IAVCEI 2013 Scientific Assembly

A Guide for Mid-Conference Field Trip

Kirishima
A-1

A-2

M2 & M3:
Aira Course
A-3

Sakurajima

A-4

Kagoshima City
A-8
A-7

A-6

A-5

M1: Ibusuki
I-1 Course
I-3
I-2A

Kaimondake

I-2B

I-5

I-4

Written by
Tetsuo Kobayashi, Ryusuke Imura and Mitsuru Okuno

Pictorial 1: Ibusuki course (M1)

Kaimondake
Ohnodake

Takeyama

Ikedako
Unagi-ike

Yamakawa coveUomidake

Chiringashima
Photo 1 Oblique view of the Ibusuki volcanic area from northeast (photo by M. Okuno).
Kaimondake
Nabeshimadake

Ikedako

Kiyomidake tephra

Photo 2 Ikeda caldera and Kaimondake volcano viewed from Shinageshi (Stop I-2B) located on
northeastern lakeside (photo by M. Okuno).

Photo 3 Outcrop of Kiyomidake tephra at Usuyama (Stop I-3)
(photo by D. Fukushima).
(i)Pictorial 2: Aira course (M2, M3)

Minamidake

KitadakeKagoshima City

Hesei cone(since 2009)Showa lava flow
(1946)

Nabeyama cone
(AD 764)

Photo 4 Sakurajiama volcano viewed from east (photo courtesy of Osumi River, National highway
Office, Ministry of Land, Infrastructure and Transport).

Karakunidake

Shinmoedake

Takachihonomine
OhachiPhoto 5 Representative peaks of Kirishima volcano taked from Takachiho Bokujo (Stop A-2)
(photo by R. Imura).

Sakurajima

Aira caldera

Photo 6 Aira caldera, now submerged (foreground) and active Sakurajima volcano (background)viewed from Stop A-3 (photo by R. Imura).
( ii )

IAVCEI 2013 Scientific Assembly
Mid-Conference Field Trip (July 22, 2013)
M1: Ibusuki Course, M2 and M3: Aira Courses
Tetsuo Kobayashi1), Ryusuke Imura1) and Mitsuru Okuno2)
1) Graduate School of Science and Engineering, Kagoshima University
2) Faculty of Science; also AIG Collaborative Research Institute for International Study on Eruptive History andInformatics (ACRIFIS-EHAI), Fukuoka University
Abstract
Kagoshima graben in southern Kyushu, Japan, includes the Kakuto, Kobayashi, Aira, and Ata calderas, andextends southward to the Kikai caldera. These calderas have post-caldera volcanoes such as Kirishima,
Sakurajima and Kaimondake. To enjoy the landscape and archaeology related to active volcanoes, we provide
three courses for 1-day trips from Kagoshima City, where the IAVCEI 2013 Scientific Assembly is held.
Key words: Kagoshima graben, Kirishima, Sakurajima, Kaimondake, archaeological museum

INTRODUCTIONGEOLOGY

The 1000-km-long southern Kyushu-Ryukyu
volcanic arc includes the active volcanoes of
Kirishima, Sakurajima, and Kaimondake on Kyushu,
and several volcanic islands to the southwest. The
Wakamiko in the Aira caldera, and Ikeda caldera andYamakawa maar in the Ibusuki area have also beenclassified as active volcanoes.
We provide three courses (M1–M3) for the
mid-conference field trip on July 22, 2013. The M1
course goes to the Ibusuki area. We will see
Kaimondake and other post-caldera volcanoes of Ata
caldera, and visit an archaeological museum. The M2
and M3 courses proceed clockwise and anticlockwise,
respectively, around the Aira caldera. Both courses
include stops at the Sakurajima and Kirishima
volcanoes, and at facilities such as an archaeologicalmuseum and a famous shrine. These trips cover thelandscape of active volcanoes of southern Kyushu(Kirishima, Sakurajima, and Kaimondake), where thevolcanic deposits and their impacts on residents canbe observed.

Volcanoes in southern Kyushu

The Okinawa trough, located behind this volcanicarc, is a marginal back-arc basin that has been active
since the late Miocene (Fig. 1). Simultaneous with its
opening, tectonic movements associated with intense
volcanism occurred and volcanism has predominatedover a wide area to the west of southern Kyushu. The
Kagoshima graben (Tsuyuki, 1969), which trends
NNE–SSW, defines the eastern margin of the
volcano-tectonic depression. Kagoshima Bay,approximately 20–30 km in width, occupies
two-thirds of the southern part of the graben.
The basement complex of southern Kyushu iscomposed mainly of the Shimanto supergroup,
Miocene silicic plutonic rocks, and Neogene–Early
Pleistocene volcanic rocks. The Shimanto supergroup,
which is made up of highly deformed Mesozoic–
Paleogene shales, sandstones, conglomerates, andminor pillow lavas, underlies the graben. The
Shimanto supergroup is broken by step faulting andoverlain by a densely welded ignimbrite which has
been dated to about 2.9 Ma (Shibata et al., 1978) and－1－

marks the beginning of formation of the volcanotectonic depression.
The Kagoshima graben contains the Kakuto, Airaand Ata calderas and other small and unidentified
depressions (Fig. 2). The Aira, Ata, and Kikai
calderas, including the Aso caldera in central Kyushu,
were first proposed by Matumoto (1943). The Kikai
caldera, which is one of the youngest Holocene
calderas in Japan, erupted 7.3 cal kBP (Kitagawa etal., 1995; Fukusawa, 1995). It is located about 30 km
south of Kagoshima Bay. Large volumes of highly
vesiculated silicic magma in the form of pyroclastic
falls and flows were repeatedly erupted from thesecalderas and formed vast ignimbrite plateaus in thearea (e.g., Aramaki and Ui, 1976). These calderas are
now partly occupied by the active volcanoes.

