地学雑誌 Journal of Geography （Chigaku Zasshi） 123 （1）0000 0000 2014 doi:10.5026/jgeography.123.0000 Eruptive Sequence and Characteristics of the Irosin Ignimbrite, Southern Luzon, Philippines Tetsuo KOBAYASHI＊, Ma. Hannah T. MIRABUENO＊＊＋, Ma. Antonia V. BORNAS＊＊, Masayuki TORII＊＊＊, Eduardo P. LAGUERTA＊＊, Arturo S. DAAG＊＊, Ericson B. BARISO＊＊, Toshio NAKAMURA＊＊＊＊ and Mitsuru OKUNO＊＊＊＊＊［Recived 28 January, 2013 ; Accepted 29 December, 2013］ Abstract A large-scale pyroclastic eruption occurred 41 cal kBP that resulted in the formation of the 11 ×11- km Irosin caldera located at the southern end of Sorsogon Province, Luzon, Philippines. The eruption consisted of two distinctive events, namely a precursory eruption and a large-scale calderaforming eruption. During the precursory eruption, ﬁne ash was ejected, and a small lava dome, Malobago, was extruded on the southeastern slopes of the present caldera outline. Other similar lava extrusion may have occurred within the caldera. After a geologically short pause（likely within 10 years） , the main caldera-forming eruption, which consisted of three distinct eruption phases, started. The ﬁrst phase involved a plinian eruption, likely within the present caldera, resulting in the northward emplacement of pumiceous tephra with bulk volume of 20 km3. Partial collapse of the plinian column repeatedly occurred during the early stage of the plinian phase. These formed intra-plinian ﬂow deposits, which are generally thin, ﬁne grained, and associated with fallout pumice layers. A strong ground shaking occurred in the Sorsogon area during the waning stage of the plinian phase, resulting in the formation of disturbed structures both at the base and in the upper horizon of the plinian deposit. This was followed by the generation of the lower Irosin ignimbrite with associated ground layers. The ﬁnal eruption was catastrophic, forming the present caldera topography and emplacing the upper Irosin ignimbrite. The upper ignimbrite is generally thick and coarse, with a coarse lithic concentration zone at the base, whose thickness is about 2 m at north of Bulusan town（Loc. D） . The 3 total bulk volume of the Irosin ignimbrite is estimated to be 25 km . A 1-m-thick co-ignimbrite ash deposit was found at the summit of the Inascan scoria cone on the western slopes of Mayon volcano. The deposit generally consists of ﬁne-grained glass shards with small amounts of phenocrysts. The coarse lowermost layer may be the distal facies of the plinian pumice fallout. The total bulk volume of the erupted tephra from the Irosin caldera could be as much as 70 km3. ＊ Graduate School of Science and Engineering（Science course） , Kagoshima University, Kagoshima, 890-0065, Japan Philippine Institute of Volcanology and Seismology（PHIVOLCS） , Quezon City, Philippines ＊＊＊ The Implementation Research and Education System Center for Reducing Disaster Risk, Graduate School of Science and Technology, Kumamoto University, Kumamoto, 860-8555, Japan ＊＊＊＊ Center for Chronological Research, Nagoya University, Nagoya, 464-8602, Japan ＊＊＊＊＊ Faculty of Science, Fukuoka University; Also AIG Collaborative Research Institute for International Study on Eruptive History and Informatics, Fukuoka University, Fukuoka, 814-0180, Japan ＋ Present Address: Christchurch City Council, Christchurch City, 8011, New Zealand ＊＊ 1 ― ― Key words： Irosin caldera, Bulusan volcano, Malobago lava dome, ignimbrite, intra-plinian ﬂow, co-ignimbrite ash, ground shaking （McDermott et al., 2005） . The volcanology of the BVC was elucidated by the geological work of Cruz（1987）and Panem and Delﬁn（1988）and by petrogenetic research by Delﬁn（1991） . Panem and Delﬁn（1988）established three distinct stages of volcanism in the BVC（Fig. 2） . The ﬁrst was a constructive pre-caldera stage, represented by highly faulted and dissected volcanic ridges consisting of the 2.14 Ma Gate volcanics, as determined by K-Ar age dating（Delﬁn, 1991） . The remnant stratovolcanoes, Mts. Tabon-Tabon I．Introduction The Irosin caldera and older volcanoes of Sorsogon Province collectively form the Bulusan Volcanic Complex（BVC）（Delfin, 1991 ; Delfin et al., 1993; Catane et al., 2005）on the southern end of the Bicol Volcanic Arc in southeastern Luzon（Fig. 1） . Bicol Arc magmas exhibit a wide compositional range（medium- to high-K basaltandesite-rhyolite suite） , which have chemical characteristics typical of subduction-related rocks Fig. 1 Index map of the study area. Filled triangles denote active volcanoes. Open triangles represent inactive and potentially active volcanoes（PHIVOLCS, 2002） . Loc. H, west of the Mayon volcano represents the Inascan scoria cone, where coignimbrite ash from the Irosin caldera-forming eruption was discovered（Mirabueno et al., 2011） . Rectangles indicate the areas for Figs. 3, 5 and 7, respectively. Open circles indicate major city and town. 2 ― ― フィリピン共和国，イロシン火砕流噴火の推移と特徴小林哲夫＊マリアハナミラブエノ＊＊＋マリアアントニアボルナス＊＊鳥井真之＊＊＊エドアルドラグエルタ＊＊アルトロダアグ＊＊＊＊＊＊＊＊エリクソンバリソ中村俊夫奥野充＊＊＊＊＊フィリピン共和国，ルソン島南東端のソルソゴあり，プリニー式噴火による堆積物の上下面に撹ン地域での 41 cal kBP の大規模火砕流噴火の結乱構造を形成した。果，11×11 km のイロシン（Irosin）カルデラがイロシン火砕流の噴火は，グランドレーヤーを基形成された。このときの噴火は，カルデラを形成底部に伴う下部ユニット火砕流の噴出ではじまっする大規模噴火とその先駆的な噴火の 2 つに区分た。そして，それに続く破局的な噴火によって上される。部ユニット火砕流が噴出し，現在のカルデラ地形先駆的噴火は，現在のカルデラ縁の南東斜面のを形成した。上部ユニットは，厚く粗粒な火砕流小規模溶岩ドームであるマロバゴ（Malobago）堆積物であり，基底部に粗粒な岩片濃集層（地点溶岩ドームの形成と細粒火山灰の噴出からなる。 D で厚さ約 2 m）が認められる。イロシン火砕流ほかにも類似した溶岩の貫入がカルデラ内にあっのバルク体積は，およそ 25 km3 と見積もられる。たかもしれない。マヨン（Mayon）火山西麓のイナスカン（Inascan）地質学的短期間（およそ 10 年以内）の休止後，スコリア丘の山頂部で，イロシン火砕流に伴う降下 3 つの噴火フェーズからなるカルデラ形成噴火がは火山灰（co-ignimbrite ash-falls）が発見されてじまった。最初の噴火フェーズは，現在のカルデラいる。ここでの火山灰層は，層厚約 1 m で細粒火内でおこったプリニー式噴火であり，バルク体積山ガラスと少量の斑晶鉱物からなる。比較的粗粒約 20 km3 の軽石質テフラを北方に形成した。プの最下部層は，プリニー式降下軽石層の遠方相にリニー式噴火の早い段階で噴煙柱の崩壊が繰り返対比される。これらの降下テフラも含めたイロシンし発生し，intra-plinian 火砕流が薄い細粒火山灰カルデラから噴出した火砕物全体のバルク体積は，と降下軽石層に挟在している。プリニー式フェーズほぼ 70 km3 に達する。の終わりごろにソルソゴン地域に強い地盤振動がキーワード：イロシンカルデラ，ブルサン火山，マロバゴ溶岩ドーム，火砕流堆積物，intra-plinian 火砕流，火砕流に伴う降下火山灰，地盤振動＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＋鹿児島大学大学院理工学研究科（理学系）フィリピン火山地震研究所（PHIVOLCS）熊本大学大学院自然科学研究科附属減災型社会システム実践教育センター名古屋大学年代測定総合研究センター福岡大学理学部・産学官連携研究機関国際火山噴火史情報研究所現所属：クライストチャーチ市協議会 10 ― ―