15
ῒΐ /* (,**/) , 1+2/ 腏腔腕腊腒腂腀腁腄腅腈腆腇腍腑腌腉腎腃腓腋腐 膺臩 臃膘膙膫 腈腈 ,**- / +, 臔膩臗,**/ - . 臔膩臬Eruptive History of Satsuma Iwo-jima Island, Kikai Caldera, after a 0./ ka Caldera-forming Eruption Fukashi MAENO and Hiromitsu TANIGUCHI ῍῍ Kikai Caldera, +1 km wide and ,* km long, is a Quaternary volcano located, nearly submerged, in the East China Sea, southern Kyushu. Two volcanoes, Iwo-dake (a rhyolitic volcano) and Inamura-dake (a basaltic volcano) on Satsuma Iwo-jima Island at the marginal part of this caldera, were formed after the great caldera-forming eruption of 0./ ka known as the Akahoya Eruption. We made a geological study to understand the eruptive history of Satsuma Iwo-jima Island after the caldera formation. The volcanic activity in the island after the Akahoya Eruption is divided into three main stages and ten sub-stages based on volcanic edifice development and tephra deposits ; the old Iwo-dake stage (stage OIo--), the Inamura-dake stage (stage In--), and the young Iwo-dake stage (stage YIo--). The old Iwo-dake stage was initiated by phreatomagmatic eruptions and pumice fallout (stage OIo-), followed by the e#usion of rhyolitic lava with continuous ejection of ash and lithic fragments, resulting in building up an old volcanic edifice (stage OIo-). In the stage OIo-, intermittent explosive eruptions also occurred. The Inamura-dake stage is characterized by the e#usion of basaltic lava flows and scoria-cone building (stage In--). After that, at the western foot of the cone, phreatomagmatic explosions occurred (stage In-) and andesitic lava flow e#used (stage In-). In the young Iwo-dake stage, the explosive eruption occurred at the beginning (stage YIo-), followed by the e#usion of multiple rhyolitic lava flows from the summit crater and the formation of hyaloclastite in shallow sea that made up the marine terrace (stage YIo-). During the past +*** years, the volcanic activity changed to intermittent ones with pumice and bomb fallout (stage YIo--). Total phenocryst contents of the Iwo-dake lavas increased up to 2 vol.through the young stages. The magma of the young Iwo-dake stage is di#erent from that of the Akahoya Eruption and the old Iwo-dake stage, bordered by the Inamura-dake stage. Key words : Kikai Caldera, Iwo-dake volcano, rhyolite lava, eruptive history, bimodal volcanism +ῐ῎ῑ῏ 膊膃腥腭腩腬腜膏膪膠臓臄腛膟膾臥腍腟自 .* km 膵腚腲臇腑臡腍腟腻膌臑膬臰腰臯腯膾腙腘腛臁 膒腥腭腩腬腎臝腝膧膥臏臆膛腛臓臠腶臋膮腚腲臇腒腡膗膡膊膃腥腭腩腬腛臁臙臛腜膱臢腑腕腌腠膢臣臮腹 臈臏腙腘腎腥腭腩腬臟腗腑腕膁膮腚膗腢腕腋腡臌臎 腖腊腡腎臐膵 ,* km, 臓臡 +1 km 腚腌腞腝臁膒腥腭腩 腬腖腊腡 (Matumoto, +3.- ; Figs. +a, b)腏腛腥腭腩腬腜 腾膎膲 +* 臤臕膇腚腣腔腡臚膲膀腛臁膉臧臜腽腚腞腠膓 膳腐腢0./ ka 腚腜 腤腥腪腫腽膣膂腖臅腟腢腡膠膯腛 腥腭腩腬臜腽腎臖膴腑腔 臊臍膯腳+312 ; 膬臩+32, ; Kitagawa et al., +33/ ; 腸臩,**,腇腁 膢臣臮腹臏臮 腹膅 臭臨膈膨腇腀 腴膼膅 膖臘膈膨腇腀 膭臱臮腹臏 臨膈膨腇腀 腓腑腕膁膱臦腺腚膻膡腒腡臉腷腽膘膋膑腎腥腪腫臜腽腱膝腚膳臋腑腔膯腑腋腽膣膿腖腊腡膬臩(+32,)腸臩(,***)Kawanabe and Saito (,**,) 32*2/12 膷臀膤膶臫膐膜膆膦膶臫 臐臡臁膄臁膄腵臬膄膕膍腼臆膄膸膚 Institute of Mineralogy, Petrology, and Economic Geology, Graduate School of Science, Tohoku Uni- versity, Aramaki-Aza-Aoba, Aoba-ku, Sendai 32*2/12, Japan. 腈腈 32*2/10 膷臀膤膶臫膐膹臒 臐臡臁膄臐臡腤腦腤膕膍腧腮腨腃 Center for Northeast Asian Studies, Tohoku Uni- versity, Kawauchi, Aoba-ku, Sendai 32*2/10, Japan. Corrsponding author : Fukashi Maeno E-mail : [email protected]

˘OIo+bˇ ˆ˙˝˛ ˚˜ ! "# ˘$%&’ (ˇ)/cm*+,- ˘ - , ,a, ,b (OIo,a, ,b). "/012 34 , 567 .8 "9ˇ ˚˜,ˇ ,m,:;,CD8 EF Gcm"H CIJ

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Page 1: ˘OIo+bˇ ˆ˙˝˛ ˚˜ ! "# ˘$%&’ (ˇ)/cm*+,- ˘ - , ,a, ,b (OIo,a, ,b). "/012 34 , 567 .8 "9ˇ ˚˜,ˇ ,m,:;,CD8 EF Gcm"H CIJ

� �� ��� � /*� (,**/)� ,� 1+�2/�

���������� ���������

� � �� � � ��

�,**-� /� +,���� ,**/� -� .����

Eruptive History of Satsuma Iwo-jima Island, Kikai Caldera,

after a 0./ ka Caldera-forming Eruption

Fukashi MAENO�and Hiromitsu TANIGUCHI��

Kikai Caldera, +1 km wide and ,* km long, is a Quaternary volcano located, nearly submerged, in the East

China Sea, southern Kyushu. Two volcanoes, Iwo-dake (a rhyolitic volcano) and Inamura-dake (a basaltic

volcano) on Satsuma Iwo-jima Island at the marginal part of this caldera, were formed after the great

caldera-forming eruption of 0./ ka known as the Akahoya Eruption. We made a geological study to understand

the eruptive history of Satsuma Iwo-jima Island after the caldera formation.