Kikai caldera

More than three large-scale ignimbrites have been
erupted from Kikai caldera (Fig. 3: Ono et al., 1982).
The oldest Koabiyama ignimbrite is found only on
Takeshima and Iojima islands, which represent thenorthern margin of the caldera. It is composed ofmany thin flow units and is densely welded, but its
eruption age is not known. The next-youngest
unwelded Nagase ignimbrite is also found only on
Takeshima, but the associated co-ignimbrite ash-falldeposit (Kikai-Tozurahara ash: K-Tz) is ca. 90–95 ka
in age and is traceable into central Japan (Machida,1999).
The climactic Akahoya eruption produced the
Koya pumice-fall deposit and Koya ignimbritearound 7.3 cal kBP. The Koya ignimbrite travelled up
to 40 km across the sea and reached the Kyushu
mainland where it generally consists of a single flow
unit traceable up to 70 km away from the source (Fig.
4). The thickness of the deposit is generally <1 m, anddecreases consistently with increasing distance fromthe source. The deposit generally shows semi- mantle
bedding, like surge deposits, and is characterized by adune bedding structure with a ground layer. It is themost representative low-aspect ratio ignimbrite inJapan (Ui, 1973). The Kikai Akahoya ash (K-Ah) is aco-ignimbrite ash-fall deposit that has been identified
more than 1000 km away from the source (Fig. 5:
Machida and Arai, 1978, 1983). K-Ah shows normal
grading and has a coarse basal layer with pumice,lithics, and accretionary lapilli. In the proximal area,

the partially or densely welded Funakura ignimbrite
occurs within the Koya pumice-fall section on thecaldera rim, suggesting that the Funakura ignimbrite
is of intra-plinian type. Although a thick deposit of
the Koya (also named as Takeshima) ignimbrite isobserved in Takeshima, it shows no welding. Many
clastic dikes associated with the K-Ah are found insouthern Kyushu, suggesting that they weregenerated during the Akahoya eruption (Naruo andKobayashi, 2002). Field evidence suggests that large
earthquakes occurred at least twice during theeruption. Tsunamis may also have occurred at thistime (e.g., Maeno et al., 2006). Numerous woodtrunks were found in pumiceous deposits of theeruption along the rivers on the northern side ofYakushima Island, 25 km south of the caldera(Okuno et al., 2013). These deposits may haveoriginated in tsunamis generated by the eruption.
Iodake and Inamuradake in Satuma-Iojima are
post-caldera stratovolcanoes. Iodake is steep andrhyolitic whereas Inamuradake is small and basaltic(Ono et al., 1982; Kawanabe and Saito, 2002).
Vigorous emission of high-temperature gas has
continued at the summit of Iodake to this day. A new
volcanic island was formed by the submarine
eruption of rhyolite in 1934–1935 (Ono et al., 1982;
Maeno and Taniguchi, 2006).

Ata caldera and lbusuki volcanic area

Two opinions have been expressed about thelocation of the source caldera for Ata ignimbrite (Figs.
4, 6–8). Matumoto (1943) thought that the Ata
caldera was located at the mouth of Kagoshima Bay
with its western margin defined by the steep scarps of
the Onkadobira fault and its eastern margin by the
steep western coast of the Osumi peninsula (Fig. 8).
On the other hand, Aramaki and Ui (1966a, b),
Hayasaka (1987), and Suzuki and Ui (1983) argued
that the caldera depression was located north of thelocation proposed by Matumoto (1943), based on
sea-floor sonic prospecting data and the radial patternof depositional ramps extending outward from thissource area (Fig. 7). The Ata ignimbrite is one of thelargest Late Pleistocene ignimbrites in Japan, and was
erupted ca. 110 ka (Matsumoto and Ui, 1997). The
flat ignimbrite plateau is well preserved especiallyalong the eastern coast of Kagoshima Bay. The
maximum thickness of the deposit is 100 m. The－2－

Fig. 2 Index map showing the locations of the main calderas andFig. 1 Geological and structural map of the Ryukyu arc,
Okinawa trough, Taiwan, and vicinity (Kimura, 1985). 1,
central graben; 2, basin occupied by Pleistocene igneous

associated active volcanoes in southern Kyushu. The Aira, Ata, andKikai calderas, including the Aso caldera in central Kyushu, were first
proposed by Matumoto (1943).

intrusions; 3, major fault and fault scarp; 4, buried major
fault and fault scarp; 5, eastern boundary of the Ryukyu
ridge; 6, trench; 7, active volcanoes; 8, submarine
intrusions or volcanoes since the late Pleistocene time.

Ata Torihama
Pumice falls

Limitation of
Koya ignimbrite

Ata Pum

ice fallsya

Ko

Fig. 3

ce
mi

ls

fal

Pu

Idealized

columnar section for

Fig. 4 Map showing the location of Kikai and Ata calderas

Quaternary tephra layers(compiled by Kawanabe and Sakaguchi, 2005). Isopach lines forfrom the Kikai caldera

Ata and Ata Torihama (Nagaoka, 1988) and Koya pumice falls

on Takeshima Island(Machida and Arai, 1978) and limitation of Koya ignimbrite(after, Nagaoka, 1988).

