Kagoshima Rift and Volcanism
An Introduction to the Geology of Kagoshima
Edited by

Kazuhiko Kano and Thomas H. Wilson

Eruption of Sakurajima volcano on October 26, 2012 viewed from Shiroyama Park, Kagoshima City

The Kagoshima University Museum, Kagoshima University

22
Preface
The Kagoshima Rift is located in southern Kyushu, Japan. The rift includes Kagoshima Bay, Sakurajima, Kirishima
and otherother active
activevolcanoes,
volcanoes,allall
of of
which
which
constituteconstitutea pleasant,a pleasant,dynamic
dynamic
and captivating
and captivatinglandscape.
landscape.
Flora andThefaunaregioninincludesandaroundseveralthe Kagoshimamineral andRiftgeothermal
are diverse energy
and representative
resources andof thishotsubtropicalsprings. toFloratemperate
and faunaoceanic
in andclimaticaroundregion.
the
KagoshimaThispublication
Rift areprovidesdiverseaandconciserepresentative
introductionof thisof thesubtropicaltectonic toand
temperatevolcanicoceanic
historyclimaticof the region
region.along
This publication
with someprovides information
additional
topics of intereston the about
tectonic
the and
Kagoshima
volcanicRift.
history
Theofefforts
the regionof all along
the contributors
with some fromadditional
Kagoshima
topics Universityof interest
about
are
greatly
the Kagoshima
appreciated.Rift. This booklet provides a concise introduction of the natural history of this region to thevisitors from overseas and other parts of Japan. The efforts of all the contributors from KagoshimaKazuhiko
University
Kano
aregreatly appreciated.

The Kagoshima Univesity Museum
ThomasKazuhiko
H. Wilson
Kano
The Kagoshima
West Virginia
Univesity
University
MuseumThomas Wilson
West Virginia University–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Contents

1. Geologic development of the Kagoshima Rift [K. Kano]…………………………………………………………3
2. Rifting and caldera-forming eruptions [K. Kano and H. Moriwaki]……………………………………………...6
3. Explosive eruptions and tephra dispersals[H. Moriwaki]………………………………………………………...94. Explosive eruptions from Sakurajima volcano [H. Moriwaki and T. Kobayashi]……………………………...10
5. Hydrothermal activity and epithermal gold-silver deposits [K. Kano]………………………………………......11
6. Submarine seismic observation using ocean bottom seismometer in Kagoshima Bay [H. Yakihara]………...12
7. Seasonal circulation and chemical variation of seawater in Kagoshima Bay [T. Tomiyasu]…………………14
8. Endemic creatures in the semi-closed water of inner Kagoshima Bay [K. Oki and K. Kano]………………16
9. Shirasu and Bora disasters [K. Uchimura, K. Oki and K. Kano]………………………………………………18
References………………………………………………………………………………………………………………..19–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––

Published on July 17, 2013 by The Kagoshima University Museum, Kagoshima University
21-30, Korimoto 1-chome, Kagoshima 890-0065, Japan.
©2013 The Kagoshima University Museum

3
3
3
130ºE

1.
1. Geologic
Geologic development
development ofof thethe Kagoshima
Kagoshima Rift
Rift
The
Kagoshima
Rift isis ofan theactiveintra-arc Rift
rift locatedlocated
1. TheGeologic
development
Kagoshima
Kagoshima
Riftanactiveintra-arc
rift
in
the
part
of
Island,Japan.
rift
Kagoshima
Riftan active
intra-arcriftThelocatedin Thethe southernsouthern
partof isKyushu
KyushuIsland,Japan.
The
rift
constitutes
the
part
the
systeminthe southernpart of Kyushu Island,
rift
constitutes
the northeastern-mostnortheastern-mostpart ofof Japan.
the rift
riftThesystemnt

Beppu

developedtheArc
constitutes
the northeastern-mostpartoftheRyukyu
rift systemdeveloped behindbehindthe volcanic
volcanic front
front
ofofthethe
RyukyuArc(Figure
1).
The
rift
valley
is
20–30
km
wide
and
extends
developed
behindthevolcanic
front
of
the
RyukyuArc(Figure 1). The rift valley is 20–30 km wide and extends

Shimabara

nic

Fro

a

Unzen

Aso

over
km
or
more
in
direction(Figure(Figurevalley20–30 km wideand extends
over 1001001).
kmThe
or riftmorein thetheisnorth-south
north-south
direction(Figure2).
Thesouthern
partinextends
extends
through direction
Kagoshima
Bay
overThe100southern
km or more
the north-south(Figure2).
partthrough
Kagoshima
Bay

lc
Vo

beneathsea
thenorthernislandKikai.
rift2). The thesouthernbeneaththesea totopartthe extends
northern throughisland ofofKagoshima
Kikai. The
TheBayrift
Kakutovalleybylow-risehighlands
withsteeply
beneathissea to theof Kikai.
riftvalleyistheflanked
flankedby northern
low-rise island
highlands
with The
steeply

Kakuto and
Hiwaki PDC

dippingTheaccommodatesorvalleyflankedwalls.
by low-risewith four
steeplydippingisparallel
parallelwalls.
The rift
rift highlands
accommodates
four
or
five
major
calderas
and
many
other
volcanoes.
dipping
parallelwalls.
The
rift
accommodates
fourorfive major calderas and many other volcanoes.

Kirishima

Aira

Ata NorthAso-4 PDC
Ito and Ata Torihama PDC
Ata PDC

Sakurajima

Kaimondake

Ata South

five major calderas and many other volcanoes.

Koya PDC

Beppu

Kikai

Strike slip
Kyushu Isl. Fault

Pacific Ocean

MiyazakiKagoshima Rift

Kyus

hu-P

Tanegashima Isl.

alau

Ridg

e

4.9 cm/yearr
Toka

Yakushima Isl.

au

i

am

Am

nds

100 kmte
Pla

Sediments

Active Fault

Volcanic debris

Cocealed fault

Alkali basic acidic rocks

Caldera rim

Non-alkali acidic rocks(mainly pyroclastic)

Distribution limit of pyroclastic
density current deposit

Non-alkali basic rocks

Inferred boundaryof Kagoshima Rift

Alkali basic rocks

d BeltFolde

AmamiOshima Isl.

