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UDO
( )
ka (
Hayakawa ( ) ( ) (Hayakawa, )
AIST, Institute of Geology and Geoinformation, Geo-
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 . . 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 ( . 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 post-caldera 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.
: Towada volcano, Ogurayama Lava Dome, stratigraphy, eruption age, eruption sequence, eruptive
episode D , eruptive episode A
km A, B, C
a, b (Hayakawa, )
( )
a, c
. ka) . ka
Hayakawa, ; ;
Tsukuba, Ibaraki , Japan.
Corresponding author : Takashi Kudo
logical Survey of Japan, Central , Higashi , e-mail : [email protected]
Takashi K
Eruption Age and Sequence of Ogurayama Lava Dome
at Towada Volcano, Northeast Japan Arc
Key words
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1 2 390
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
“Towadako-toubu” published by the Geographical Survey Institute is used.
(a) (b) (c)
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of Towada volcano. Ages : *Machida ( )
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Fig. . Eruptive history during the post-caldera stage
and Hayakawa and Koyama ( ) ; **Kudo and
Sasaki ( ) ; ***Kudo ( ) ; ****Kudo
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Table . Modal compositions of eruptive rocks from the eruptive episodes A to G in Towada volcano.
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cal kyr BP G
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A a
(Hayakawa, )
km G
a, c Hayakawa, (vol. )
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Table . Representative whole-rock major element compositions for eruptive rocks from the
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Fig. . Whole-rock TiO -SiO and MnO-SiO diagrams for eruptive rocks from the eruptive
episodes C, E and G. The other eruptive rocks during the post-caldera stage of Towada
volcano are plotted in the outside of these frames.
C, E G TiO -SiO MnO-SiO
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96
h G
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Fig. .
of Ogurayama (loc. ). Its location is shown in
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Fig. . Modal compositions of eruptive rocks from the eruptive episodes A to G in Towada volcano.
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E . cal kyr BP ;
98
Fig. . Whole-rock TiO -SiO and MnO-SiO diagrams for the Ogurayama Lava Dome and related rocks.
. TiO -SiO MnO-SiO
. cal kyr BP
D . cal kyr BP
D
(Hayakawa, )
Fig. . A sketch of a fallen tree near the summit of D
D D.
c
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Hayakawa ( )
. cal kyr BP
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Fig. . Correlative stratigraphic columns for representative sections. Locations of each section are shown in Fig. a.
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100
Fig. . Stratigraphic columns for Nakanosawa Ash and Herai Ash at each type locality.
Locations of each section are shown in Fig. a.
. a
(Hayakawa,
)
)
( )
a
; Self and Sparks, ; Walker,
mm
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et al.,
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101
Fig. .
the thickness of tephra in centimeters. A part of thickness data for Oguni Pumice (marked by *) is
quoted from Hayakawa ( ).
. D D (cm)
* Hayakawa ( )
++
+32/
+++32/
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assuming a relation . presented by Haya-
(S) (T)
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. .
(S) and the thickness (T), for Nakanosawa Ash
and Herai Ash. Broken lines are iso-volume lines
kawa ( ) (see text in detail).
.
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kawa ( )
Hayakawa ( ) D Hayakawa ( )
103
. : : : Self and Sparks, ; Walker,
. km ; ;
; Morrissey and Mastin,
. g cm . g cm
. g cm . g cm
cm
Hayakawa ( )
cm cm
. km
. km a
. g cm . g cm . g cm
. g cm
DRE . km
. cm
D
cm cm G
mm
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. wt. SiO
mm
vol.
vol.
A C
vol.
(armored lapilli)
D
V et al.,
TS V T S
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Fig. . Whole-rock TiO -SiO and MnO-SiO diagrams for the eruptive rocks from
the post-caldera stage of Towada volcano.
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106
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Kano ( )
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no. .
( )
.
Hayakawa, Y. ( ) Pyroclastic geology of Towada vol-Ecano. , .
C( )
E C ( . . cal kyr .
( )BP)
D.
Horiuchi, K., Sonoda, S., Matsuzaki, H. and Ohyama, M.
( ) Radiocarbon analysis of tree rings from a . -cal
kyr BP pyroclastically buried forest : a pilot study.D, .
. cal kyr BP DHoughton, B.F., Wilson, C.J.N., Smith, R.T. and Gilbert,
J.S. ( ) Phreatoplinian eruptions. In
D
Hunter, A.G. and Blake, S. ( ) Petrogenetic evolution
of a transitional tholeiitic - calc-alkaline series : Towada
volcano, Japan. , .
( )
p.
( )
.D
Kano, K., Yamamoto, T. and Ono, K. ( ) SubaqueousA
eruption and emplacement of the Shinjima Pumice, Shin-
jima (Moeshima) Island, Kagoshima Bay, SW Japan.
, .A( )
.
( )
no. .m
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.
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.
( )
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Kuno, H., Ishikawa, T., Katsui, Y., Yagi, K., Yamasaki,
Volcanoes
et al.
Bull. Earthq. Res. Inst.
Radi-
ocarbon,
Encyclopedia of
J. Petrol.,
J.
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107
M. and Taneda, S. ( ) Sorting of pumice and lithic Morrissey, M.M. and Mastin, L.G. ( ) Vulcanian erup-
fragments as a key to eruptive and emplacement mecha- tions. In (Sigurdsson, H. ed.),
nism. , . Academic Press, .
( ) ( )
. p.
( )
( )
. .
( ) ( )
. no. .
( ) ( )
no. . .
Lipman, P.W. ( ) Mineral and chemical variations ( )
within an ash-flow sheet from Aso caldera, Southwestern .
Japan. , . Self, S. and Sparks, R.S. J. ( ) Characteristics of wide-
( ) spread pyroclastic deposits formed by the interaction of
silicic magma and water. , .
. ( )
( )
p.
( ) .
. ( )
( ) .
( )
. no. .
( )
no. . ashes, and their water-flushed origin.
( ) , .
p.
Encyclopedia of Volcanoes
Japan J. Geol. Geogr.,
Contrib. Mineral. Petrol.,
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