(Machida and Arai, 1978) are also shown. The value is in cm.

－3－

Fig. 6 Map showing the distribution of Ata
ignimbrite (Suzuki and Ui, 1982, 1983). Thick andthin broken lines show the rims of the Ata caldera, as
proposed by Matumoto (1943) and Hayasaka (1987),
Fig. 5 Isopach map of the Kikai Akahoya ash (K-Ah) (Machida andrespectively.

Arai, 1978, 1983). The value is thickness (in cm).

Fig. 7 Direction of depositional ramp structure of Ata ignimbrite(Suzuki-Kamata and Ui, 1988). Solid arrow: confirmed direction by field
mapping and altimetry. Open arrow: estimated direction from topographic
maps and photographs.

Fig. 8 Geomorphologicalmap of the Ata caldera(Nagaoka, 1988). 1:
Alluvial plain and fan, 2:
Crater, 3: Conical volcano,4: Lava dome, 5: Lavaflow, 6: Caldera wall, 7:
Cliff, 8: Pyroclastic surge
deposit surface, 9: Ikeda
ignimbrite surface, 10: Itoignimbrite surface, 11:Tashiro ignimbrite surface,
Ata ignimbrite surface, 13:
Mountain and hill, 14:Landslide.

－4－

Ikedako ash

Ikeda ignimbrite

Ikeda pumice fall

Osagari scoria
Ikezaki ash
Fig. 9 Simplified geological map of the Ibusuki volcanic area (compiled
by Okuno and Kobayashi, 1991). Ok: Onkadobira fault scarp, On:
Ohnodake volcano, Ky: Kiyomidake volcano, Ik: Ikeda caldera, Un:
Unagi-ike maar, Ym: Yamakawa maar, Nb: Nabeshimadake volcano, Km:

Fig. 10 Columnar section of the 6.4 cal kBP

Kaimondake volcano, 1: Post caldera volcanoes, 2: Ikeda ignimbrite andepisode of the Ikeda caldera (Kawanabe andrelated pyroclastic surge deposit, 3: Fault scarp, caldera wall and crater

Sakaguchi, 2005). 1: Volcanic ash, 2: Scoriarim.

fall, 3: Pumice fall, 4: Ignimbrite and surge,

sh
ia
ak
ez

ag

Os

a

ori

sc
ari

Ikeda pumice fall

Ik

Ike

dak

o as

h

5: Stratified fine ash, 6: Humic soil.

Fig. 11 Isopach map showing distribution of the 6.4 cal kBP episodeFig. 12

Geological map of theof the Ikeda caldera (Kawanabe and Sakaguchi, 2005).

Nabeshimadake volcano (Okuno andKobayashi, 1991).

－5－

most distal deposits are >100 km from the caldera(Watanabe, 1985).
Post-Ata volcanism (small stratovolcanoes andlava domes) has been mainly concentrated in the
Ibusuki volcanic area (Figs. 8 & 9). The Onkadobira
fault is part of a large system of faults that define thewestern margin of the Kagoshima graben. The largest
eruption on this stage is represented by the
Kiyomidake tephra (7.4 km3 in bulk; Okuno et al.,1995), which reached the northern part ofTanegashima Island. The 14C age for this tephra isapproximately 48 kBP (Okuno et al., 2012). Someother tephra layers on this stage erupted fromsubmerged vents in Kagoshima Bay (Nagaoka et al.,1991).
A climactic fissure occurred in the central part of
the Ibusuki area ca. 6.4 cal kBP, resulting in theformation of the ~4-km-diameter Ikeda caldera.
Sequential fissure eruptions aligned ESE formed fourmaars. The largest of these was the Yamakawa maar,
which was formed by a base surge eruption. The
caldera-forming eruption was initially phreatic, thenbecome magmatic. The deposits thus formed were,from the bottom to top, Ikezaki ash, Osagari scoria,
Ikeda plinian pumice, and Ikeda ignimbrite (Figs. 10& 11: Ui, 1967; Kobayashi and Naruo, 1986). The
Ikeda ignimbrite moved through topographic lows
and reached the sea. Many littoral vents penetrating
the Ikeda ignimbrite were exposed in the coastal cliff.
The Osagari scoria is andesitic whereas ejecta ofsubsequent eruptions were all rhyolitic. Shortly after
the formation of this caldera, phreatomagmatic
eruptions occurred from the caldera floor andproduced the Ikedako ash, which was composed
mainly of an accumulation of thin ash layers withaccretionary lapilli. The Ikedako ash is 10 m thickaround the caldera rim. Alternating thin layers of ash
gave a false impression that they were lacustrine inorigin. Many clastic dikes have been found in the
Ikedako ash (Naruo and Kobayashi, 1995). The total
erupted volume of magma erupted is estimated to be
5 km3. After a long period of dormancy, a small lava
dome, Nabeshimadake, was formed on thesoutheastern caldera rim around 4.8 cal kBP (Okuno
and Kobayashi, 1991; Okuno, 2002). As the dome
grew, its northern half slid down into the caldera due
to a sector collapse of the caldera rim (Fig. 12).

