cm across (Fig. ), and commonly rock fragments ( mm across). The andesitic lithic thickly coated with a pale brown, fine-grained ash. clasts and pumice clasts contain phenocrysts of plagio-The pumice consists of fresh volcanic glass and crystals clase, hypersthene, augite, and opaque minerals. The of plagioclase, hypersthene, augite, and opaque miner- pyroclastic flow deposit has a ground surge layer ( als. Table lists the whole-rock major-element chem- cm thick) at the base, which includes charcoal frag-ical composition of the pumice (sample numbers Ten- ments and partially carbonized wood. Radiocarbon A and - B), which contains wt. SiO . dating of a partially carbonized wood sample (number The ash ( vol. of the tephra) consists of altered SP- , Fig. ), determined by Beta Analytic (Miami, rock fragments, plagioclase, augite, hypersthene, mag- USA), yields an age of years BP (Table ). netite, and fresh volcanic glass. The altered rock frag- Calibrated calendar ages, obtained using the IntCal ments are petrographically identical to the larger lithic calibration (Reimer ., ; Talma and Vogel, fragments. The volcanic glass is vesicular and shows a ), are and cal BP ( , pumice-like habit (cf., Yoshikawa, ; Machida and probability). The Rausudake pyroclastic flow deposit
et al
et al
et al
Stratigraphy and radiocarbon ages
+ # 0 ,**- # , + . , + , * * +3/, - . 1 + +31* / + 2 + ,*** ++ , /* -* +/ 1 2 * 3 +320 / +* . ,/ -* 2 ,**3 +* +/ * / + + +* -* 3 / / , -+ + + 0, 0- , 0* , 1 ,-+* 0* , *. ,**. +33- ,.0* ,,3* ,,1* ,+0* , 3/ +310 -m f f sf s
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is m. (B) Crater chains on the summit of the Tenchozan Volcano. The northern chain trends southwest Fig. . (A) Distribution of the Tenchozan Lavas and the Rausudake Lavas. Outcrops of the Ten-a tephra are
indicated by solid circles. Location numbers of the outcrops correspond to those in Fig. . Contour interval northeast, extends for m, and consists of explosion craters. The southern chain trends southwest northeast to west east, extends for m, and consists of four explosion craters. Contour interval is m. Locations of Figs. A and B are also shown.
section (Fig. ). At locations and , the Ten-a tephra
is capped by a reworked pyroclastic deposit (reworked laminated, and consists of fresh volcanic glass and crys-Rausudake pyroclastic flow deposit), m thick. The tals of plagioclase, hypersthene, augite, and opaque Ten-a tephra directly overlays the reworked pyroclastic minerals. The volcanic ash corresponds to the volcanic flow deposit. The textures and components of the ash below Rausudake pyroclastic flow deposit at loca-Ten-a tephra are described in Section - . The tephra tion , because they are identical in mineral assemblage is overlain by a dark brown surface soil, cm thick. (plagioclase, hypersthene, augite, and opaque minerals) At locations and , the geological sections show and stratigraphic location (below the Ten-a tephra). similar sequences to those observed in the upper part of The ash is therefore inferred to have derived from Rausu Volcano (Goto, ). Radiocarbon dating of covers a debris-flow deposit. At location , the tephra a soil sample, collected from the topmost one centimeter contains a higher concentration of pumice ( vol. of the soil layer, located immediately below the volcanic of the tephra) than that observed at location . At ash, yields a conventional radiocarbon age of
location , the Ten-a tephra consists mainly of lithic years BP and a calibrated calendar age of cal
fragments. BP ( , probability, sample number TEN- ;
At location (Ichino-ike Pond, km east of Rausu- Table ). The second buried soil is dark brown and ko Lake), the geological section consists of (from lower also consists of peat. The Ten-a tephra is pale brown to upper) : a buried soil ( cm thick), a volcanic ash and composed mainly of fresh to altered andesitic frag-( cm), a second buried soil frag-( cm), the Ten-a tephra frag-( ments up to cm across. Radiocarbon dating of a soil cm), and the surface soil ( cm). The lowermost sample, collected from the topmost one centimeter of buried soil is dark brown and consists of peat. The the second soil layer, yields a conventional radiocarbon overlying volcanic ash is pale gray, fine-grained, non- age of years BP and a calibrated calendar age
+** , 1 +2** +* 1** ,* - -+ 1 . / , - , + -* , -,**3 . ,* + ,,0* 0* / ,-0* ,+-* , 3/ /. 0 , , +* , - / + +* +3-* .* s
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clastic deposit. Note the person for scale. Scale
Volcano. (A) An explosion crater of the northern
140 Yoshihiko G
Fig. . Ten-a tephra (Ten-a) at the type locality ( m east of Shiretoko Pass ; Loc. in Fig. ). The tephra is cm thick, pale brown, and composed of lithic fragments, pumice and ash. It overlies the Rausudake pyroclastic flow deposit, which is capped by a reworked pyro-bar is cm long.
