Bull. Volcanol. Soc. Japan
Vol. .3 (,**.) No. ., pp. ,*+ῌ,*/LetterOn the Eruption Age of the Hijiori Caldera, Based onMore Accurate and Reliable Radiocarbon Data
Isoji MIYAGIῌ
(Received October ,+, ,**- ; Accepted May ,., ,**.)
Eight +.C ages of the Hijiori pyroclastic ﬂow deposit (stage + and ., Kawaguchi and Murakami , +33.) are
reported. Calibrated +.C ages of the stage + pyroclastics fall in a relatively narrow range, while those stage . are
more ﬂuctuated. The larger ﬂuctuation may be attributed to nature of samples collected (lower organic carbon
concentration and poor state of sample preservation relative to the stage +). No signiﬁcant di#erence in the +.C
age is recognized between the stage + and stage .. It is concluded that the eruptive activities of Hijiori volcano
reached its peak shortly after the outbreak at about +,,*** y.B.P. (calendar age).
Key words : caldera, Hijiori, hot dry rock, +.C, radiocarbon+. Introduction
Hijiori caldera (Sugimura, +3/-) is located in the
Northeastern Japan arc at about +/ km NE from theGassan volcano (Fig. +). This area is about .* kmback arc side from the volcanic front of NE Japan arc(the Moriyoshi volcanic zone, Nakagawa et al., +320).
Pyroclastic ﬂow deposits of the Hijiori dacite with 0.῍, wt.ῌ SiO, (Ui, +31+ ; Kawaguchi and Murakami,+33. ; Murakami and Kawaguchi, +33.) covered thearea of about +* km from the caldera center with estimated maximum thickness of about +/* m. The estimated total volume of pyroclastic ﬂow deposit, about+.. km, (Sugimura, +3/-) is comparable with thevolume of depression, *.2 km- (Sugimura, +3/-)ῌ+.,km- (Ui, +31+). Coincident pumice falls are distributed toward the east. At a location /* km awayfrom the caldera, total thickness of the pumice falls is
about ,* cm (Miyagi in preparation, thicker than previous reports by Yonechi and Kikuchi, +300 and Machida
and Arai, +33,).
Present activity of the Hijiori caldera is as hot springs. Although there is no high temperature volcanic
steams, nor active fumaroles, the caldera lake deposits
have undergone remarkable hydrothermal alteration.
A Japanese administrative organization NEDO, New
Energy and Industrial Technology Development Organization, conducted a series of experiments (a long term
ﬂow test for two years) on geothermal electricity power
plant that will make use of hot dry rock as a heat source(e.g., Sato and Okabe, ,**+).
The Hijiori caldera is one of a “new active volcano”
῎ Geological Survey of Japan/AIST, Tsukuba Central
1, +῍+῍+ Higashi, Tsukuba, Ibaraki -*/῍2/01, Japan.
e-mail : miyagi.iso+.***@aist.go.jp

of Japan, because CCPVE (Coordinating Committee
for Prediction of Volcanic Eruption, JMA/MLIT)
reorganized deﬁnition of “Active volcano”, on ,+ January ,**-. Now the deﬁnition includes those volcanoes
erupted in the last +* thousand years (Ui et al., ,**,).
The Hijiori volcano is watched not only by the nations
as a new active volcano but also by archaeologists since
the coincident pumice fall can be used as a time marker.
Ages of the Hijiori pyroclastic ﬂow were reported to
be around +* thousand years (+*,1.*῍3,12* y.B.P. +.C,
Ui et al., +31- ; 22**῍+2** years, Fukuoka andKigoshi, +31+).
Recent develops in radio carbon dating techniques
have greatly improved the accuracy and reliability. In
those days, ideal conditions were assumed for the +.C
dating that +.C concentration in the atmosphere to beconstant through the past until the present, and that the+.
C concentration in all parts of the living plants to beconstant. However, production rate of +.C in the highatmosphere varies with time, also, +.C concentration inbiosphere can be modiﬁed by isotopic fractionation
through biochemical reactions. Radiocarbon dating
nowadays is able to calibrate the temporal change of +.Cconcentration using a measured +.C/ +,C ratio in treerings of known age, and the modern datings correct the
carbon isotopic fractionation using +-C/ +,C ratios ofeach sample (Stuiver and Reimer, +33- ; Stuvier et al.,+332).
To obtain up-to-date age of the Hijiori pyroclastic
ﬂow, radiocarbon specimens should be re-measured,rather than re-calibrated, because the correction forisotopic fractionation requires +-C/ +,C analysis for thesame sample that analyzed the +.C/ +,C ratios.
An essential beneﬁt of re-measureing is that a moredetailed stratiﬁcation of the pyroclastic ﬂow deposit