Kaimondake volcano is undissected and consists ofa basal stratovolcano and a small central volcano (Fig.
13: Fujino and Kobayashi, 1997). The first eruption
occurred near the coast at ca. 4 cal kBP, and two
eruptions were documented in AD 874 and 885. The
volcano was active for about 3000 years, during
which time 12 major eruptions occurred. These werenamed Km1 to Km12 in ascending order (Fig. 14).
The mode of eruption of this volcano was mainly
scoriaceous subplinian type which is frequently
associated with phreatomagmatic events. Lava flows
were often associated with the scoria eruptions.
Among the 12 major eruption deposits, Km1, Km9,
Km11 and Km12 were voluminous. During the mostrecent eruption (Km12b), a central volcano formed in
the summit crater of the basal stratovolcano. This
central volcano is not a simple lava dome, but amound of complex volcanic materials with acomposite structure (Fig. 13). It consists of a basal
scoria cone associated with fluid lava flows, later
capped by a viscous lava dome and subsequently
penetrated by a volcanic plug around the summit.
Numerous hot springs are used as spa resorts in thisarea. The “Suna-mushi” sand steam bath and the
Yamakawa geothermal power station are located near
the coastline of Fushime (see description of Stop I-4).
The system in the Fushime geothermal field ischaracterized by a very high-temperature (>350°C)
reservoir, which is located in a fractured zone thatdeveloped around a dacite intrusion (Okada et al.,2000).

Aira caldera

The Aira caldera occupies the northern end ofKagoshima Bay. Because it is controlled by regional
fault systems, it is rectangular, rather than circular inshape. Its northeastern portion, which has a deeper
flat bottom at 200 m below sea level, is called the
Wakamiko caldera (Shimomura, 1960). The
southeastern rim consists of remnants of alternating
ignimbrites and lava flows, ranging from 2.9 Ma to
0.2 Ma in age (Kaneoka et al., 1984). This sequencesuggests that intense volcanism, with large-scale
pyroclastic eruptions and minor lava flows, hasrepeatedly taken place in the caldera region.
At 29 cal kBP, a great pyroclastic eruption
occurred within the Aira caldera (Aramaki, 1984).
The first phase of the eruption was plinian, forming－6－

Fig. 13 Volcanic edifice of Kaimondake. (a) View from the east
(photo by K. Katahira). (b) Schematic cross section of the central
cone of the Kaimondake volcano (modified from Fujino andKobayashi, 1997).

Nabeshimadake tephra (ca. 4.3 kBP)
Ikedako ash (ca. 5.6 kBP)

Fig. 14 Idealized columnar section of tephra layers from
Kaimondake volcano (Fujino and Kobayashi, 1997; Kawanabe
and Sakaguchi, 2005).

Fig. 15 Distribution of the Ito ignimbrite (Yokoyama, 2000).

Fig. 16 Isopach map of the Aira-Tn ash (AT), withthickness in cm (Machida and Arai, 1983).

－7－the Osumi pumice fall (Aramaki and Ui, 1966a;
Kobayashi et al., 1983) with a dispersal axis to thesoutheast. This dispersal pattern indicates that thesource vent is located near the present site of
Sakurajima. The bulk volume of the deposit has been
calculated to be about 98 km3 (Kobayashi et al.,1983).
The final catastrophic eruption generated a vastignimbrite plateau over southern Kyushu (Fig. 15).
The most distal deposits are found 90 km north of thecenter of the caldera (Yokoyama, 2000). Aira Tn ash(AT: co-ignimbrite ash from the 29 cal kBP eruption),first described by Machida and Arai (1976, 1983), is
found up to 1400 km from the vent. Indeed, it covers
all of Japan and has become a good marker key bed
for tephrochronology (Fig. 16). The total volume ofmagma erupted during the Ito eruption is about 110
km3 (Aramaki, 1984). Vigorous submarine fumarolicactivity, called "Tagiri" (Ossaka, 1991; Ishibashi et al.,2008), occurs on the floor of the Wakamiko caldera.