The volcanic activity in the island after the Akahoya Eruption is divided into three main stages and ten

sub-stages based on volcanic edifice development and tephra deposits ; the old Iwo-dake stage (stage OIo-�-�),the Inamura-dake stage (stage In-�-�), and the young Iwo-dake stage (stage YIo-�-�). The old Iwo-dake

stage was initiated by phreatomagmatic eruptions and pumice fallout (stage OIo-�), followed by the e#usion ofrhyolitic lava with continuous ejection of ash and lithic fragments, resulting in building up an old volcanic edifice

(stage OIo-�). In the stage OIo-�, intermittent explosive eruptions also occurred. The Inamura-dake stage is

characterized by the e#usion of basaltic lava flows and scoria-cone building (stage In-�-�). After that, at the

western foot of the cone, phreatomagmatic explosions occurred (stage In-�) and andesitic lava flow e#used (stageIn-�). In the young Iwo-dake stage, the explosive eruption occurred at the beginning (stage YIo-�), followed bythe e#usion of multiple rhyolitic lava flows from the summit crater and the formation of hyaloclastite in shallow

sea that made up the marine terrace (stage YIo-�). During the past +*** years, the volcanic activity changedto intermittent ones with pumice and bomb fallout (stage YIo-�-�). Total phenocryst contents of the Iwo-dake

lavas increased up to 2 vol.� through the young stages. The magma of the young Iwo-dake stage is di#erentfrom that of the Akahoya Eruption and the old Iwo-dake stage, bordered by the Inamura-dake stage.

Key words : Kikai Caldera, Iwo-dake volcano, rhyolite lava, eruptive history, bimodal volcanism

+� � � �������� �� !"#$%&'() .* km!*+,-.� /'(01234� 56� 7%89#:;����<=>?@ABC#!DEFG+,-HIJKL� ������#:MN�OP.QRS� TUVWA� XA89<����YZ.Q[G+K\Q]I^_`aI<� b* ,* km, !/ +1 km+Rc>:;����`aI (Matumoto, +3.- ; Figs. +a, b)J d#�����

efg +*h�i+jkIlgm#:nopq+cSrst\� 0./ ka+� uv�wxqyz{`|(\I }#����pq<~�.k ���}�� +312 ; 3���+32, ; Kitagawa et al., +33/ ; ��� ,**,�J TUVWAVW� ������� ��� ������� ��VWA ��������.Q[O��+�LHI��q���<�v�wxpq��+sF.k}.]qy�`aIJ 3�� (+32,)� ��� (,***)� Kawanabe and Saito (,**,)

��32*�2/12 ���� ¡¢�£� b/:¤:¤¥�¤¦§¨B¤©ªInstitute of Mineralogy, Petrology, and Economic

Geology, Graduate School of Science, Tohoku Uni-

versity, Aramaki-Aza-Aoba, Aoba-ku, Sendai 32*�2/12,Japan.

���32*�2/10 ���� ¡«¬b/:¤b/v­v¦§®¯°±Center for Northeast Asian Studies, Tohoku Uni-

versity, Kawauchi, Aoba-ku, Sendai 32*�2/10, Japan.

Corrsponding author : Fukashi Maeno

E-mail : [email protected]

Page 2: ˘OIo+bˇ ˆ˙˝˛ ˚˜ ! "# ˘$%&’ (ˇ)/cm*+,- ˘ - , ,a, ,b (OIo,a, ,b). "/012 34 , 567 .8 "9ˇ ˚˜,ˇ ,m,:;,CD8 EF Gcm"H CIJ

�� ����������� ����� +.C������� ��������� ��������� /.,

ka� �!"�� #�$� -.3 ka%&"'()������� �*+� ,., ka�,���������� �!"�-./��&%����(0����123 ���4�56��7� 8�9��:��;<=�>?@ A��BCDE�3/FG� �<=HIJ=�KL(M��NOP��( (Kazahayaet al., ,**, ; Shinohara et al., ,**,)0 '-� �������Q�R?����S (Saito et al., ,**+)@T������U2 (Saito et al., ,**,)VW%&� 12�XY�Z[��\]\�7�� 5��^��\]\� _��T��\]\�3�`ab�V���(/cd&P��((Saito et al., ,**, ; Kazahaya et al., ,**,)0�������������@\]\�7�e�� fS�gh��&%�iP��j(�� ���@������k�lm�������V���n� '-� ����\]\� �)��\]\� � o!"#V �pq�����M��r��NOP��V�0����st#V�u @� +3-.�-/�v$��%���� XY�Z[�&w�123V� #9j(./�x���(0 '&w�lmV��k�y��� z(

��� �)C� �;{|�}~/�o�9�*9j(0 #.9'�9�� � �`�� ;�&�VW�&��#+��3/�� XY�Z[�����������?��-���/������k����c�L(0

,� ��������,�+ � �����������,��%�XY�Z[��-��Z[��5��.�� %-/�0�� (HJ)%&1���2�� (EZ)�%���Z[�3��4V&�/���iP��((Figs. +b, c)0 &��� Fig. ,�� �56���(7e�� Fig. -�xL0 '-� XY�Z[��:V /� ���������`�� Table +�'/�-0%-��89;������:&e��;L(�<=���Z[��?���;�9� �Z[�3�a?���(0 �Z[����� �Vn/3 -��>�?��@������AOP��(0 #�@���� B%/�,��%��;L(������@�� �K-ab ; �C�D� +32,�� B%��+�;L(� ���@���K-Ns ; �C�D� +32,�� #��� ������¡�¢E�@�� �K-Kyp, K-Ky ; £¤� +31- ; �C�D�+32,� 9j(0 .P&����@���F���� #P¥P� G +.* ka �HI�D� ,**+�� G 3/ ka(Machida,