202

Isoji MIYAGIsample and a constant half-life of //02 years (so called
Libby half-life). “Calibrated +.C age” as well as “conventional +.C age” means a corrected +.C age for thee#ect of carbon isotopic fractionation in +.C and +,C.
The correction depends on measured +-C/ +,C ratios ofeach sample. “Calendar age” includes both the carbon
isotopic fractionation e#ect and the +.C production ratechange using the calibration data by Talma and Vogel(+33-).
The measured +.C age, calibrated +.C age, and thecalendar age are expressed in y.B.P. (year Before Present, AD +3/*).

Fig. +. Map showing the location of Hijiori caldera,
includes neighboring major towns (Sendai,
Yamagata) and a volcano (Gassan).

(Kawaguchi and Murakami, +33.) is now available.
The volcanic activity of Hijiori caldera is subdivided
into ﬁve stages ; +) pyroclastic ﬂow without welding, ,)pyroclastic ﬂow with welded base, -) lava dome, .)pyroclastic ﬂow of the maximal, /) local and small scale
tephric deposit that covers the lake deposits(Kawaguchi and Murakami, +33.).
,. Samples and methods
Samples for radiocarbon dating (charred woods,
woods, peat, plant material and organic sediments)
were collected from the body of pyroclastic ﬂow deposit
or from a carbon rich horizon directly covered by the
pyroclastics. Locality of the outcrops (latitude andlongitude) is reported in a geodetic frame of the Tokyo
Datum. Samples collected in this report will represent
the age of the ﬁrst eruptive stage +, and the mostprominent stage .. After preparations, +.C concentration of the samples was measured with beta-ray counting using liquid scintillation counter in case that a plenty
amount of carbon were available, otherwise +.C, +-C,
and +,C were measured with AMS. All the analyzer inthis report is BETA ANALYTIC INC. (.23/ SW 1.
Court, Miami, FL, U.S.A. --+//). The author collected all the samples.
-. Result
In the following descriptions, “measured +.C age”
stands for a calculated age using +.C/ +,C ratio of theNo. +.
Measured +.C age : +*,,/*῍0* y.B.P., d +-C῎ῌ,/.3ῑ
Calibrated +.C age : +*,,.*῍0* y.B.P.
Calendar age : +,,-0*ῌ++,02* (, s) y.B.P.,+,,--*ῌ++,2-* (+ s) y.B.P.
Intercept age : ++,3/* y.B.P.
Analysis (preparation) method : Beta counting (acid/alkali/acid)
Analyzed material : Charred wood bulk
Sampling date : - Aug. ,**+
Sample number : IM,**+*2*-J (Beta-+0-./.)
Location : N-2῏-.ῌ/0.2/ῐ, E+.*῏*2ῌ/*.**ῐ
(+.1 km south from the Hijiori caldera center)Occurrence : Charred woods are accumlated near thebottom of the Hijiori pumice ﬂow deposit in stage +.
Analyzed sample is a piece of charred wood the diameter of which is about +/ cm.
No. ,.
Measured +.C age : +*,-0*῍+** y.B.P., d +-C῎ῌ,..2ῑ
Calibrated +.C age : +*,-1*῍+** y.B.P.
Calendar age : +,,2.*ῌ++2.* (, s) y.B.P.,+,,13*ῌ++,3,* (+ s) y.B.P.
Intercept age : +,,-,* y.B.P.
Analysis (preparation) method : Beta counting (acid/alkali/acid)
Analyzed material : Charred wood bulk
Sampling date : , Aug. ,**+
Sample number : IM,**+*2*,B (Beta-+0-./-)
Location : N-2῏-1ῌ*..1/ῐ, E+.*῏+.ῌ*+..3ῐ
(0.0 km ENE from the caldera center)Occurrence : This outcrop comprises layers of stage + to.. No charred material was found except the stage +.
The charred wood from the Hijiori stage + layer
analyzed is about +* cm in diameter.
No. -.
Measured +.C age : +*,--*῍/* y.B.P., d +-C῎ῌ,2.+ῑ
Calibrated +.C age : +*,,2*῍/* y.B.P.
Calendar age : +,,/2*ῌ+,,/,* (, s)+,,-1*ῌ++,2-* (, s), +,,-/*ῌ++,3** (+ s) y.B.P.
Intercept age : +,,+**, +,,*+*, ++,31* y.B.P.