Sakurajima volcano

Sakurajima volcano	  (e.g., Iguchi et al. , 2013),
which began to erupt around 26 cal kBP (Okuno,2002) on the southern rim of the Aira caldera, iscomposed of two adjacent stratovolcanoes, theKitadake (1117 m) and the overlying Minamidake(1040 m), and small parasitic volcanoes (Fig. 17).
Nakadake, a small peak between Kitadake andMinamidake is also thought to be a parasitic cone
formed near the summit crater. A small plateau
called Hakamagoshi which consists of basement
deposits (remnants of the caldera rim), is located on
the western coast of the volcano.
The Kitadake volcano consists largely of
pyroclastic rocks, whereas the lower slope ispredominantly lava flows. The western part has beenthoroughly dissected by deep gorges. In contrast,some areas have flat surfaces of thick pumice
deposits and are cut by deep valleys. Parasitic
volcanoes of Kitadake volcano are mainly
distributed on the eastern and western flanks. On thewestern flank of the Kitadake, three major lava
domes trend in this direction. The chemicalcompositions of these lava domes is dacites(SiO2=67 wt%). They probably erupted from thefissure vents almost simultaneously.
The Minamidake is an active stratovolcano thatbegan to grow on the southern slope of Kitadake ca.
4.5 cal kBP. The first well-documented record of alarge eruption dates to AD 764. This was a phreatomagmatic eruption occurred on the southeastern
coast of the volcano. It generated the Nabeyama tuff
cone, whose eastern half was eroded by sea waves
soon afterward and buried by the succeeding lava
flow (Kobayashi, 1982; Okuno et al., 1998; Miki,1999). The Ohira lava flowed down north from thesummit crater in ca. AD 950 (Kobayashi et al.,2009). The formation of Nakadake was estimated tooccur ca. AD 1200 (Kobayashi, 2010).
Figure 18 shows the distribution of large-scale
tephras in historical time. A series of large-scale
fissure eruptions occurred in P4 (AD 764:
Tenpyo-Hoji era), P3 (1471–1476: Bunmei era), P2(1779: An-ei era) and P1 (1914: Taisho era), startingwith plinian activity from newly opened fissure ventsand ending with the emission of lava flow.
Pyroclastic flows inevitably accompanied these
eruptions. During the An-ei eruption, submarine
eruptions continuously occurred in the northeasternoffshore area for about 2 years. The seafloor washeaved up by the intrusion of magma beneath thesubmarine deposits, producing a submarine
cryptodome that is 100 m high. Four islets arepreserved on the surface of the cryptodome. Two ofthem (Shinjima and Nakanoshima islets) arecomposed mainly of submarine deposits withoverlying blocks of giant pumice that were settled
during the first phase of the An-ei submarine
eruption (Kobayashi, 2009).
The Taisho eruption began on January 12, 1914,
from the western and eastern fissure vents. Although
activity in the western craters ceased within 2 weeks,lava emission from the eastern vents continued formore than a year. The eastern lava flow filled theSeto Strait by the end of the month, and connected
the island to the Osumi peninsula.
During October 1939, small-scale pyroclastic flow
eruptions occurred on the eastern slope near thesummit. In 1946 (Showa era), 0.18 km3 of lava
(Ishihara et al., 1981) was emitted from the same
parasitic crater.
A sudden explosion occurred in the summit craterof Minamidake in 1955, and summit eruption
intermittently occurred thereafter. The mode oferuption was mainly vulcanian. Sometimes, however,－8－Fig. 17 Geomorphological map of Sakurajima volcano (Aramaki and Kobayashi, 1986). 1: volcanic fan, 2: recent ejecta; 3:
Showa lava (S: 1946), 4: Taisho lava (T: 1914–1915), 5: An-ei lava (A: 1779), 6: submarine cryptodome formed by the
intrusion of magma (1779–1781), 7: Bunmei lava (B: 1471–1476), 8: Nagasakibana lava (N: AD 764), 9: Minamidake lava, 10:
Kitadake lava, 11: agglutinate, 12: pyroclastic flow deposit, 13: tuff cone, 14: lava dome, 15: crater, 16: cliff, K: Kitadake, M:
Minamidake, H: Hakamagoshi

Fig. 19 Annual number of explosions from Minamidake
Fig. 18 Isopach maps of pumice fall deposits in historicalsince 1955 (data from Kagoshima Local Meteorologicaltime, with thicknesses in cm (Kobayashi and Tameike, 2002).

Observatory).

－9－

an ash-laden cloud poured out continuously with no
explosive eruption for a few days, especially during
active periods. Figure 19 shows the annual numberof explosion of Minamidake crater since 1955. The
most active period of the summit crater in the 20th
century was in 1985, when 474 explosions occurred.
Volcanic activity declined after 2002, but in 2006 theShowa crater resumed activity for the first time in 70
years. It became very active after 2009. The numbersof explosions in 2009, 2010, 2011, and 2012 were548, 896, 996, and 885, respectively. In 2013, 365
explosions had occurred by the end of June. A new
volcanic edifice, the Heisei pyroclastic cone, hasbeen formed on the eastern slope of the
Minamidake.
The rocks of the Sakurajima volcano are composedof olivine-bearing and/or olivine-free pyroxene
andesite and pyroxene dacite, and show relatively
narrow compositional variation ranging from 67 to 62
Wt% SiO2. However, the SiO2 content of lavas andpyroclastic rocks erupted from lateral vents has
regularly decreased from 67 to 57 Wt% in the last
500 years. In 1946, the SiO2 content suddenly
increased to about 62 Wt %. It has remained almost
constant with some minor fluctuation, since that time.

Kakuto and Kobayashi calderas

Two basins lie to the north of Kirishima volcano:the Kakuto basin to the west and the Kobayashi
basin to the east (Fig. 2). They are interpreted to beof caldera origin on the basis of the distribution of
the two great ignimbrites, i.e., the Kakuto andKobayashi ignimbrites, and the negative Bouguer
anomalies. The western, northern, and eastern
margins of Kakuto caldera are clear, but southernpart is obscured by the overlapping younger
Kirishima volcano. The Kobayashi caldera is older
than the Kakuto caldera, and its rim is not as wellpreserved. The Kakuto ignimbrite is rhyolitic incomposition, with pyroxene and hornblende. The
Kobayashi ignimbrite was formed by the activity offelsic magma and is characterized by biotite
phenocrysts. Co-ignimbrite ash falls of the Kakuto
and Kobayashi ignimbrites have been identified incentral Honshu, Japan. The stratigraphic positions ofthese ash falls suggest that the Kakuto andKobayashi eruptions occurred 330–340 ka and

520–530 ka respectively (Machida and Arai, 2003).
The Kakuto caldera was filled with lake water until
about 29 cal kBP, and the lacustrine deposits are
distributed in the basin (Aramaki, 1968).