+333)� 0./ ka �Kitagawa et al., +33/ ; ¦C� ,**,� ���J�Kd&P��(0V��XY�Z[�§L�¨©%�ª«%�U2��G /2* ka �¬M�D� ,***� ��A�-/iP��(� I���@�� �K-Sd ; HI�­¤�+33,� 3XY�Z[��>�?��@��/cd&P��(�� �,��%� B%���;���V�0 '-� N%��� G +-®2 ka�O-( +.C���xLO

.37

¯� �° (K-Km)��;L( ��C�D� +32, ; ¦C�+330�0 O¯� �°� +-®2 ka�G 0***�±�O-��� Pt#�@��-3�/cd&P� #����²2�Z[��³%�,� � (Sz-S)�*2L(�Kobayashi and Hayakawa, +32. ; ¦C�D� +33.�0 �Z[�Q��� ��������/)�������U2�� N�;�/L(� �°�� �Z[�3��9������@���A���( (Figs. -, .)0

,�, � �������������C�D (+32,)�� �Z[�3���U2L(��

��������� �°�� _�` ������ ��` ������ 5�` ����� ��¦C�D (,***)� ¦C (,**,)�K-IoL, K-In, K-IoU�´��µ¶���(0 Kawanabe and Saito (,**,)�� ����� ��� ���·R��;L(���� �° (K-Iw)/;S§L�¸'� �° (K-Sk)/�´

Fig. +. Maps showing the location (a) and topo-

graphy (b) of the Kikai Caldera. Figure (c)

shows the topography with outcrop distribution

in Satsuma Iwo-jima. For the abbreviations in

(c), see the text.

�C ¹�Tº»¼72

Page 3: ˘OIo+bˇ ˆ˙˝˛ ˚˜ ! "# ˘$%&’ (ˇ)/cm*+,- ˘ - , ,a, ,b (OIo,a, ,b). "/012 34 , 567 .8 "9ˇ ˚˜,ˇ ,m,:;,CD8 EF Gcm"H CIJ

��� ������� ����������K-Sk-l, K-In, K-Sk-u����� ������ ,�� ,��.��� 2�������� ���� ! "#� $%&'(���������� K-In-+)*+ ,� ,��,���� ! -.�/0�*1� 2�������34�5���6��� 7��� ���� 89&�:;<=> ?@A��BC�D��5��E��! FE�� 89GGH�IJK�����L�?@A�LMBC;5����� 2�;7����NOPQRSTU�VWX;Y��# (Table +)!���L�Y��� Z [�\]S�^;_���`�� ���5a�bcdefgh" ij��� 2�;_���VWXk����L"#��������lm�Y��#! def�� nop;q�rs2���� etguv�#2�� >�wxyz[{�RSVW�|}"#�~�Xg��#2�;�>!���L�?@A�LM;7��> �� VWX��"�lm�� NOPQRS (Akahoya Eruption)� �X89&�VWX (Old Iwo-dake stage)� $%&�VWX

(Inamura-dake stage)� �X89&�VWX (Young Iwo-

dake stage)�Y�Em ! 2��;6��� �X89&���������� stage OIo-�,�� ,����� $%&���������� stage In-���� .����� �X89&���������� stage YIo-���� .�����Y��# (Table +, Figs. -, .)!�VWX����L�� �X89&������ stage

OIo-�� ����> �X89& +a�+b, ,a�,b (OIo+a�+b, OIo,a�,b)� .�� $%&����� stage In-������> $%& +, ,, - (In+, In,, In-)� -�� �X89&����� stage YIo������> �X89& +, ,a�,e, -a�-c, .a�.b (YIo+, YIo,a�,e, YIo-a�-c, YIo.a�.b)� ++�g��6� ! ���������E�� �������Lg����� #�� ,a, ,b� ��*��Y��#! "#� In,� In-������ i��*���� ��5a�f�g������2�g� !

,�- ���89&�� ��O���� S¡¢��£¤�¥� ¦"1�§¦¨©�ª«�� ¬¦­ 1**m�®¯��:

Fig. ,. Geological map of Satsuma Iwo-jima (modified from Ono et al., +32,).

°±89G�)² O���³=XTU�RS´ 73

Page 4: ˘OIo+bˇ ˆ˙˝˛ ˚˜ ! "# ˘$%&’ (ˇ)/cm*+,- ˘ - , ,a, ,b (OIo,a, ,b). "/012 34 , 567 .8 "9ˇ ˚˜,ˇ ,m,:;,CD8 EF Gcm"H CIJ

Fig

.-.

Colu

mnar

sect

ions

of

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outc

rops

inSats

um

aIw

o-j

ima.

Loca

tions

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ns

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�� ������74

Page 5: ˘OIo+bˇ ˆ˙˝˛ ˚˜ ! "# ˘$%&’ (ˇ)/cm*+,- ˘ - , ,a, ,b (OIo,a, ,b). "/012 34 , 567 .8 "9ˇ ˚˜,ˇ ,m,:;,CD8 EF Gcm"H CIJ

�������� ����� ���������������� ������ �!"#�� $%!&'��� (Fig. /)�()*�+,�-.�����/012�3�!� 4�5�� 678�9:;<12� 68��4=� ���12>�!?@�=�� 68�12>��� A��!()�/012�B�=�CD� 12��68EF!G.��� (Figs. ,, /)� 4�HI�J

4

KLF

MNL�OKP QR=� �SK��TUJ�12�VWX�!� +,YZ�[\��4 �]�.�����()��SK�$%!&'�CD� ^_�����`�abc�d��ef=����� ���� >�ghCijH��k�Ll�mn��� ���������o��p�� ���� 4=���SK����IoLAqF����L (Fig. /)� rq68��sX���(IoLA, B, C)�tuvwxN�#�� y�z{������s��JKwx |}f=�� H�� �q�78��sX��� (IoLF)�*12�z{ ����!� ~��t�N��#������ $%��F����!�

��������e�� rq68���iDU����^������� g�f=�� �78�� ��������D�!D� �����78���� 6����� �.���� >�� (Fig. ,)��� IoLAqF