On the Eruption Age of the Hijiori Caldera, Based on More Accurate and Reliable Radiocarbon Data

Analysis (preparation) method : AMS (acid/alkali/
acid)
Analyzed material : Wood
Sampling date : - Aug. ,**+
Sample number : IM,**+*2*-G (Beta-+0-...)
Location : N-2῏-.῍/1.+*ῐ, E+.*῏*2῍.3./-ῐ
(,.2 km SSW from the caldera center)Occurrence : The sample was collected from an organic
material-rich layer with good continuity and /ῌ+* cm inthickness, which is sandwiched between reddish Tertiary silt and Hijiori pumice ﬂow deposit in stage +.
No. ..
Measured +.C age : +*,--*῍/* y.B.P., d +-C῎ῌ,2.*ῑ
Calibrated +.C age : +*,,2*῍/* y.B.P.
Calendar age : +,,/2*ῌ+,,/,* (, s),+,,-1*ῌ++,2-* (, s), +,,-/*ῌ++,3** (+ s) y.B.P.
Intercept age : +,,+**, +,,*+*, ++,31* y.B.P.
Analysis (preparation) method : AMS (acid/alkali/
acid)
Analyzed material : Wood
Sampling date : +. Sep. ,**+
Sample number : IM,**+*3+.B (Beta-+0-./+)
Location : N-2῏--῍./.12ῐ, E+.*῏+*῍/-./2ῐ(/.- km SSE from the caldera center)Occurrence : Analyzed sample was collected from thelayer (+ cm in thickness) of organic carbon rich material, which layer is sandwiched between a Tertiary silty
basement and the Hijiori pumice ﬂow deposit (stage .)with pale blue sandy basal part of about / cm thickness.

203

Analysis (preparation) method : AMS (acid/alkali/
acid)
Analyzed material : Plant material
Sampling date : +. Sep. ,**+
Sample number : IM,**+*3+.A (Beta-+0-./*)
Location : N-2῏--῍./.12ῐ, E+.*῏+*῍/-./2ῐ
Occurrence : Location of this outcrop is the same as the
No. .. Analyzed organic particles were less than + mmin size and dispersed in pale blue sandy ash covered by
the pumice ﬂow deposit in stage ..
No. 1.
Measured +.C age : ++2/*῍/* y.B.P., d +-C῎ῌ,0..ῑ
Calibrated +.C age : ++,2-*῍/* y.B.P.
Calendar age : +.,*1*ῌ+-,/.* (, s),+.,*.*ῌ+-,0-* (+ s) y.B.P.
Intercept age : +-,2-* y.B.P.
Analysis (preparation) method : AMS (acid washes)
Analyzed material : Organic sediment (silt)
Sampling date : +, Sep. ,**+
Sample number : IM,**+*3+,U (Beta-+0-..0)
Location : N-2῏.*῍,+.+/ῐ, E+.*῏*3῍.1.3-ῐ
(1.- km N from the caldera center)Occurrence : The sample was collected from an organic
material-rich layer with poor continuity and less than +cm in thickness. This layer is on reddish or purplish
Tertiary silt, and is covered by the Hijiori pumice ﬂow
deposit in stage ..