Kirishima volcano

Kirishima volcano is a generic designation for acluster of Quaternary volcanoes situated at theboundary of Kagoshima and Miyazaki prefectures insouthern Kyushu (Fig. 2). More than 20 volcanoes
and craters are concentrated in an oval-shaped area 30
× 20-km area. Kirishima is one of the active
volcanoes in Japan (e.g., Nagaoka and Okuno, 2011),
and many records of eruptions exist in historical time.
The Kirishima volcano consists of new and old
volcanic bodies; new bodies are exposed on thesurface, and old bodies are slightly exposed at thebase (Fig. 20). In this guidebook, the former bodies
are termed the younger Kirishima volcano, and thelatter are termed the older Kirishima volcano. The
radiometric ages of the former are 300 ka, and thoseof the latter are 1.2 Ma to several hundred thousand
years (Imura and Kobayashi, 2001). The Kakuto
ignimbrite dates to about 340 ka, separating theyounger and older Kirishima volcanoes.
The older Kirishima volcano products are now
exposed only at the western foot of Kirishima.
However, these volcanics predominantly underlie thepresent Kirishima volcano (Fig. 20). The activity of
the younger Kirishima volcano began at about 300 ka.
Small stratovolcanoes with poorly preserved volcanic
topography, such as Kurinodake and Shishikodake,were formed in the early stage of the younger
Kirishima volcano. After a few tens of thousands ofyears of relative dormancy, volcanic activity resumed
about 100 ka and has continued until the present. This
activity formed many small stratovolcanoes,
pyroclastic cones, maars, and lava flows.
During the past 29,000 years, the activity of the
Kirishima volcano occurred in a NW–SE-tendingzone (e.g. Imura, 1992). In the last 10,000 years, mostmajor eruptions have occurred in the southeasternpart of the Kirishima. The Takachihonomine volcano(1574 m), which is famous for its relation to a myth,
has a triangular summit. It began erupting ca. 8 cal
kBP. The sharp peak is an exogenous lava dome thatfilled the summit crater. The Ohachi volcano is asmall stratovolcano with a large summit crater that－ 10 －

Fig. 20 Geological map of the Kirishima volcano (simplified from Imura and Kobayashi, 2001).

Fig. 21 First subplinian eruption of the
Shinmoedake on the evening of 26 January,
2011 (photo by R. Imura).

Fig. 22 Lava filled the summit crater ofShinmoedake (photo by R. Imura).

－ 11 －

was generated on the western slope of
Takachihonomine. Ohachi has been the most active
volcano in historical time. Another active volcano
Shinmoedake (1421 m) has a large summit crater andfissure vents on its western slope. Shinmoedake
resumed activity in 1716 after a long dormant period.
Its most recent eruptive activity occurred in 2011.
The rocks of the older Kirishima volcano are
predominantly augite-hypersthene andesites, withlittle or no olivine phenocrysts. The rocks of theyounger Kirishima volcano, on the other hand, range
from olivine basalt or pyroxene andesite with orwithout olivine, to hornblende-bearing pyroxene
dacite (Sawamura and Matsui, 1957; Kobayashi et al.,1981).
More than 10 highly reliable records of eruptions
since AD 742 are known to exist. The records contain
information about the numbers of casualties, burnt
temples, shrines, and houses, and damage to
agricultural produce and livestock. Most recorded
eruptions are related to Ohachi and Shinmoedake, but
a small lava flow, Ioyama formed at the northwesternfoot of Karakunidake in 1768. Eruptions in AD 788
and 1235 at Ohachi (Tsutsui et al., 2007) and in AD
1716–17 at Shinmoedake (Imura and Kobayashi,1991) were large enough to cause complete changes
in the outlines of both volcanoes. The 1235 eruptionof Ohachi was the largest in historical time for the
Kirishima volcano.

Eruptive activity of the Shinmoedake volcano
in 2011On the morning of January 26, 2011, an eruption
occurred at the Shinmoedake volcano (e.g., Nakada
et al., 2013; Miyabuchi et al., 2013). The eruption
became subplinian beginning around 16:00 (Fig. 21),with accompanying continuous air shocks. Although
the eruption subsided after 18:00, violent pumice
eruptions occurred from around 02:00 until dawn on
the following day. Pumice eruptions occurred again
in the evening of January 27. On January 26 and 27,
a volume of pumice and volcanic ash fell on
Miyakonojyo city and other leeward areas. Car
windows were broken by volcanic ejecta at around
7–8 km distance from the crater.
On the morning of January 28, a lava dome with adiameter of tens of meters was observed in the crater(Nakada and Joint Observation Team of Kirishima