Okin
awa
Trou
gh

iwan

ji-Ta

a Isla

Shin

N

0

30ºN

Late Pliocene–Holocene

Shimabara

Kuchinoerabujima

FigureFigure 2.
2. ThisThis geologic
geologic map
map shows
shows thethe distribution
distribution ofof
Daito Ridge

Spreadingaxis

5.6 cm/year

Late
Late Pliocene
Pliocene toto Holocene
Holocene rocks,
rocks, thethe Kagoshima
Kagoshima Rift
Rift
and
other
geologicfeaturesin
Kyushu.
The
shadedreliefand other geologic features in Kyushu. The shaded relief
geologic
geologic map
map isis adopted
adopted fromfrom Hanaoka
Hanaoka etet al.
al. (1996).
(1996).
Distributionsofmajor
pyroclastic
density
currents
Distributions of major pyroclastic density currents areare

Reverse Fault

Volcanic
front
MiyakojimaIsl.

adoptedadopted fromfrom Machida
Machida andand Arai
Arai (2003).
(2003). Active
Active faults
faultswere
were obtained
obtained fromfrom thethe GeologicalGeological Survey
Survey ofof JapanJapanwebsite:website: http://riodb02.ibase.aist.go.jp/activefault/.
http://riodb02.ibase.aist.go.jp/activefault/.

Rhuk

Normal
Fault

yu T
renc
h

Okinawa Isl.

East Cina Sea

IshigakijimaIsl.
Iriomote Ils.
6.3 cm/year

FigureFigure 1.
1. Tectonic
Tectonic map
map ofof thethe Ryukyu
Ryukyu Arc.
Arc. Kagoshima
Kagoshimaisis locatedlocated inin thethe northern
northern part
part ofof thethe arc
arc behindbehind thethe0

100 kmvolcanic
volcanic front.
front. Modified
Modified after
after Letouzey
Letouzey andand Kimura
Kimura(1985).
Geologicshadedmap
isadoptedfrom
Hanaoka(1985). Geologic shaded map is adopted from Hanaoka
etet al.
al. (1996).
(1996).

4
The origin of the Ryukyu Arc volcanic front can be(Letouzey and Kimura, 1985) in association withtraced back to 6 Ma. The edge of the subducted

uplifting of the fore-arc side of Ryukyu Arc (Ujiie, 1994).

Philippine Sea Plate sank to depths exceeding ~110 km

Rifting eventually reached Kyushu and formed the(Figure 3). At these great depths, the mantle and/or crust

currently active Shimabara-Beppu and Kagoshima

were partially melted (Tatsumi, 1986). The melt

intra-arc rifts.

produced a magmatic plume that began its rise to the

The beginning of the Kagoshima Rift is poorly

surface. The resulting volcanic front migrated eastward

constrained but presumably initiated no later than 800 ka

toward the Pacific Ocean, and reached its present

as inferred from the deposition of shallow marine toposition 1 or 2 Ma (Figure 4). Fore-arc stress changed

coastal plain sediments of 800–500 ka. Marine to blakishfrom compression to extension about 4 Ma and has

deposits of the Kokubu Group (Kagawa and Otsuka,

remained unchanged since then.

These stress changes

2000) accumulated in the inner bay coastal areas. The

are revealed by the fault sets developed in the deposits ofthickness of this deposit suggests that a 50 m sea level

the Miyazaki basin on the fore-arc side of Kagoshimarise occurred during the Pleistocene time (Miller et al.,Rift (Yamaji, 2003). These events collectively suggest2011) but the thickness of this deposit exceeds sea levelthat back-arc rifting started ~4 Ma in association withrise, implying significant crustal subsidence. The Kekuraslab rollback (Yamaji, 2003). The back-arc rifting was

Formation (correlative to the Kokubu Group) is thicker

initially slow but rapidly propagated ~2 Ma from thethan 800 m on the western flank of Sakurajima volcano

southwest to the northeast of the Okinawa Trough(Hayasaka, 1987).

Figure 3. Distribution of epicenters in southern Kyushu(left) and distribution of hypocenters projected on thevertical sections (below) crossing Kirishima, Sakurajima
and Kaimondake volcanoes, for the earthquakes larger
than magnitude 0 that occurred during 10 years from2002 to 2012. Compiled by K. Goto (Nansei-Toko
Observatory for Earthquakes and Volcanoes, Kagoshima
University) by using the earthquake database managed
by his observatory.

5

Holocene

Geologic
Age

Late
MiddleKirishima
volcano group

Aira PDC 30 ka

Ata Torihama PDC
240 kaShiroyama Fm

Nansatsu
Volcanic Rocks
2.59
3.60

Late
11.63
Ma

Kakuto PDC 330 ka
Hiwaki PDC 600 ka(= Shimokado PDC)

Yoshino PDC 450 ka
Pre-Aira volcanics

Early

Late

Sakurajima
volcano

0.78

5.33

Early

Main part

Forearc area
(Osumi Peninsula)

Aira caldera

Early

Pleistocene

Intra-arc area (Kagoshima Rift)

Ata PDC 110 ka
0.13

PlioceneWest coast

Koya PDC 7.3 ka
0.01

MioceneBackarc area (Satsuma Peninsula)

Hokusatsu
Volcanic Rocks

Hokusatsu
Volcanic Rocks

Koriyama Fm(freshwater lake)

Nagano Fm(freshwater lake)

Kokubu Group(non- to marine)

Terukuni PDC 3 Ma

Hokusatsu
Volcanic Rocks
Nansatsu
Volcanic Rocks
Shibisan Grd

Osumi Granite
Takakumayama Gr

Figure 4. Miocene to Holocene stratigraphy in southernmost Kyushu Island. Modified from Uto et al. (1996)

Seismic profiles across the deep inner area oflayers are displaced by many minor normal faults (Chujo

Kagoshima Bay reveal that the sediments and associatedand Murakami, 1976; Hayasaka, 1987) and provide

volcanic products of that time are confined mainly

evidence for active rifting (Figure 7).