Fig. . Crater chains on the summit of Tenchozan chain viewed from the southwest. The diameter of the crater is m. This crater is filled with cold water, forming a -m-wide lake. Note the person (left) for scale. The mountain behind the crater is Rausu Volcano. (B) An explosion crater of the northern chain viewed from the northeast.
This crater is filled with water in springtime, Fig. . Pumice in the Ten-a tephra. The photograph forming a shallow -m-wide lake, but is dry from was taken after washing the sample in the
labo-summer until autumn. ratory. Before washing, the pumice had been
coated by a fine-grained ash.
The tephra is inferred to have formed by
phreato-of cal BP ( , probability, sample
number TEN- ; Table ). The Ten-a tephra is over-lain by the dark brown surface soil, cm thick.
The Ten-a tephra increases in thickness and maxi-The thickness distribution and maximum grain size of mum grain size toward the summit of Tenchozan Volca-the Ten-a tephra are shown in Figures A and B, no, suggesting the tephra was erupted from the volcano. respectively. The maximum grain size was calculated
as the average long-axis diameter of the three largest magmatic explosions because the tephra is composed of lithic clasts. These data suggest that the tephra in- non-juvenile lithic fragments that are interpreted to creases in thickness and maximum grain size toward the have formed by fragmentation of the pre-existing volca-summit of the Tenchozan Volcano. The bulk volume no surface, and fresh subrounded pumice, interpreted to of the tephra, calculated following Hayakawa ( ) be juvenile pyroclasts formed by the fragmentation and and using the -cm isopach, is . m . ejection of magma. The volume ( . m ) and grain-size distributions of the Ten-a tephra (high
pro-. Discussion
Origin of the Ten-a tephra Distribution and volume
1 - 1 -. +** + , /* /* -,/* /0 / ,* +30* +2+* , 3/ /+ , +* 2 2 +32/ / . , +* . , +* . . + - . s
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Fe O * total iron as Fe O . L.O.I. loss on ignition. Sample locations (latitude, longi-Table . Whole-rock major-element compositions of andesites from the Tenchozan Lavas (samples Ten- , - , - , - , - , - ) and pumice from the Ten-a tephra (Ten- A and - B), as determined by X-ray fluorescence (Rigaku RIX- ) at Shimane University, Japan, following the analytical method proposed by Kimura and Yamada ( ).
tude) : Ten- ( N, E), Ten- ( N, E),
Ten-( N, E), Ten- ( N, E), Ten- (
N, E), Ten- ( N, E), Ten- A and - B (
N, E).
Fig. . Grain-size histogram of the Ten-a tephra, sampled from m east of the Shiretoko Pass (the type locality ; Loc. in Fig. ). The sample was sieved at intervals of (where log , with being the grain size in millimeters), using a set of sieves ranging from to ( mm to / mm). The sieving was carried out in a water bath because the samples were partly cohesive. The sieved samples were dried and weighed to . g on a laboratory balance.
portion of grains smaller than mm ; Fig. ) are consis-tent with phreatomagmatic explosions (see Morrissey
., ). The pumice in the tephra is thickly coated with a pale brown, fine-grained ash, implying steam-rich eruptions. The crater chains at the summit of the Tenchozan Volcano are composed of a number of small explosion craters (diameter : m in the northern chain, m in the southern chain) and are consis-tent with steam-driven, multiple-vent-forming, explo-sive eruptions (cf., the eruption at Usu Volcano : Yokoyama ., ; the eruption at Usu Volca-no : Ui ., ). The crater chains are thus in-ferred to have formed during the same eruptions as those that produced the Ten-a tephra. Volcanic Explosivity Index (VEI, Newhall and Self, ) of the eruptions is .
Previous geochronological studies (Orlova and
Panychev, ; Okuno ., ; Xu ., )
suggested that the radiocarbon age of a buried soil located immediately below a mass-flow or pyroclastic deposit represents the emplacement age of the deposit. The radiocarbon ages of the buried soil immediately below the Ten-a tephra (sample TEN- ) and a volcan-ic ash (sample TEN- ) may represent the emplacement ages of the tephra and ash. The TEN- age for the Ten-a tephra ( years BP) and the TEN- age for the volcanic ash ( years BP) are consistent with the stratigraphic positions of these deposits (Fig.