Volcano, 2011). On the morning of January 31, thespreading of lava over the crater was confirmed (Fig.
22). An explosive eruption occurred on the morningof February 1, and volcanic bombs dropped in thearea within 3.2 km from the crater. Furthermore, ashock wave broke windows in buildings in
Kirishima city, Kagoshima prefecture, and injured aperson.
During February 1–8, vulcanian eruptions repeatedevery few hours or days, and a volcanic ash plume
was observed continuously, then gradually became
intermittent. Very strong explosions occurred on
February 14 and April 18. Many lapilli were ejected
leeward within a broader area, causing damage suchas breakage of window glass. Since then, no eruption
sufficient to cause major damage has occurred.
DESCRIPTION OF FIELD STOPS
M1: Ibusuki Course
The Ibusuki volcanic area (Photo 1) occupies thewestern half of Ata caldera, as proposed by
Matumoto (1943). This course provides special
views of Lake Ikeda, Kaimondake, and otherpost-caldera volcanoes within the caldera. At
Usuyama, which is located on the northern foot ofKiyomidake volcano, we can observe thick tephra
layers in a roadside outcrop. We will also visit the
COCCO Hashimure Archaeological Museum whichis exhibiting archaeological materials with specialreference to the Kaimondake volcano disaster.
ACRIFIS-EHAI provides a photo sharing site(http://ehai-app.rd.fukuoka-u.ac.jp/IAVCEI2013/)
for IAVCEI 2013. Geological map by GSJ is alsoavailable on this site.
Stop I-1: Senganbira Lookout
On the way to Ibusuki, we have a panoramic viewof the Sakurajima volcano and Kagoshima City from
Senganbira Lookout, along the Ibusuki Skyline
mountainroad(http://www.kagoshimakendourokousha.or.jp/). Along the trail to the viewpointfrom the parking lot, we can observe the Kikai
Akahoya and Ikedako tephras in small outcrops. The
Akahoya tephra consists of ignimbrite and overlying
co-ignimbrite ash-falls. Small charcoal fragments
and accretionary lapilli can be found in this tephra.

－ 12 －

Kaimondake
Akamizudake“Suna-mushi”

Fig. 24 Photograph of the view from the Fushime coast (Stop I-4 )(photo by M. Okuno).

Takeyama

Fig. 25 Photograph of the volcanic neck Takeyama (background) and the Yamakawa
geothermal power plant (foreground) viewed from northwest near Stop I-4 (photo by M.
Okuno).

Fig. 23. Columnar section of tephra layers at Usuyama (Stop I-3)(modified from Kawanabe and Sakaguchi, 2005). Hn: Hananoki
tephra, Us: Usuyama tephra, Ky: Kiyomidake tephra, Mz: Mizusako tephra, Yd: Yadoribai tephra, A-Os: Osumi pumice fall, Iw:
Iwamoto tephra, Sz-S: Sakurajima-Satsuma (P14) tephra, K-Ky: Kikai Koya tephra (=K-Ah), Ik: Ikedako tephra, Km: Kaimondake
tephra. 1: Loam, 2: Humic soil, 3: Scoria, 4: Pumice, 5: Sandy ash and lapilli, 6: fine ash, 7: stratified ash (including accretionary lapilli),
8: ignimbrite.

－ 13 －

Stop I-2: View from the Ikedako lakeside
We can see the topographies of Nabeshimadake(Fig. 12) and Kaimondake (Fig. 13) volcanoes fromthe lookout on the Ikedako lakeside (Photo 2). We
can use the rest room and shop at Stop I-2A the
Ikedako Paradise Lakeside Station (http://www3.
synapse.ne.jp/ikepara/).
Stop I-3: Roadside outcrop at Usuyama
An agricultural roadcut provides an impressive
tephra section on the northern slope of Kiyomidake
volcano (Photo 3). Lava and overlying Kiyomidake– Ikedako tephras can be observed (Fig. 23). We candetermine the detailed depositional structure (e.g.,slumping) of the thick Kiyomidake tephra, thelargest tephra that formed during post-caldera
volcanism.
Stop I-4: Fushime geothermal field
Here, we can enjoy wonderful views of the cliffs
of Ikeda ignimbrite, the sandsteam bath
“Suna-mushi” (http://www.seika-spc.co.jp/sayuri/),the Yamakawa geothermal power station(http://www.kyuden.co.jp/effort_geothermal_t_yam
agawa.html) and Takeyama (Figs. 24 and 25).
Takeyama is a pyroxene dacite volcanic neck with afission track age of 60±30 ka (Kawanabe andSakaguchi, 2005).
Stop I-5: COCCO Hashimure ArchaeologicalMuseumThis museum (http://www.minc.ne.jp/cocco/)
exhibits archaeological information including that
related to the Kaimondake volcanic disaster. It
provides foreign-language (English, Chinese andKorean languages) explanation on paper and on iPaddevices. You can also obtain this information on
your own device by downloading the application.
M2 and M3: Aira Courses
M2 proceeds clockwise around the Aira caldera,
and M3 proceeds counterclockwise. We list here therepresentative stops of M2, which first ascends to the
Kirishima area. After lunch, we will descend south
along the eastern side of the Aira caldera, and finallyenter the Sakurajima area. M3 goes first to
Sakurajima, and then proceeds counterclockwisearound the Aira caldera. We must travel in many