�o���� �SK����N @�m (IoL) ���L� NL� �SK������� M�� ; 0�++P �� IoL

��k��.L�,������D� >����4=�l ���L�4=N�� �SK������ y=������ ��������� MO��5� +32,P f=�pL!� � �iD� 6��r���F��tu������¡���� ¢£¤¥¦§¨©£ª (Pichler, +30/) M�SK¢£¤¥¦§¨©£ª� ; IoHyP � y=���«¬��� M�SKt�«¬�� ; IoTeP !­)X�4 !�����.L (Figs. /, 0)� ¢£¤¥¦§¨©£ª��$%��®�Ap�¡�� (IoHy-l) ��� (IoHy-u)

�o��p�� �SK�+,���J� ^���F�=�!� ()>��¯?�p��SK�������1°�� M¢£¤¥¦§¨©£ª�±²P �� �³´ab

Table +. Geological units in Kikai Caldera area

µ��S¶�CF�·¸¹§���º»��1¼ 75

Page 6: ˘OIo+bˇ ˆ˙˝˛ ˚˜ ! "# ˘$%&’ (ˇ)/cm*+,- ˘ - , ,a, ,b (OIo,a, ,b). "/012 34 , 567 .8 "9ˇ ˚˜,ˇ ,m,:;,CD8 EF Gcm"H CIJ

� ��� ; 1�-� 2�+� ���� � �������������������

,�. ���� �!� "#$ ,-* m�%&'�()*� +,-�./0(123+45�623789�� ���!�� �:;<= In+>-�?@9� stage In-�>��ABCDE (InIL)�?@9� stage In-�*F�� GDE (InSL)� ,DE (InEL)!� H�I� stage In-�>�����*F� (Table +)�JKLM!� NOP<�2QR�S����TU��2VWXY�Z/ [\9�0]�� ����^���_� ���� ������`a7b�c9�

-� �������-�+ ����� +a, +b (OIo+a, +b)

����d/NOP<ef-�ghi (HJ)Tjklm (ST)�n]M��oL� 4p�qr (OU)*�stL�un*_�� v�wx�!yzp�{>�|}~2({� (OIo+a)� H����Rz��� (OIo+b)LM��� ,��s�0��!g� /* cm��*F�� OIo+a

!� ��2��wx�����7�]�j��� ����7��*�����n]M�� ��� cm���� x�� ¡�¢£� /�v�F�� ¤0� p�¥

Fig. .. Photograph of the outcrop at Sakamoto (ST). OIo+a>,b, In+>-, and YIo+>.a are derived from the

Old Iwo-dake volcano, the Inamura-dake volcano, and the Young Iwo-dake volcano, respectively.

Fig. /. Geomorphological map of Iwo-dake volcano.

Shaded area at the western foot of Iwo-dake

shows the distribution of volcanic bombs (YIo.a, b) ejected from the Kintsuba crater in stage

Io-�.

¦§ ¨©ª3«¬76

Page 7: ˘OIo+bˇ ˆ˙˝˛ ˚˜ ! "# ˘$%&’ (ˇ)/cm*+,- ˘ - , ,a, ,b (OIo,a, ,b). "/012 34 , 567 .8 "9ˇ ˚˜,ˇ ,m,:;,CD8 EF Gcm"H CIJ

������������� �������������� OIo+b���������� !��"#����� $%&'�(�) / cm*+,-�

-�, ����� ,a, ,b (OIo,a, ,b)

."/01234����� ,�567�.8"�9� ���,�� ,m, :;,< +m*=,-�OIo,a, ,b�<�>?@A�B>CD8�� EF� G cm"H CIJ� ��� KHJL ,��) -* cm��"�BM��G����� �BM� OIo,a=N" A� K�9 -* cm��L ������ sag structure� ��������=O,P�Q�� =�"RS� �T,����Q�� �BM��"U��VW�X�YZQ[\ �A�,-���,]� sag structure

"�^$_�`a�N�� �BM����bc���d�� O� .��6�\ Gmm��G cm"&'�efa�

.� �����

.�+ ��� + (In+)

Kawanabe and Saito (,**,)"K-In-+�g�d��>?�B>��h��iO�9dG cm��"j+klmn���Y\ op���qr2sC�t9�u?�v�=����Q�� wx.�6�\ !�Q[\ �9�"#By (YW), 2* cm, ��� (HJ), /*

cm��,-�klmn�� "#By,�>?�B>Ci# -$����� ��� !� (ST),� ,$�Y� klmn"z:�)� - cm��,{|%&� u?�}?"<"�efa� In+�9dG +* cm"'~�B>��T��6��

.�, ��� �� (InSL)

�(�� (+32,)")*���+a� ���)��, ,y� 1**m�6�\ !a�-�A*�, 9d +�Gm,-� ���"), )y.,� ��a,*� (InEL) ���*� (InIL)��6�� InSL

����"z+N��f� �S�{ZQ*�"��j+klmnCiJ��� �/,� InSLC In+�0� stage In-�"1������

.�- ��� , (In,)

Kawanabe and Saito (,**,)"K-In-,"+� +� Kj+klmn�L �g�d�� /01234,� ,$"j+klmn��h��iO>?�B>�, .� !a� �9� "#By, ,m, ���, -* cm��,-� �|� "#By,� =N"klmn��+mC�� ���qr2s� z<9�� klmn)� +�,� + cm*+,-� =�,�z: - cm�Pa� ��� !�,�� ,�- cm"9d,-� In,

����"D��2�{<", klmn��g�d�� klmn�� InSL"���=C�t� �/,� In,[{�klmn�"�2C stage In-�����

.�. ������ (InEL)

�(�� (+32,)",*���+a� �������"��� !a�-�A*�, z:�9� +* m

�Pa� ���3,�� ����"�"���C),������ ),.,�klmn�"U�C�t�B84^�� InEL�klmn�"^%�wx0�p����Q[\ K�(�� +32,L In,�0� stage In-

�"1��,-�.�/ ��� - (In-)