between the eastern and western bay-shores. Sediment
thickness in the rift interior ranges from 0.8 to 1.6 km.
Recent sediments are underlain by the Late Cretaceous
accretionary sedimentary complex of the Shimanto
Group (Figure 5). Gravity modeling of the subsurface
structures across Sakurajima volcano also suggests thebasement of the rift is 2.5 km below sea level (Figure 6).
The subsidence rate during the past 800,000 to 500,000
is thus estimated to be 1–3 mm/year in the inner bay area.
Boundary faults of the rift have been poorly identified
on the ground surface, perhaps due to the surface
sedimentary processes. Sediment accumulation rateduring the last 100 years has averaged ~ 1 cm/year in the
inner bay and exceeds the subsidence rate. Acoustic
profiles across Kagoshima Bay reveal that acoustic[K. Kano, K. Oki and K. Uchimura]

6
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Figure 6. A) Residual Bouguer anomaly observed along
a profile6.across
Sakurajima
volcano.
B) Subsurface
FigureA) Residual
Bougueranomalyobserved along
a profile across Sakurajima volcano. B) Subsurface

Figure 5. Geophysical data are presented along four
profiles 5.acrossthe Kagoshimanear Sakurajima
FigureGeophysicaldata are Riftpresented
along four

Figure 7. A reflection seismic profile across the southern

volcano. acrossA) Shotthelocations(openRiftstars)near
andSakurajima
temporaryprofilesKagoshimaside of 7.Sakurajima
aftertheHayasaka,
FigureA reflectionvolcano
seismic(modified
profile acrosssouthern

earthquake
stations(solid andcircles)on avolcano. A) seismographShot locations(open stars)
temporary1987)ofreveals
packagesof sediments
throughside
Sakurajima
volcano(modifieddeposited
after Hayasaka,

structure inferredfrom gravity
alongonthea
earthquake
seismographstations anomalies(solid circles)time. reveals
Normal packages
faults are
confirmed depositedin areas through
where
1987)
of sediments

NW-SE inferredprofile. from
Modified
Yokoyama
structuregravityfrom
anomalies
along andtheacoustic
layers are
displacedor sharply bent.
time.
Normalfaultsare confirmed
in areas where

Ohkawa(1986).
anomalyB) P-wave
NW-SE
profile.Bouguer
Modifiedfrom map.
Yokoyama
andacoustic layers are displaced or sharply bent.

velocity models.Bouguer
Modifiedanomaly
from Miyamachi
et al.
Ohkawa(1986).
map. B) P-wave(2013). models. Modified from Miyamachi et al.
velocity

2. Rifting and caldera-forming eruptions(2013).

The fore-arc
side of southern
Kyushu is currently
2. Riftingand caldera-forming
eruptions
being
to thesoutheastKyushu
or south-southeast
The displacedfore-arc sideof southern
is currently
against displaced
the Philippine
Sea southeast
Plate. GPSorderived
horizontal
being
to thesouth-southeast
displacement
rates measured
along
fore-arc
area ofagainstthe Philippine
Sea Plate.
GPSthederived
horizontal
southern Kyushu
8) along
duringthea one-year
perioddisplacement
rates(Figure
measuredfore-arc areaofsouthern Kyushu (Figure 8) during a one-year period

7extending from May 1996 to May 1997, for example,

Aira caldera is located on the southern side of thevary from 1–5 cm/year (Nishimura et al., 1999). This

Kirishima volcano group and has a diameter of 20 km

rate corresponds approximately to the subduction rate of(Aramaki, 1984). This caldera is the largest in thethe Philippine Sea Plate (Figures 1 and 2). Crustal faults

Kagoshima Rift and formed about 30 ka with anproduced by rapid crustal extension accommodate theexplosive eruption volume of 450 km3(Machida and Arai,rise of large amounts of magma generated through rapid2003). Eruption of the Ito PDC represents the climactic

plate subduction beneath the area. In the past 800,000

stage of eruption from this large caldera. The Ito PDC

years, Kakuto, Aira, northern Ata, southern Ata andspread over southern Kyushu and the overriding

Kikai calderas formed from north to south along theco-ignimbrite ash known as Aira Tn tephra.

Kagoshima Rift (Figure 2).

deposits from this eruption have been located as far

Ashnorth as Hokkaido.
After

this

catastrophic

event,

thepresent-day

Sakurajima volcano (located within the Aira caldera)
began erupting about 22 ka under water near thesouthern caldera rim. Wakamiko caldera was later
produced about 15 ka in the northeastern floor of Airacaldera by submarine explosive eruption. The eruption
volume from Wakamiko caldera was relatively small (12km3) and the eruption plume likely collapsed under thesea by ingestion of ambient seawater.
Northern Ata caldera (Figure 2) is 16 km by 12 km in
Figure 8. GPS displacements during one year extending

dimension and is located 200 m below sea level. This

from the week of from May 17-31, 1996 to May 17-31,

caldera is presumed to have formed 110 ka by an1997. Modified from Nishimura et al. (1999). Geologic

explosive eruption of over 320 km3 of volcanic material

shaded relief map is adopted from Hanaoka et al. (1996).

that included the Ata PDC (Aramaki and Ui, 1966;
Machida and Arai, 2003). The caldera structure, however,Kakuto caldera, 5 km by 15 km in dimension, isremains poorly defined by acoustic profiles, being

located in the northernmost Kagoshima Rift. This

disturbed by lava domes and intrusions (Hayasaka, 1987;

caldera was produced 330 ka by the explosive eruption

Nagaoka et al., 1990).

3

of 100 km of material including the Kakuto pyroclastic

Southern Ata caldera is 24 km by 14 km in dimension

density current (PDC). The origin of the Hiwaki PDC (=and is presumed to have formed 240 ka by explosive

Shimokado PDC) that erupted 600 ka is somewhat

eruption of the Ata-Torihama PDC and associated fallout

uncertain and may have resulted from eruptions ofdeposits. The total volume of the caldera-forming

Kakuto caldera or Aira caldera to the southwest. An area

eruption exceeds 100 km3. Nevertheless, the caldera

characterized by a low gravity anomaly located on the

structure remains poorly defined in topography, andeast side of Kakuto caldera is associated with theacoustic profiles are too complicated to resolve the

Kobayashi caldera. Eruption of this caldera formed thecalderaKasamori fallout tephra 520 ka.