). The TEN- age for the Ten-a tephra ( d d et al et al et al et al et al Eruption age , - , -, + +- +. +/ +0 +2 ,* + + ,*** +330 +- .. *, .2 +./ */ ., +. .. *, .2 +./ */ ., +/ .. *, .3 +./ */ -0 +0 .. *, .2 +./ */ -- +2 .. *, .+ +./ *0 +0 ,* .. *, .. +./ */ ,- + + .. *- +1 +./ *0 ,. 0 +** + , + 0 . 0. + +0 * *+ , 0 ,*** +** ,/* 1* +** +3+* +31- ,*** ,**, +32, -+33- +331 ,**. /+ /. /+ +3-* .* /. ,,0* 0* 1 /+ +3-* .* . , f f f
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Fig. . Stratigraphic sections of the Ten-a tephra. Location numbers of the sections correspond to those in Figure . The section at location is shown at small and large scales. Radiocarbon ages are also shown.
Table . Results of radiocarbon dating.
C age* is based on Libby’s half-life ( years), uncorrected by C values. Ages are expressed in BP (years before AD ) with an error range of . **The conventional C age includes C correction. Ages are expressed in BP with an error range of . ***Calibrated calendar ages were calculated from the conventional C ages, using the program developed by Talma and Vogel ( ), based on the IntCal calibration database (Reimer
., ). Ages are expressed in cal BP with an error range of ( probability) and ( probability). Sample TEN- is a dark brown soil (grain size . mm), collected from the uppermost cm of the -cm-thick buried soil layer located immediately below the Ten-a tephra at Ichino-ike Lake (Loc. in Figs. and , latitude N, longitude E). Acid wash pretreatment was done before radiocarbon dating. Sample TEN-is a dark brown soil (grain size . mm), collected from the uppermost cm of the -cm-thick buried soil layer located immediately below a volcanic ash at Ichino-ike Lake (Loc. ). Acid wash pretreatment. Sample SP- is a partly carbonized wood (size cm), collected from a ground surge layer at the base of the Rausudake pyroclastic flow deposit, located m east of Shiretoko Pass (Loc. , N, E). Acid-alkali-acid wash pretreatment.
years BP) is also consistent with the stratigraphy at tephra was emplaced at . . ka. This age is consis-location , where the tephra overlies the Rausudake tent with the well-preserved morphology of the crater pyroclastic flow deposit, which yields a radiocarbon age chains on the summit of the volcano. Therefore, it is of years BP (Fig. , sample SP- ). The inferred that Tenchozan Volcano erupted at . . ka, TEN- sample ( years BP) yields a calibrated resulting in the formation of the crater chains. calendar age of cal BP, suggesting the Ten-a
et al ca ca +. +-+. +-+. 1 , + , //02 +3/* + + +33- *. ,**. , 3/ + 02 /+ * / + -0 , 1 .. *+ .2 +./ *0 +2 /. * / + +* 0 , . . 0 +** + .. *- +1 +./ *0 ,. + 3 + ,-+* 0* 1 , + 3 /+ +3-* .* +30* +2+* d s d s s s
Fig. . (A) Thickness distribution of the Ten-a tephra (in cm). (B) Grain-size distribution of the Ten-a tephra (in cm). The maximum grain size is determined by the average of the long-axis diameters of the three largest lithics. The tephra increases in thickness and maximum grain-size toward the summit of Tenchozan Volcano.
ῌ῍ ῎ ῌ ῌ ῌ ῌ ῌ ῌ ῌ ῌ ῌ ῌ ῌ ῌ ῌ ῌ ῌ ῌ
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This research was supported financially by the Muroran Institute of Technology. The author thanks H. Takizawa (Shiretoko Sanko-sha), Y. Yokoyama, A. Sawada (Muroran Institute of Technology), N. Gouchi (Shiretoko Museum) and all the sta at the Shiretoko Museum for their help in the field. H. Sasaki (Kokusai Kogyo) and M. Nakagawa (Hokkaido University) are thanked for constructive discussion. We are grateful to I. Miyabuchi (Kumamoto University) and an anonymous referee for reviewing the manuscript. I. Miyagi (AIST) is thanked for editing the manuscript. Part of this work was performed as part of compiling a geological guidebook, Geology of the Shiretoko Penin-sula, issued by the Hokkaido Shinbun, Shari Town, and the Shiretoko Museum.
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B -Acknowledgements References ,**/ ,**2 -3 .3 ,**--0* ,*** ,,** 1/ 2/ +32. +,, +,-+32, , +,-+ +,-2 +31* +331 .1* .1. -. +33--03 ,**3 -11 +,, +-0 +32/ ,**. *. /*1 /3, * ,0 +3/, +*,3 +*/2 +,/ +./ +33-+. -+1 -,, +32/ ,*** ,**, ,*** -1 0- +*/ ++1 +330 ,**. +. /0* /01 0, 1, +320 # 21+ 212 ,*** .-+ ../ # .1 ./ 21 + /**** +,--/ +. 0* .0 ,, -/ .1 ,,- ,,. 3+ 3, ῌ OTO
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+31-+310
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