Kawanabe and Saito (,**,)"K-In-,"=� ,�K�������j+klmn�L �g�d�� �5����"#By4,"�9�z: +./m�Pa� ���z+�"9d ,�- cm"j+klmn� ��"9dG +*

cm�� + m*="�������� [{�z=�"9d� +* cm"j+klmn���Y� =��+�"j+klmn�6 E,%&��5�� =�"j+klmn�� ��������="RSC¡W¢X����5�-� ���������Gmm�G cm"7�"£O\, D� , cm*+"klmn �B> ����Y� O��) +* cm��"M89��� sag

structureC^%a� ���������"klmn�%&��¤|¥£�Q�� ��� !�,� ��������C¦] +�":?"§��B>"#��%� =�"j+klmn�u?�v���¨fa�In-� �S�{ZQ In,�"���TCiJ���In,�©;� In-"�2pC stage In-�����

.�0 �������� (InIL)

9d +*Gm"ªB�A*�, InSL[{� InEL���A�«Y� 3y�¬����� ���)y.,���<�Q�� ��=�,�� >�­®¯°X, O� ?@ -�/m"*��6CAa� �*�� ���klmn�"±(C�� =�����bc"'~�B>��²�6� ���qr2C¦|� ���p�klmn�^%�³� B´�CD,-� �/,�"�2C stage In-�����

/� �������/�+ ����� + (YIo+)

���qr2sC�t9�u?�v�"=���� ���B8�E,"#µ�,]� !� (ST)=�,� @) , m�Pa>�¶?"M89��� sag struc-

tureC���B> [{��·��Y���, .8��Q E�Q��5�¸���¹�9� +*�-*

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Page 8: ˘OIo+bˇ ˆ˙˝˛ ˚˜ ! "# ˘$%&’ (ˇ)/cm*+,- ˘ - , ,a, ,b (OIo,a, ,b). "/012 34 , 567 .8 "9ˇ ˚˜,ˇ ,m,:;,CD8 EF Gcm"H CIJ

cm���� ������� ������������������� (HO)��� sag structure !"#$%&'�()*+,- ./0 1* cm1 �������

/�, ����� ,a�,e (YIo,a�,e)

Kawanabe and Saito (,**,)K-Sk-u-,�234��56789:;��� /��<=>?@AB���� (YIo,a, ,b, ,c, ,d, ,e)C�D�� EFG (HJ)� HI(ST)��� JK�L� - m���� stage YIo-�MNOP�QRSTUV�LWXY8Z�[�� XY8�\]�^�C�_�`� ab�c@d efghi���`]jk'�� ���U��0lmm,m�no�� pq�rs'�t]�[�� V_��L4l +* cm

��D���� (YIo,e)�� lmmuW�vQv=>?@w����x�C�D����� (HO)��� yzR{|N^}~����f

YIo+()*+� ���@���]�_���

���%&�C�D�� �@���E��L� -* cm

���� l cmpq�rs'�� �����%&��L4lmm<+ cmu�C�D��������#$W�� ����#$��� �QR����L4lm��'�� ���Q<�@�����fw�,mC�D�� o��pq n�� t��%&��� �QR�,D (IoLD)��f���%&�������$�����C�� YIo,a�234���

/�- ������� ������ (IoHy-l)

�QR�<�����  '�¡¢�£?8}¤¢¥��� ,q¦§¨© (0�-) �]�'�]� stage YIo-�og��MNSTPª+��[�� ^�� (IoHy-l)

V«�L� +/ m��'��t���:¬�­®8}���#� E¯D°±� �²� D³�i�,m.l cm<l +* cm��1 ]� ´|���<³� .lmm<l cm��1 µB�¶"4�� t����K]

Fig. 0. A marine terrace (a) consists of volcanic conglomerate (IoTe) and hyaloclastite (IoHy-l) at south-coast of

Iwo-dake. A volcanic conglomerate (b) covers a hyaloclastite layer including a block of pseudo-pillow lava

(c) with radial cracks due to rapid cooling. Figure d shows a sketch of the pseudo-pillow lava.

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Page 9: ˘OIo+bˇ ˆ˙˝˛ ˚˜ ! "# ˘$%&’ (ˇ)/cm*+,- ˘ - , ,a, ,b (OIo,a, ,b). "/012 34 , 567 .8 "9ˇ ˚˜,ˇ ,m,:;,CD8 EF Gcm"H CIJ

���������� � � �������������������� ������������ � ��� +m!"#�$��� �����%&�'���()� ����*+�,�-.��()/� (������ pseudo-pillow (Figs. 0c, d ; Watanabe

and Katsui, +310)�0()12��� (3,�-&4����� pseudo-pillow1$567�/� 8�9���:;��<�=>?@��-A>B)�CD��67�� �E� +32, ; 8�F�E� +330# )GH��� � � IJ�K�� (IoTe)��L�()M�� N�OPQ���� )RS3�T��0� !"3UV�W)XY� (367�1=>?@��-A>B) !� �Z3"�� [#\&���� �$� %]3����%[3^�_&1W`���� [#\&')a]b.'M��^�_&1W`���1$c� (��W%&1d�������� =>?@��-A>B3e1����()If3e ISD*+# �/� =>?@��-A>B),g3��� hmmih cm-.# M��/j0k��1lm�

/�. ����������� �� (IoHy-u)

2no=>?@��-A>BQ� (IoHy-l)3pq3"1��e/�K31�J���_J�&/�45�r]���� �$� %]���� [#\&3����s6�� Z��tuv��^��[#\&'1wca]b.� ����$��� xg 2no��7 A# �y�/8&���3cM��z������ =>?@��-A>B3e1�m� xgM�9�):&����$��� ;�{'�<|m}~���

/�/ ������ A (IoLA)

2no��7 A (IoLA)/� �l���Y2no���3���W����0 (Figs. 1a, b)� 2no=>?@��-A>B"� (IoHy-u)1��M`�e=����0()� >?<}�%@�NA�����,�-.�����()M�� 8��/ IoHy-u3xg�0� ) !D�� ��786�/7�_&�}�� ���*B�/� �C ,m� m��)�Z�1�z��}~���h cmih +* cm3����'D�� (�� pseudo-pillow1$5=>?@��-A>B�=����

/�0 ����������� (IoSP)

(3�Q�\�/ ISC*+�3c���� (Fig.