Subsequent eruptions occurred along the northwestern

structure(ChujoandMurakami,1976).

additional

area of the presumed caldera. These included formation

eruptions along the southern rim of the caldera producedof the Ibusuki volcano about 30 ka, Yamakawa,deposits referred to as the Kirishima volcano group

Narukawa and Unagiike maars and Ikedako caldera,(Figure 4). Since then, Shinmoedake and other volcanic

which opened about 5.5 ka, and Kaimondake volcano

centers erupted intermittently and grew over the southern(Figure 2), which has been active in the last 4 ka.

Soon

after

Kakutocalderaformed,part of Kakuto and Kobayashi calderas.

Located 50 km to the south of the southern Ata8caldera, Kikai caldera is 25 km by 15 km in dimension

between 7–4 ka at the culmination of Holocene marine

and almost completely submerged under the sea (Ono ettransgression. Subsequent uplift of the geomorphic andal.,depositional surface is centered in the western part of the1982).

The

islands,

Satsuma-Iwojima

andShowa-Iwojima (or Shin-Iwojima) are post- caldera lava

caldera.

domes that grew over the caldera rim. Small submarine

movements associated with the Quaternary volcanic

volcanic edifices also occur on the caldera floor 400–500

activities of the Aira caldera volcanic field provide us

m below sea level. Kikai caldera presumably formed

with fundamental data to evaluate future eruptions.

3

about 95 ka by explosive eruption of 100 km ofThefindings

forthelong-term

crustal[K. Kano and H. Moriwaki]

volcanic material. This was one of the Earth’s most
explosive eruptions. Volcanic deposits from this eruption
include the Koya Pumice Fall, Koya PDC (famous for its
low aspect ratio) and Akahoya Ash Fall (K-Ah) about
7.3 ka. The volume of the latest large-scale eruption
reached an estimated 170 km3(Machida and Arai, 2003).
PDCs have repeatedly erupted from the major calderas
spread throughout southern Kyushu. Co-ignimbrite ashes
have been transported by westerly winds to northeast
Japan and form useful stratigraphic markers throughout
much of Japan. In the Kagoshima Rift, large-scale
explosive eruptions of VEI (volcanic explosivity index)
6–7 have recurred at regular intervals of ~50,000 years
during the past 300,000 years (Figures 4 and 9) andpresumably caused serious damage to flora and fauna inecosystems throughout Japan. Clear evidence ofsignificant environmental impacts is lacking with thepossible exception of one case from southern Kyushu. In
this case, pottery excavated from settlements above the
7.3 ka K-Ah boundary differs from that found in earlier
settlements. We cannot deny that such large-scale
explosive eruptions will continue in the future and will
have catastrophic impact on the southern Kyushu region.
Intermittent magma accumulation in the underlying
reservoirs of volcanic centers in the region is best
exemplified by the Aira caldera (Figure 2) that formed
about 30 ka. Its initial formation was followed byeruptions of Sakurajima volcano, Wakamiko caldera, anddomes of unconfirmed age. Leveling along the coastal
areas of Aira caldera reveals uplift of the area during thepast 200 years. Subsidence temporarily occurs in
response to moderate scale eruptions of Sakurajima
volcano; however, long-term uplift persists. Overall,

Figure 9. Distribution of tephras erupted in the past

there is a net uplift of between 7 to 15 m above sea level~110,000 years from southern Kyushu Kagoshima Riftof Holocene coastal plain sediments across the region

region. Modified from Moriwaki (2010).

(Figure 10). The coastal plain sediments were deposited99
3. Explosive eruptions and tephra dispersals
and extensive
Quaternary
tephras were
3. Numerous
Explosive eruptions
and tephra
dispersals
deposited
acrossarea (Figure9). Sources
included
Numerous
andtheextensive
Quaternary
tephraswere
the
Kakuto,Aira,
and Kikai
volcanicincluded
fields,deposited
acrossthe Ata,area (Figure9). Sources
Kagoshima
and Ata,even and
someKikaifrom eruptionsin thethe Kakuto,RiftAira,
volcanic fields,
TokaraIslands
the rift.
Kagoshima
Riftsouthwest
and even ofsome
fromEruptions
eruptionsinclude
in theplinian,sub-plinianand caldera-forming
types. include
Of theTokara Islands
southwestof the rift. Eruptions
eruptions
identified so
far,caldera-forming
those that occurredin theplinian, sub-plinianandtypes.
Of lastthe~100,000
years are much
eruptions identified
so far,better
thosedocumented.
that occurred in the last~100,000 years are much better documented.

Figure 10. Uplift of the 7 ka isobase in the Aira caldera
andits environs
domal
reliefin caldera
excessFigure10. Uplifthasof athe7 ka shapeisobasewith
in theAiraof
The center
the dome
is mostatand10itsm.environs
has ofa domalshapewithlikely
relief locatedin excessthemarkedpresencea largeof 10point
m. Thecenterbyofred
the star.
domeTheis mostlikelyof
locatedatmagma
beneaththe Theareapresence
is inferredfromthe pointchambermarked by
red star.
of a large
gravity
fromareaMoriwaki(2010).frommagma anomalies.
chamber Modified
beneath theis inferredgravity anomalies. Modified from Moriwaki (2010).

Figure 11. Isopach map of major tephras depositedFigure 12. Isopach map of major tephras deposited fromduring30 kys.map
Thicknessis givenin centimeters.
Figure the11.lastIsopachof majortephrasdeposited30
to 12.
110Isopach
ka. Thickness
is givencentimeters.
Figuremap of major
tephrasindepositedfrom

ModifiedMoriwaki(2010). is given in centimeters.
during thefrom
last 30
kys. Thickness

Modifiedfromka.Moriwaki(2010).
30 to 110
Thicknessis given in centimeters.

Modified from Moriwaki (2010).

Modified from Moriwaki (2010).