buses simultaneously, so all buses may not visit all
stops.
Stop A-1: Kirishima Shrine “Kirishima Jingu”
Kirishima Jingu (http://www.kirishimajingu.or.jp/
index.html) is a Shinto shrine located at the southernfoot of Kirishima volcano (Fig. 26). The Kirishima
area plays an important role in Japanese mythology.
Here, the god Ninigi-no-Mikoto, grandson of the sun
goddess Amaterasu, is said to have descended fromheaven to rule over the earth. Devoted to
Ninigi-no-Mikoto, Kirishima Shrine was built during
the 6th century on a different site from where it stands
today. It has been destroyed numerous times by
volcanic eruptions, but it has always been rebuilt. The
current red-painted main building of the shrine wasconstructed in 1715, and survives to this day.
Stop A-2: Takachiho Bokujo (Takachiho Farm)This location provides an excellent overview of the
Kirishima volcano (Photo 5). It is like a museum of
volcanology because we can observe many types of
volcanoes, volcanic products and associatedlandforms. The ejecta of the 2011 Shinmoedake
eruptions covered not only Shinmoedake but also
other leeward volcanoes. Takachiho Farm
(http://www.takachiho-bokujou.co. jp/) is 35 hectaresof green land, where you can try milking a cow andtouching animals. The farm also produces original
dairy products including bread, ice cream, yogurt, andmeat.
Stop A-3: Uenohara Jomon-no-mori Archeological Site
The Uenohara Jomon-no-mori ArchaeologicalMuseum (http://www.jomon-no-mori.jp/) on the Airacaldera rim is the remains of a 9500 BP(corresponding to 10.6 cal kBP) settlement. We canlearn the history of the Sakurajima eruptions andhuman occupation for about 10,000 years at this site.
We will visit the reconstructed ancient dwelling
cluster, the tephra sequence observatory, and the relicpreservation dome, where pit dwellings (Fig. 27),
stone formations, earthen pit ovens, and other parts ofthe village dating to 10.6 cal kBP have been
excavated and can be observed in their original state.
If the weather is good, we will have a wonderful view

－ 14 －

Fig. 26 Kirishima Jingu (photo by
R. Imura).

Fig. 27 Reproduction of thesettlement at Uenohara Jomonno-mori (photo by R. Imura).

Fig. 28 Minamidake viewed from
Arimura lookout (photo by T.
Kobayashi). The Showa crater isbehind the right shoulder of
Minamidake. A growing slope ofthe Heisei pyroclastic cone canalso be seen.

－ 15 －

of the Kirishima volcano to the north, and Sakurajima
volcano and Aira caldera to the south (Photo 6).

days, the Kaimondake volcano can be seen to thesouth.

Stop A-4: Traditional Vinegar Factory
A traditional vinegar factory is located on the
gentle slope of the eastern caldera wall
(http://kurozurestaurant. com/). The moderate climate
and large quantities of clear water render this area
suitable for the brewing of Kurozu, high-quality
vinegar, Kurozu is a trademark meaning black
vinegar, although the product is not black, but a densebrownish yellow, just like amber. Kurozu is brewed
for about 2–3 years in a large pot designed especiallyfor this purpose. We can see how the vinegar is madeand taste it. If you wish, you can also buy some as agift. Kurozu vinegar contains large quantities ofamino acids, so it is believed to be very good for
health.

Stop A-7: Sakurajima International Volcanic
Sabo Center
The establishment of an international
comprehensive information/ research/ training center
was first proposed at the Kagoshima International
Volcanoes Conference in 1988. The resulting
Sakurajima International Volcanic Sabo Center (http://www.qsr.mlit.go.jp/osumi/kyoutsuu/sabocente)
opened in 1999. The sabo facilities and lahars in
Sakurajima are displayed on the exhibition floor.
There, we can experience the horror of lahar andlearn what to do in the case of a disaster.

Stop A-5: Tarumizu Michi-no-eki Roadside
StationThe Tarumizu Michi-no-Eki Station (http://ekitarumizu. com/index.php) is located on the east coastof Kagoshima Bay. We can see the east side of
Sakurajima volcano with Showa crater (Photo 4), andalso see the An-ei islets, which were generated
offshore to the northeast of Sakurajima during theAn-ei eruption (1779–1782). This station boasts a hotfoot bath for travelers to rest their feet while enjoying
the view of the Sakurajima and Kirishima volcanoes
and the Kagoshima Bay. We can see many rafts
floating on the sea, where yellowtails are cultivated.
Between Stops A-5 and A-6, we pass the junction
between Sakurajima and Osumi peninsula. This area
was once a 400-m-wide strait, but it was buried bylava flows from the eastern fissure vents during the
Taisho eruption (1914–1915).
Stop A-6: Arimura Lookout: Historical Lava
Flows
This stop is located on the Taisho lava flow, which
erupted in 1914. We can see Minamidake, where theShowa lava flow erupted from a parasitic vent in1946, directly in front of us. You may be able to see
eruption plumes from the Showa crater (Fig. 28).
Extensive ramp structures are present in the blocklava around the lookout site. A concrete facility at the
parking lot is a shelter against projectiles. On clear

Stop A-8: Sakurajima Visitor Center
Sakurajima Visitor Center (http://www.sakurajima.
gr.jp/svc/english/) located next to the Sakurajima
ferry terminal, is a small museum where you canlearn about Sakurajima. Travelers visiting Sakurajima
see this first. The center simply exhibits and explains
history and nature to allow a deep understanding of
Sakurajima. In the nine-part pavilion, the history of
Sakurajima and the transitions of plants, sightseeing
information and disaster- prevention activities areavailable. You can experience the active volcano
Sakurajima through the hi-vision theater, the diorama
and our computer programs. A corner shop offers
goods that you can buy only there.
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－ 20 －

IAVCEI 2013 Scientific Assembly

A Guide for Mid-Conference Field Trip

Kirishima
A-1

A-2

M2 & M3:
Aira Course
A-3

Sakurajima

A-4

Kagoshima City
A-8
A-7

A-6

A-5

M1: Ibusuki
I-1 Course
I-3
I-2A

Kaimondake

I-2B

I-5

I-4

Written by
Tetsuo Kobayashi, Ryusuke Imura and Mitsuru Okuno