-)� �� -* cm-.�� E� +* cm!Q3%]<}�^�3�\M��� Q=>?@��-A>B�(IoHy-l)3E"�0k�� �����M��IJ�K�� (IoTe)�W(3�),�^��\�$��()

M�� IoTe�J3EFM� � /��,��32noG��) !�Y� (3���/����H�I32no����)/JK�Y���

/�1 ��������� (IoTe)

��/h +* cmi+m-.�� Q=>?@��-A>B�1L����~�K0k��0 (Figs. 0a, b)���.� 2no�g3���s�_J��� 45� W��/'M��$� r]���� %]�\� ^��\� .:&� ���M���¡>v/E�h cmihm�� (��/¢�£NN¤1d��¥������

/�2 ������ B, C (IoLB, C)

2no��7 B, C (IoLB, C)/� �o�yM�7���7O����� 2no(M�)I3¦31�m�P��)W� �IN3Q�� ��� IoLB/� ISA*+�y3If�2no=>?@��-A>BQ��2no��7 A (IoLA)1L������ (Figs. 1

a, 1b)� �D -*m!"345���7�� Q����� ��� �§M��/ IoLA)/¨�z� ©R�7�/������ %@� />?����Sh3������ �� e/4�.���

/�3 ������ D, E (IoLD,E)

2no(ªT��������0� �T�/UV���7*�1W«��W33� �6"�/pq�¬c� xg/���Y�� 2no��7D (IoLD)

/� )­X (HO)�y�®YoG��� YIo+, YIo,a1���I� �� (Figs. 1c, d)��¯M������H"3 YIo, ����),��3G��)RS���2no��7 E(IoLE)/� IoLD}zWZ°[� . vol.

�!"\±�� IoLD)/¨���7�0�/�+* ������ F (IoLF)

s�E*B<}�ªi]I3�^�'Dm7O����� IoLAiE<}�²³0k�1�~�W´����0� �6"�/pq� .:&�¬µ��� _h`3 flow lobe/� ¶�W�� /*m1|S� ��45�·&�0�

/�++ ����� !" (IoTa)

2no�^M��T�'Dm²³0k� Fig. ,�3 l# �� ¸¹�}��/�� -*m!"� m� E�h cmM�h +* cm345�r]���� �$� r]���1$5� %&/��r��M��� 2no(If�/��Qº��$.3}�\ �h cm-.# �$��� 8&�&1$c� 2no��7)abM����0()M�� ��7��3cd4��) !�Y� ef�M�®Yo� �T�/� IoTa1�g)mg�*0k� Fig. ,�3 k# ������

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/�+, ����� -a�-c (YIo-a�-c)

YIo-a�-c�� ������� �������������������� !"#$%&�K-

Sk-u-- (Kawanabe and Saito, ,**,)�'(�)�� YIo-a

�� + cm*+��,-./0mm�0 cm�1�2�345�6728-9:� 8��;<� 0* cm=>���� +?��@AB8C-DEF� �?�GH�IJK�LMNOP�� ���QR�STUV (HJS)��� 6W���XBYI��� Z�6W����� �[ +

m=>�� \]�^ -* cm�_P�`abc-de�.:� fg�2�672���� ���hR (IOW)���0 cm�`abcijB�6W���Fkljm��Z�6W�� ���7n�opP�`abcijB�K-Iw-P+ (Kawanabe and Saito, ,**,)�'([m�� qr� ���stuvw (IOT)��� 8� -* cm=>�x

+6728X�Ikljm��Z�0 +* cm�y��� ,

z�@AB2��345�{g6728 (YIo-b, -c)F��P�� |m}m�yM~�:-���GH672�NOP�D��� @AB����9p�kljmB��l� Zmj�;I stage YIo-�� �X�����

/�+- ����� .a, .b (YIo.a, .b)

����h7�ij7R�i�I���IJK� bcJ��67�� ��-�<X�� ,z*���8ijB�F� +y� ,z (YIo.a, .b)�:F@AB8X�Ik��)�� ���w���� YIo.a, .b|m}m�� sag

structure-���\]^ 2* cm�_P��4�a67�Fkljm��67��������#�g��?���2�bcg��S�B����m�d�<a���� Sag

structure�� ��2��bc� ���,� �g���������[mIJK� ���S����� 67�� 

Fig. 1. Figures a and b show a photograph and a sketch of the outcrop at south-coast of Iwo-dake (ISA),

respectively. Iwo-dake hyaloclastite (IoHy-l) and Iwo-dake lava A (IoLA), which is a feeder of IoHy-l, are

covered by Iwo-dake lava C (IoLC). Figures c and d show a photograph and a sketch of the outcrop at

Higashi Onsen (HO), respectively. Inamura-dake East lava (InEL), YIo+ and YIo,a are covered by

Iwo-dake lava D (IoLD), talus and fan deposits. Locations of outcrops are shown in Fig. /.

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Page 11: ˘OIo+bˇ ˆ˙˝˛ ˚˜ ! "# ˘$%&’ (ˇ)/cm*+,- ˘ - , ,a, ,b (OIo,a, ,b). "/012 34 , 567 .8 "9ˇ ˚˜,ˇ ,m,:;,CD8 EF Gcm"H CIJ

��������������� (Fig. /) ��������������������������� �!�" YIo.a, .b� #��$�� cm%&���'()*�+,-���.����./ 0123��45$� ���678�9:$� ;�+<=;>" ?@AB�C� D@�E��FG H� +*

cmI8����JK/L>� (K-Iw-P, ; Kawanabe

and Saito, ,**,)"

0� �������MN��� OP� QN����RSR�TU ��.V>LWX��� Y;Z[N��\��*�]^+_`�" %& a�bc��]^*�dbTU+]8 (Fig. 2, Table ,)"

0�+ �� ����OIo+b�e&�� (stage OIo-�)� fgh��ih���*$ j0kl�\� mnTU+<8" oH +

mm9:�pq;ro��Ys$ Fe-Tit.��,���" u� OIo,a���F (stage OIo-�)� stage OIo-

� mnv�wxyz{$!�� ���|}~ �����|+�8�ro���� - mm�+Ys /�fgh��ih�2����$!� (Figs. 2a, 2b, Table ,)"ro����� j0kl�\� stage OIo-�$ /�1

vol. �$!���/ stage OIo-�$�� ,* vol. �I8� (Fig. 3)" fgh��)�ih���V>L# j0kl�\�$���$!���/ stage OIo-��\�$��� ;�"