10Volcanic activity in the last 100,000 years has been

and Tameike, 2002). In the past 26,000 years, ash falls

quite intense with more than 100 explosive eruptions.

from plinian scale eruptions of Sakurajima volcano

These include ~30 eruptions from Kirishima volcano

repeatedly buried local dwellings. The ash served as thegroup (Kakuto volcanic field), ~25 eruptions from Airasource of many lahars especially in areas of thicker ash

volcanic field, ~25 eruptions from Ata volcanic field, ~5accumulation.

eruptions from Kikai volcanic field, and ~10 eruptions
from Tokara Islands (Figure 9). On average, eruptions
occur approximately once every 1000 years. Tephra
air-falls accumulated mainly to the east of individual
sources being carried by prevailing westerly winds(Figures 11 and 12).
[H. Moriwaki]

4. Explosive eruptions from Sakurajima volcano
Sakurajima volcano is currently the most explosive
volcano in the rift and the most recent three large
explosive eruptions are known to have impacted
surrounding populated areas. The first of these explosive
eruptions occurred about 26 ka on the southern rim ofAira caldera. This eruption occurred only 3,000 years

Figure 13. Tephra and soil layers exposed at Takatoge

after the catastrophic eruption of Aira caldera. Since then,Pass, 15 km east of Sakurajima volcano. Modified from

Sakurajima volcano has repeatedly erupted producing 17

Moriwaki (2010).

tephra deposits (Sz-1~17 in Figure 9). 12 of 17 tephras
have been identified in the Osumi Peninsula on the eastside of Sakurajima volcano. They are intercalated with 5
tephras sourced from other volcanoes; two (Tkn and Yn)from other vents of Aira caldera, three (K-Ah. Ik andMi) from Kikai caldera, Ikeda caldera, and Miike craterof the Kirishima volcano group (Figure 9). An important
outlier marker tephra, K-Ah derived from the 7.3 kaeruption of Kikai caldera occurs in the succession of
Sakurajima tephras exposed at Takatoge pass (Figure
12).
The thick tephric loess deposited between Sz-14 andSz-15 represents a long period of dormancy (Figure 13).
Explosive eruptions resumed about 13 ka to produce the
Sakurajima Satsuma (Sz-14) tephra with estimatedvolume of 20 km3. It was the largest among all of thetephras from Sakurajima volcano and likely caused
severe damage all over southern Kyushu. The Sz-13

Figure 14. Isopach map of the fallout tephra produced

tephra, which overlies the ruins of the Uenoharaby the 1914 eruption of Sakurajima volcano. Modified

3

settlement, was the second largest (1.3 km ). Ash fallfrom the original map prepared by Kanai in 1920. Unit

more than 30 centimeters thick buried many farms andthickness is syaku (classic length unit in Japan), whichhouses making life difficult for many years (Kobayashi

equals to 30 cm.

11
Tephras from Sakurajima volcano are also distributed

occasionally accompanied by CO2 emissions and minor

mainly to the east or northeast, carried by the prevailingamounts of H2S, CH4 and other gases (Ossaka et al.,westerly winds (Figure 12). The influence of the1992).

westerly wind pattern in this region is highlighted by thewell-known location to observe these processes in action

pattern of thickness variations in the fallout tephra from

(Figures 15 and 16).

The

submarine

Wakamikocalderaisathe 1914 eruption (Figure 14). At present, Sakurajima
eruptions are dominantly vulcanian. Ash falls precipitate
many lahars on Sakurajima Island during periods ofheavy rain. In order to reduce the volcanic hazard, itwould be necessary not only to intensify hardcountermeasures but also to conduct careful hazard
mapping mitigation efforts along with significant
disaster prevention education outreach programs.
[H. Moriwaki and T. Kobayashi]

5. Hydrothermal activity and epithermal gold-silverore deposits
Hydrothermal activity has been prevalent in the

Figure 15. Distribution of gold-silver deposits in

Okinawa Rift System since 4 Ma, especially in thesouthern Kyushu. Modified from Watanabe (2005).

Okinawa

Geologic shaded relief map is adopted from Hanaoka etTroughandassociatedminor

rifts.

Hydrothermal activity has been concentrated along theal. (1996).

back-arc side of southern Kyushu. Hydrothermal
systems are closely associated with volcanic activity inthe region. Epithermal gold-silver deposits are found in
the area. These deposits formed initially along the west
coast of southern Kyushu. Their distribution spread into
areas very close to the present volcanic front, following
its easterly migration during the past 2 Ma (Figure 15).
The back-arc area of southern Kyushu, thus includes
highly productive gold mines of Kasuga, Iwato, Akeshi,
and Kushikino, which formed 4–3 Ma and of Fuke,

Figure 16. A) Submarine emission of hydrothermal

Ohkuchi, Hishikari, Ohnoyama, Iriki and Hanakago,

water and gas from the floor of Wakamiko caldera

which formed 2–1 Ma (Watanabe, 2005). Among these,(courtesy of H. Sakamoto, Kagoshima University); B) an

the Hishikari mine is the best mine, estimated to carry

acoustic profile across the submarine Wakamiko knoll in

250 tons of gold. The ores occur in quartz veins of Late

the Wakamiko caldera, showing volcanic gas plumeCretaceous

volcanic

from the eastern shoulder of the dome. The acoustic

succession emplaced 1–2 Ma. The ores contain 25~70

profile was recorded by the research vessel Keiten Marug/t on average and are remarkably superior to the worldof Kagoshima University in 1980.

sedimentary

rocks

and

the

average of 2 or 3 g/t. Hot water with temperatures of
60–65ºC often gush out of these ore veins, providing
direct evidence for their hydrothermal origins.

Hydrothermal water is discharged from the caldera
floor, 200 m below sea level, and a submarine lava dome.

Currently active hydrothermal activity is located along

Hydrothermal activity forms chimneys (Figure 17A),

the Kagoshima Rift. Hot water spring emissions arestibnite crust (Sb2S3) (Figure 17B) and other minor

observed even under water in some areas and are

sulfides of arsenic, gold, silver and copper together with

12
talc or other secondary silicates (Ishibashi et al., 2008).
Tagiri (which means boiling in Japanese) is well known
on the sea surface. It consists of gas bubbles, mainly of
CO2, that rise up intermittently from submarinehydrothermal vents. Massive sulfide deposits in the area
are estimated to contain approximately 900,000 tons of
antimony.