No. /.
Measured +.C age : +*-+*῍/* y.B.P., d +-C῎ῌ,3.,ῑ
Calibrated +.C age : +*,,.*῍/* y.B.P.
Calendar age : +,,-/*ῌ++,1+* (, s),+,,--*ῌ++,2+* (+ s) y.B.P.
Intercept age : ++,3/* y.B.P.
Analysis (preparation) method : AMS (acid/alkali/
acid)
Analyzed material : Peat
Sampling date : +- Sep. ,**+
Sample number : IM,**+*3+-FB (Beta-+0-..3)
Location : N-2῏-3῍*1.0.ῐ, E+.*῏+*῍*1.1+ῐ(-.+ km NNE from the caldera center)Occurrence : Analyzed sample was a black peat layer
with about + cm in thickness which is sandwiched between the Tertiary purplish silt and Hijiori pumice ﬂow
deposit in stage ..

No. 2.
Measured +.C age : 1,2-*῍/* y.B.P., d +-C῎ῌ,..1ῑ
Calibrated +.C age : 1,2-*῍/* y.B.P.
Calendar age : 2,1.*ῌ2,.0* (, s),2,0-*ῌ2,/0* (+ s) y.B.P.
Intercept age : 2,0** y.B.P.
Analysis (preparation) method : AMS (acid washes)
Analyzed material : Organic sediment without charcoal,plant
Sampling date : . Aug. ,**+
Sample number : IM,**+*2*.D (Beta-+0-../)
Location : N-2῏-2῍.1.-1ῐ, E+.*῏++῍-2./0ῐ(/.. km NE from the caldera center)Occurrence : The sample analyzed is an organic
material-rich layer with good continuity and +ῌ, mm inthickness, which covers Tertiary orange silt. About +*cm from the top of the silt is more reddish in color,probably representing an oxidation by heating with theoverlying Hijiori stage . pumice ﬂow deposit.

No. 0.
Measured +.C age : 3,2+*῍/* y.B.P., d +-C῎ῌ,3.+ῑ
Calibrated +.C age : 3,1.*῍/* y.B.P.
Calendar age : ++,,+*ῌ++,++* (, s),++,+3*ῌ++,+0* (+ s) y.B.P.
Intercept age : ++,+2* y.B.P.

.. Discussions and summary
In spite of smaller analytical errors (῍/*ῌ+** years,
while ῍+2*ῌ-.* years by Ui et al., +31-), total range ofthe measured +.C ages in this report was wider (++,2-*
ῌ 1,2-* y.B.P.) relative to the range +*,1.*ῌ3,12* y.B.P.
by Ui et al. (+31-). However, weeding out three

204

Isoji MIYAGIsamples, No. 0, 1, and 2 will reduce the ﬂuctuation.
All the three dropped samples correspond to those put
in parenthesis in Fig. ,, indicating that the thickness oforganic carbon rich layer is less than + cm. Because
they have relatively lower carbon concentration andsmaller (silty sediment except No. 0) organic carbon
particles, it is rational to remove them. The author
supposes the lesser carbon concentration in the dropped
samples from the stage . to be related with a possible
damage in ecological system due to the forerunningstage + activities.
Measured +.C ages of qualiﬁed samples (No. +ῌ/) fallin a narrow range between +*,-0* and +*,,/* y.B.P.,
with good agreement with +*,1.*ῌ3,12* y.B.P. by Ui etal. (+31-). However, the “Measured +.C ages” do not
represent true eruptive ages because they still need to becalibrated as mentioned earlier.
After the corrections for both the carbon isotopic
fractionation e#ect and the +.C production rate change,“Calendar ages” become available. All the intercept
ages of the screened samples fairly focus around +,,*//
y.B.P. (arithmetical mean) and fall within an overwrapped + s probability range from +,,--* to ++,3,* y.
B.P. (Fig. ,). Thus no signiﬁcant di#erence isrecognized in the formation age between the stage + andstage . of the Hijiori pumice ﬂow deposits. To summarize, the eruptive activities of Hijiori volcano reached
its peak shortly after the outbreak at about +,,*** y.B.P.
(calendar age). The coincident air fall pumice layers
can be used as a good time marker in archaeologicalstudies.
Acknowledgements
Drs. T. Yamamoto and J. Itoh are acknowledged fortheir advice on sampling methods for radiocarbon
dating how to prevent secondary alteration after the
sampling. I thank Dr. H. Takahashi for discussions
about radiocarbon dating methods. I also appreciate
the constructive comments provided by an anonymousreferee and Dr. Ui.
References
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