0�, ������OP��Y;�\�� OP�� * (InSL)'()

OP�� * (InEL)$a�=���ih�������!* (Fig. 2e, Table ,)$ oH +�- mm�ro����+��,-" e&���j�� ���ro�����y/�����G ����+,-�"#:������������y�!�" ��  *� oH .

mmI8�ro���+�8� �����fgh��ih�¡�*$!� (Fig. 2f, Table ,)" pq;ro��$¢b�£ InSLG InELTUb;ro���������;>"OP��\�� j0kl��¤�¥¦§�\�

�fgh��ih�¡�* ; Fig. 2d� ,��� %�¨E}~+#Vro�&'�������;>"

0�- ��� ����QN���©N����ª�j«¬3�­��� ���ro��� �+ mm9:� +�8�®��� (1�3vol. �)�fgh��ih���*$!�" Q/>�\�¯°����±'()²7$�³´µ¶/ �#Q

Fig. 2. Microphotographs (cross nicol) of volcanic rocks. Scale bar indicates + mm. Pl : plagioclase, Cpx :

augite, Opx : hypersthenes, Ol : olivine. (a) Two pyroxene rhyolite with clear plagioclase (pumice in K-Tk at

HJ), (b) two pyroxene dacite with dusty plagioclase (ballistic bomb in OIo,a at HJ), (c) two pyroxene

rhyolite with clear plagioclase (lava flow in IoLE at IOW), (d) two pyroxene andesite with large dusty

plagioclase (banded pumice in K-Tk at TK), (e) olivine two pyroxene basalt with plagioclase aggregates (lava

flow in InSL at INS), (f) two pyroxene andesite with dusty and clear plagioclase (lava flow in InIL at IM).

For the abbreviations of sampling locations, see Fig. +c.

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�� YIo.a (stage YIo-�)��� ������ -./

mm��� ���� +,�+- vol. ���� (Figs. 2, 3,

Table ,)� ��� +3-.�-/��������� +1

vol. ����� � ��� �������� ���� !����"#$� %��������&�'�(���)*�+ �������",� � -./����/����0�1��234�5� 6����������7���8�+���

1� � � �9:;<=>��?�@�A��0�1� B+�����CD�� +.C�E��F��G " HI�JK�+32, ; L�JK� ,*** ; Kawanabe and Saito, ,**,M� %���N�����OP=>��A��Q� ��R�ST�0�1�U��V��+1���� �W��� XY<N�� �Z[� OP =� ����0�1!�+��"\]6���1� ���CD��^A�_X`a�b* ��#c�de�0�1f�gh�1�$�

1�+ ������%����A��� /., ka HL�JK� ,***M �ij��� stage OIo-�k��A����l2`a"$m���%no�pq�� (OIo+a)� B�pq��� rsrtuv&$�'wx`_2`apq��gy�+ � z�{�|���� OIo+b"pq��{� A���

}�$~�� �stage OIo-��%����@� A��� (�)�

pq���O�Z (OIo,a, ,b)�*+,�� HOIo,a�-M ���)*�+ ��.� �`��s/���� H�0JK� +310M ��� ��OP�O12��� ����� "�&34q����]60 HI�JK�+32,M� B�B��� A�ij�5� OP�-����� �]@���A��=>�+� z+�'�� ��)�&$)��"$m��B�]���1� � �����O��9:;<6��� OIo,a, ,b��1pq���gy�+ ���JK (,**+)�� �9�����?����"� n����> HTiO,, K,O, Zr, Rb�QM ��8�+ ����,����9�����%���s:`��CD�s:`�� ����s:`������ B�]���� %�������� �9������ 6���" +* vol. ���5¡� "� �>)���9�����7¢s:`��£�+� �9�����¤¥rsr�85� A��G¦��gy�+ �§¨����©�9��¥¨� %�2��� ����� (IoLAªIoLC)�� «��� ����¬�:��6%�� ���� B+���� � IoLD����CD�A��]­®¯°±� ��²³´;�µ���N��¶��·]��°±��$¸¹(���

Table ,. Model Compositions of Representative Samples from Satsuma Iwo-Jima

�� ºJ<©»¼82

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IoLD������ �������� ����� YIo+, YIo,a�� �� (Figs. 1c, d)� ���������������� IoLC�� �!"#� �� IoSP

$�%��&'� SiO,&�� 0+�1+ wt ������()*�+,� � -.��� /0123&�4� 5��6�7� �8�91�:� ;< =>���IoLA?IoLC�� -.�@� A�>BC� DE��FGH�OIo,a, ,b �I�"JK@� sag structure �L

M���� N.��OP -.�>QR ST!3"P,�>#$U4�D� %VWX@� YZ�&���W��

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,**,: ��3� )`a .b \'.�c L U4��stage In-������ K@d����ef� �>@*�� +!,>�H\'-g�./hi�4��stage In-���� jk�ef�>>l3� �01��2HKmn1o��� (In-)D3'�4�=>���p/p4qr56D67,�l >89U4�� ���Kmn1o��� IU� :;��� = s K7���@� <=3"P,�t>?DV�� stage In-���� @�<= =K7��uvw+!D,, ���stage In-� K7> stage In-� K7D A l �F���XH��@&�F�D� xWa>l� stage In-

�;j� �� \'Ay�� <=3z{,�>BC�4������ �!"#� �� �|}~�K �B�p/p>��W^ 5��6(� +320 ; ��6(� ,**+:�Sr T�C�V�D�+� (Notsu et al., +321)� =4� =>��� �� \'�l��,�p/p�� �|}~�K;j3��|09�k3��U4�-,�p/p t>?DF��

1�- ������-.� \'�� �� \'���� ,., ka[5_�6(� ,*** ; Kawanabe and Saito, ,**,: 3�],�� stage YIo-��� �E (ST) YIo+3��4��H3� !��1��56��K�./hi�4�� YIo+

���4 �F�lGa� <k3�I�H�ID6J, ��=>��� stage YIo-��� K���.�3���L6�W\'>89U4�� � � stage