Figure 18.

Oil seepage from a mud sample dredged

from a hydrothermal vent area in Wakamiko caldera.
Courtesy of T. Yamanaka (Okayama University).

6. Submarine seismic observation using ocean bottom
seismometer in Kagoshima Bay
Nansei-toko
Volcanoes

Observatory(NOEV),

for

Earthquakes

andKagoshima

University,has

conducted seafloor seismic observations using ocean
bottom seismometers (OBSs, Figure 19). Analysis ofearthquakes recorded by this sensitive recording arrayhas been used since 1990 to improve our understandingof the active tectonic processes around Kyushu and thenorthern part of Ryukyu Islands.

Figure 17. A) View of a chimney on the Wakamikocaldera floor and B) stibnite-bearing sulfide ore
recovered from the floor. Courtesy of JAMSTEC and T.
Yamanaka (Okayama University).

Wakamiko caldera is also a place of hydrocarbon
generation (Figure 18). The mud and mudstone, andeven mineral precipitates dredged from the caldera floor

Figure 19. Deployment of an ocean bottom seismometercontain 5–50 mg/g bitumen from dried samples. The(OBS) in Kagoshima Bay.

characteristic hydrocarbon parameters of the mud andmudstone samples suggest that the bitumen formed

The OBSs are high sensitivity ground motion sensors

primarily through heating of land derived organic mattercapable of detecting the frequent low magnitudein the ambient hydrothermal system (Yamanaka et al.,earthquakes that occur in the vicinity of Sakurajima

2000). Methane released from fumaroles is also likely tovolcano. Data collected from the OBSs provide detailed

be hydrothermal in origin.

information about hypocenter location and earthquake[K. Kano]

focal

mechanisms.

OBS

observations

have

been

13
undertaken in collaboration with SVRC (Sakurajima

emissions created considerable noise so that the

Volcano Research Center, Kyoto Univ.) since 2007

seismometer location was not useful for detection of low(Figure 20).

magnitude earthquakes produced by the movements of

Figure 21 shows example waveforms produced by amagma beneath Sakurajima volcano.

volcanic tremor that occurred beneath Sakurajima
volcano in November 2011. The tremor was detected at
OBS-2 but not by the nearby land-based seismometer at
FUK located on the opposite shore. The seismic
recordings (Figure 20) indicate that the volcanic tremor
reverberated within the water column throughout the
inner bay.
In 2007 we deployed an OBS at station N1 of
Wakamiko caldera (OBS-N1 in Figure 20). Soon after
deployment, we saw gas bubbles rising from the release
point and noticed that the OBS station was very close toa highly active fumarole. Signals recorded on thisFigure 20. Submarine seismic stations OBS-1, 2, N1,

seismograph were not detected by any of the otherN2, 4 and S1 (operated by NEOV) and on-land seismic

stations in the network including the nearest station,

stations KUR and FUK (operated by SVRC) and KG

OBS-N2, 430 m east of OBS-N1 (Figure 18). At this(operated by the Japan Meteorological Agency).

point, it was apparent that ground motions in the vicinity

OBS-N1was the only OBS deployed close to one of theof this station resulted from fumarole activity. Fumarole

seafloor fumaroles.

Figure 21. Ten minute long record of ground motion produced by a volcanic tremor sourced from Sakurajima volcano.
The display consists of three-component observations from OBS 1, 2 and 4 and the vertical components measured byland-based seismometers FUK and KUR. Note the improved detection capabilities provided by the OBSs.

Nonetheless, some very interesting variations ofactivity (N1 RMSAs) was also compared to sea level

seismic activity related to fumarole emissions werechanges observed by the Japan Meteorological Agency’s

observed on this station. Since the data were quite noisy,

Kagoshima Tidal Gauge at station KG during the period

it was decided to compute the root-mean-squaresof observation. At station N1 the maxima and minima of

amplitudes

forthe RMSAs (LA and HA in Figure 22) are preceded by

one-minute time windows. Variations of RMSAs

low tide and high tide (LW and HW in Figure 21) bythrough time (Figure 22) revealed previously unknown

about one hour. Correlation between the RMSAs and the

behavior associated with daily fumarole activity.

water-level peaks was not observed at station S1.

(hereinreferredtoas

RMSAs)The RMSA observation period (Figure 22) extended

The comparison (Figure 22) reveals that variations offor three days from September 25 to 27, 2007. RMSAs

tidal gravity correlate positively with variations inof the vertical components of ground velocity at stations

fumarole activity: maxima and minima in the tidal

N1 and S1 shared no similarity (Figure 21). Fumarole

gravities are synchronized with variations in the RMSA.

14
To our knowledge, the relationship between tidal andenters the inner bay area and returns to the straight

fumarole activity has never before been reported or

which forms a connection to the outer bay. The surface

observed.

water is warm in summer and cool in winter, and theresulting density contrast between shallow and deep
water makes vertical water circulation pervasive during
the winter but limited during the remaining three seasons(Figure 23).

Figure 22. RMSAs observed over a 3-day period atstation OBS-N1 (blue dots) and station S1 (small black
dots across the bottom of the plot). Tidal water levelchanges observed at station KG are shown by the black
dotted curve (second from top). The red curve depicts
theoretically calculated changes in tidal gravity. Labels
LW and HW indicate low and high water times,
respectively.

The positive correlation between RMSAs and tidal
gravities (accelerations) remained consistent over a
one-month long observation period. The correlationsuggests that the upward tidal forces enhance the activityof the submarine fumaroles. Time lags between the
RMSAs and tidal accelerations are quite small. The
response of fumarole activity to tidal acceleration isalmost immediate.

We speculate that fluids circulatingin the underlying hydrothermal system are pressurized

Figure 23. Longitudinal profile of seasonal waterand discharged from fumaroles by increasing tidal

circulation in Kagoshima Bay. Modified from Chaenforces.

(1983).
[H. Yakihara]
The prolonged period of stagnation extending from

7. Seasonal circulation and chemical variation ofspring to autumn produces oxygen depletion in the deep

seawater in Kagoshima Bay

water of the wider bay interior. Volcanic gas emissions

Kagoshima Bay has a depth that exceeds 200 meters

are dissolved and hydrothermal water is mixed with the

Circulation of seawater isambient deep water. Gas emissions are mainly composed

slow, especially in the inner bay since the straightof CO2 (70–90 vol.%: Ossaka et al., 1992). This leads toleading to the sea is quite narrow and shallow. Seawater

acidification and enriched concentrations of mercury,enters from the Pacific Ocean through a 10 km wide

antimony, arsenic and other concomitants especially in

inlet and circulates anti-clockwise through the bay

Wakamiko

before returning to the Pacific. Some of the seawater

extremely small, some circulation of these toxic

and a narrow bay mouth.

caldera.

Althoughconcentrations

are

15
elements does occur in association with hydrothermal

fumaroles during continued geothermal activity (Figure

activity and gas movements. Mercury up-take by

25). In February and June of 2012, seawater samples

plankton and other creatures could be especiallywere collected every 50 m from the surface to 10 m

problematic since this element will accumulate in fish.

above the bay floor with Niskin sampling bottle. For

Mercury in excess of the allowable amount, 0.4 mg/kg

total mercury measurement, the water samples wereby law was actually detected in 1970’s from 10 fish

filtered with 0.45-µm-membrane filter, and the total

species caught in Kagoshima Bay. Since then, local

mercury was measured by cold vapor atomic absorption

fishermen have established self-imposed controls onspectrophotometry after BrCl oxidation under UV

selling problematic fish species. Public concern has alsoradiation.

been raised about the source of mercury in the bay
ecosystem. Since there were no factories using large
amountsofmercuryaroundKagoshimaBay,organomercurial pesticide was initially suspected as apotential source. The pesticide was used until the early
1970s for the prevention of rice diseases. Rivers andstreams draining rice-farming areas are believed to havecarried the pesticide into the bay.
In 1977 a geothermal gas seep was found on the waterbottom of Wakamiko caldera. Subsequent analysis of
sediments retrieved from the area revealed the presenceof 200 mg/kg of mercury in sediments near the
fumaroles. These findings suggested that the submarine
fumaroles

are

the

major

sourceofmercury

contamination in Kagoshima Bay (Figure 24).
Subsequent surveys (Sakamoto et al., 1995; Ando etal., 2010) have disclosed that 1) bottom sediments of

Figure 24. Distribution of total mercury in the bottom

Kagoshima Bay contain mercury (II) sulfide (0.2–200

sediments of Kagoshima Bay. Modified from Sakamotoppm as Hg), mercury (II) oxide (<0.04–11 ppm), organic(1985).

mercury (0.04–0.9 ppm) and residual mercury (0.07–39ppm); 2) mercury sulfide is abnormally concentrated in

Total mercury concentration in the water samples

the sediments proximal to the submarine fumaroles ofcollected at the gas seeping site was 0.49±0.43 ng L-1

the Wakamiko caldera floor where fumarole mercury

and the values showed no association with depth inand hydrogen sulfide gas could be condensed to

February. In June, the thermocline extends from theprecipitate on the sediments; and 3) that filtered seawater

surface to a depth of about 75 m.

and suspended matter collected from the fumaroles

decreased sharply from depths of about 110 to 130 m.

contain total mercury of 7.6 to 65.0 ng/L on average,

This drop in pH may be caused by the fumarolic activity.

which is much lower than that measured in the surface

Significantly higher concentrations of total mercury

seawater collected in the bay mouth (1.0 to 0.5 ng/L on

were observed at depths of 150 and 190 m (Figure 25).

average). Together, these studies suggest that Mercury in

Near surface concentrations of mercury are much lower

the seawater and bottom sediments is most likely

above the pH

sourced mainly from submarine volcanic gas.

concentrations were lower than 0.5 ngL-1. During thetransition

During June, the pHzone. In all samples,

The distribution of mercury in the inner part ofwarmer summer months, deepwater chemistry is

Kagoshima Bay was analyzed in detail to elucidate thesignificantly impacted by geothermal activity, which isbehavior of the mercury released from the submarine

interpreted to produce the decrease in pH value and16
increase in Hg concentration. These observations

8. Endemic creatures in the semi-closed water ofsuggested that fumaroles are the main sources of

inner Kagoshima Bay

mercury in the study area. The formation of a seasonal

The Wakamiko caldera accommodates some unusual

thermocline may have significant influence on thecreatures endemic to the semi-isolated deepwater

dispersion of the mercury released through theenvironment. Especially noteworthy is the tubeworm

geothermal activity.

Lamellibrachia satsuma that swarms 80–110 m below[T. Tomiyasu]

sea level in the areas proximal to the fumaroles of a lava
dome located on the eastern caldera floor (Figures 26
and 27). This creature has a tube that is 8 mm or less indiameter with a length between 0.5–1 m. Their presencein the area is unusual since Lamellibrachia are generally
found in much deeper waters. The tubeworm carries
endosymbiotic bacteria in an internal organ called thetrophosome. The tubeworm does not have an alimentary
canal. It obtains nutrients directly from organic matterproduced by the symbiotic sulfur bacteria in thetrophosome. These bacteria live by oxidizing hydrogensulfide in the ambient water. Sulfur utilized by symbiotic
sulfur bacteria in the tubeworm is, however, not directly
derived from fumaroles but produced from bacterial
sulfate reduction using hydrothermal methane as anelectron donor. Methane (5–20 vol.%) is released inmuch greater amounts than hydrogen sulfide (0.1–1.4
vol.%) (Ossaka et al., 1992). Tubeworms commonly live
beneath the photic zone and inhabit areas surrounding
submarine vents of thermal water and/or gas in much
deeper water; the Lamellibrachia satsuma of Kagoshima
bay live at the shallowest water depths in the world.

Figure 25. Vertical profiles of water temperature, pH
and total mercury content in water collected in February
and June at a site of the fumarolic area in Wakamiko

Figure 26. A swarm of Lamellibrachia satsuma on thecaldera. Modified from Tomiyasu (2012).

mountainside of the submarine lava dome, Wakamiko
Knoll. Courtesy of JAMSTEC.