YIo-��� -.� ���\'.�� �+!(IoLA?IoLF) ,3�^� OP @� )�:DL U4��stage YIo-����OP�^lM3T!��K7 5N

N�: ��+!D,,�� @� O���+!,D�A3,4������������� (IoHy-l)�L ,�� �� 1***P� �4��' 5)Q� ,**,: ��BC�>� � R� "P�� � � �M��AS T��+�, ��� +!, IoLA?IoLC�,,�N \'�� �K7�OT! z',� �<?U=�M �� IoLD?IoLFD,,�� =4��+! VW¡��� �lN����������� (IoHy-l)

� 3 vol. �� NN���,,�+! (IoLA?IoLC)

� 3�+* vol. �� � � \'�,,�+!� ++�+0

vol.��F^� \'¢£>¤3� IoLA��¢3X¥, �� (Table ,, Fig. 3)� =4��+!¦>|09�Y§= YIo,a?,e> @��Z� YZW[C��` �W�D� YIo,a?,e�� -.� ¨©�¦>, ��lIa� \.�3]^b3ª�� ,,��+!32H¨©�¦>BC�4��stage YIo-��� YIo-a 5$�%��&«�@* Km,>� |09�Y§=3��4�jkK@d: ��]��� kT3�I�H�I¬D"P��=>��� +!,���>,� stage YIo-�;j� K���.�c�� \'>BC�4�� ��� �@_3"P��$�%� (K-Iw-P+ ; Kawanabe and Saito, ,**,)

�� stage YIo-� �+!, IoLF�� �^�YIo-a3[CU4�� |09�Y§= ¨©�D 3.*­.* yBP (Kawanabe and Saito, ,**,)� @_ ¨©�D++-*­.* yBP (Kawanabe and Saito, ,**,)� @* Km,���D +***­2* yBP 5_�6(� ,***: �F�=>��� YIo-a �P`���� +***P�>BC�4�� = Km,�K;j�� ^b K@dJa�1���KDF��l � ®b )`W56��K��=, �W��stage YIo-���¯)c�K@�J%�WX� jk¨©� ��./Ni�4�\'.�F�� �W¨©��� ,d (YIo.a, .b)"P��D� X°�l:;�QR�<*3±�4 (Fig. /)� |09�Y §=3�Q>²X:;³�� C´�µ®bW�K�F��� = \'�e /**�0**P� 5_�6(� ,*** ; Kawanabe and Saito,

,**,: >U4 ��� YIo.aA �� VW¡�� +/�+1 vol. ��� stage YIo-� +!, VW¡� �">fg�3X¥, ��� ��� +3-.�-/P ¶g�h�K�,,�,i!�+! VW¡�� +1 vol. ��·C� = W¡� ¸j��"��P��¹k, ���-.�,i! �!"#� �� �|}~�K>�W�/012�ºa^ 5��6(� ,**+:� ��� Sr T�C��|}~�K ,i!�p/p�^lVaW�

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(Notsu et al., +321)� ������ ���� � ��������������� �������������� �!"�#$����%�&���'� � ��(��%)*+,��

2� � � �-.�/01���� �!23����$����

(stage OIo-�4�)� ��� (stage In-�4�)� ����� (stage YIo-�4�)������5+,��$������ � 678��%9:�! (stage

OIo-�)�;� <=>��>�!�?� @��AB��!CD��(��CE�F@GH� (stage OIo-�)������ I�'� '��(%JKL�M�NF

(stage In-�4�)� OC��� � 678��%PC'� '��( (stage In-�4�)%?Q���R���������� �ST�� +***�<���>�!%

'��(��BCE�F@GH�� (stage YIo-�4�)� UV +***�<� '��(��"+W� <=>�XY�9:�!�Q����Z�� (stage YIo-�4�)� ����������� ��� �!"�#$����%�&���'� � ��(�� ����[S�� \�]��^_�`a��?��

� ��bc�dT��e�B� f�ghf��i�bcjklm�n� op��qr�s����tu� vwx�tuy��� �g�hbcz�{|�}~�� ������~�� g� �p� f��i�bcjklm�;���p� ����\�bc������p����>v���tuy��� ��������p� �� �p� !��"�����p�vwx�� ���� �g�#¡���� ¢£¤¥¦¤��$_��� %§¨��©�g�"ª&�By��� 2«�$_�¬­+®¯�w�'�y°�

� �

����±²³´µ (+330)¶·¤��'� '��x+,�(¸!³%)(*� !C� .+� +1+�+13.

Kawanabe, Y. and Saito, G. (,**,) Volcanic activity of theSatsuma-Iwojima area during the past 0/** years. Earth

Planets Space, /., ,3/�-*+.Kazahaya, K., Shinohara, H. and Saito, G. (,**,) Degass-ing process of Satsuma-Iwojima volcano, Japan : supply

of volatile components from a deep magma chamber.

Earth Planets Space, /., -,1�--/.Kitagawa, H., Fukusawa, H., Nakamura, T., Okamura,

M., Takemura, K., Hayashida, A. and Yasuda, Y.

(+33/) AMS +.C dating of the varved sediments from

Lake Suigetsu, central Japan and atmospheric +.C change

Fig. 3. Variations of modal abundance of phenocryst in volcanic rocks for the past 0/** years. The rock of the

stage OIo-�is more crystal-rich (¹,* vol.�) than that of other stages (º,* vol.�). In the Young

Iwo-dake stage, the modal abundance increased gradually with decreasing age.

»% ¼±²³´µ84

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(+33.)����# �`���aEbcdH�e�6fgh +.C�R� ij�'(�(� � 47(�k�(5# ,1# +23�+31.\� D (+330)���6�� ����6fgh +.C

�R 4��5�lm'(fghn��opqr�st4�5# 23�+*3.\� D� �'!�^�_� (,***)���6��uvuwuMx�yz +*"�{6����� |#L!

"# +,# 3�,-.\� D (,**,)���E�$dHyz% -"�{6���6�R(U!"� �� !"# .+# ,,/�,-0.^�}~��m&'���(. (+32,)�)*��7�67n� 7�7n!"�s 4/"�6 +�+5# 7n���# 2* p.

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