17
feed on increasingly abundant planktons. Their life spanis only 3 months so that their numbers drop off into theautumn and winter as water bottom oxygen levels
decline.

Figure 27. Close-up of Lamellibrachia satsuma on themountainside of the submarine lava dome, Wakamiko
Knoll. Courtesy of JAMSTEC.

Figure 28. Periclimenes thermohydrophilus resting on a
tube worm in a water tank. Courtesy of Kagoshima
Aquarium.

Figure 29. A) Ciona savignyi kept in a water tank.
Courtesy of T. Yamamoto (Kagoshima University). B) A

Periclimenes thermohydrophilus is a small shrimp,
which inhabits areas near fumaroles along with theswarm on Wakamiko caldera floor. Courtesy of
JAMSTEC.

swarms of Lamellibrachia satsuma (Figure 28). Its
ecology remains poorly disclosed. A shell named

Benthic foraminifera in Kagoshima Bay appear

Solemiya tagiri inhabits the area called Tagiri where

diverse, and constitute five communities A, B, C, D andhydrothermal water and volcanic gas are discharged

E (Figure 30; Oki, 2000). Community A is distributed infrom fumaroles. It survives using the energy produced

areas under the influence of open-sea water. Community

by symbiotic sulfur-oxidizing bacteria living in the

B is distributed in areas of water with slightly lower

epithelial cells of its gill.

salinity,lowtransparency

and

high

nutrient

The sea squirt known as Ciona savignyi normallyconcentration and suggests inflow of sewage discharged

inhabits shallow sea environments but also inhabits thefrom surrounding populated areas, mainly Kagoshima

deep floor of Wakamiko caldera (Figure 29). This

City. Community C is distributed in the boundary areas

creature has the ability to uptake organic particulates andbetween water masses of different salinities and waterplanktons by filtrating water at a rate of 2–3 liters antemperatures. Community D is distributed on the basinhour and can survive even in deep water where food isfloor. Community E is distributed on the basin floor oflimited. Their number increases in the spring when they

the bay head area characterized by acidic water. All the18
specimens of Community E are represented by the

9. Shirasu and Bora disasters

species having the agglutinated test, or in other words, inShirasu and Bora are local (southern Kyushu) names

this area, the species provided with the calcareous test

for an unconsolidated mixture of pumice and ash

are entirely absent (Oki and Hayasaka, 1978; Oki, 1989).

(pyroclastic flow deposit) and well-sorted accumulation

The absence of calcareous foraminifera is assumed to beof pumice (pumice fall deposit), respectively. Shirasu

closely related to the acidic water in this area of the baymeans white sand in Japanese and appropriately

caused by the fumarolic activities in Wakamiko Caldera.

represents its appearance. Bora is a local dialect word,
meaning idiot, or more appropriately, uselessness. This
name was employed because it has no ability to retain
water

and

contains

littlenutrients

requiredfor

agriculture.

Figure 31. Collapsed cliff of a Shirasu plateau caused by
the heavy rains of August, 1993. A residential area, Aira
New Town, was extensively developed at that time.
Photo by Kokusai Kouku.

Figure30.

Distributionofbenthic

foraminiferal

communities in Kagoshima Bay. See Oki (1989) for
constituent species of the communities.

It is evident that submarine fumarolic activity inWakamiko caldera creates a unique ecosystem in which
unusual fauna thrive. The unique creatures found in thesubmarine caldera floor are likely to accumulate mercury
as well as other elements derived from the fumaroles.
Lamellibrachia satsuma, for example, were found to

Figure 32. A cavern produced in Shirasu by piping ofcontain mercury along with much smaller amounts ofwater at Nakoshi, Kagoshima City, during the heavy

gold and silver. Mercury concentrations observed in therains of August, 1993. Photo by K. Oki.

anterior muscles of the body and the posterior
trophosome were 238 ng/g and 164 ng/g, respectively.
5

5

Derived mainly from repeated caldera forming

These concentrations represent 2.2×10 and 1.5×10

eruptions, thick accumulations of Shirasu and Bora weretimes those of the filtered ambient seawater (Ando et al.,formed in and around the Kagoshima Rift during the past2002).

800,000 years. Destructive lahars and slope failures have
[K. Oki and K. Kano]

occurred repeatedly in the steeply incised plateaus made

19
mainly of Shirasu, especially during heavy rain falls

Maruzen, Tokyo, 139 p. (in Japanese).

(Figure 31). Shirasu surface soil beds are likely to slide

Hayasaka, S. (1987) Geologic structure of Kagoshima Bay, South

under their own weight, particularly when bed thickness

Kyushu. The Association for the Geological Collaboration in Japan,

exceeds tens of centimeters during 80–100 year intervals

Monograph 33, p. 225–233 (in Japanese with English abstract).
Ishibashia, J., Nakaseama, M., Seguchi, M., Yamashita, T., Doia, S.,of time.
Bora sandwiched between Shirasu along with internal

Sakamoto, T., Shimada, S., Shimada, N., Noguchi, T., Oomori, T.,

cracks in Shirasu intervals facilitates water flow andKusakabe, M., Yamanaka, T. (2008) Marine shallow-water

weathering to clay. Heavy rains easily transport finerhydrothermal activity and mineralization at the Wakamiko crater inparticles, enlarge interstices and cracks and accelerate

Kagoshima bay, south Kyushu, Japan. Journal of Volcanology andwater flow through the enlarged ‘pipes’ (Figure 32).

Geothermal Research, vol. 173, p. 84–98.

Slope failure of Bora and Shirasu beds is particularly

Kagawa, A. and Otsuka, H. (2000) Stratigraphic sequence andcommon during periods of heavy rain. In addition,

volcano-tectonic event deposits of the middle Pleistocene Kokubu

water-saturated Shirasu lanslides often liquify during

Group on the coastal area of the north Kagoshima Bay, South

movement and are transformed into Uki Shirasu, a sort

Kyushu, Japan. Journal of the Geological Society of Japan, vol. 106,of dense lahar.

762–782 (in Japanese with English abstract).

[K. Uchimura and K. Kano]

Kobayashi, T. and Tameike, T. (2002) History of eruptions andvolcanic damage from Sakurajima Volcano, southern Kyushu,

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