フィリピン共和国,イロシン火砕流噴火の推移と特徴  [in Japanese] Eruptive Sequence and Characteristics of the Irosin Ignimbrite, Southern Luzon, Philippines  [in Japanese]

Access this Article

Search this Article

Author(s)

Abstract

 フィリピン共和国,ルソン島南東端のソルソゴン地域での41 cal kBPの大規模火砕流噴火の結果,11 × 11 kmのイロシン(Irosin)カルデラが形成された。このときの噴火は,カルデラを形成する大規模噴火とその先駆的な噴火の2つに区分される。<br> 先駆的噴火は,現在のカルデラ縁の南東斜面の小規模溶岩ドームであるマロバゴ(Malobago)溶岩ドームの形成と細粒火山灰の噴出からなる。ほかにも類似した溶岩の流出がカルデラ内にあったかもしれない。<br> 地質学的短期間(およそ10年以内)の休止後,3つの噴火フェーズからなるカルデラ形成噴火がはじまった。最初の噴火フェーズは,現在のカルデラ内でおこったプリニー式噴火であり,バルク体積約 20 km<sup>3</sup>の軽石質テフラが北方に拡散した。プリニー式噴火の早い段階で噴煙柱の崩壊が繰り返し発生し,intra-plinian火砕流が薄い細粒火山灰と降下軽石層に挟在している。プリニー式フェーズの終わりごろにソルソゴン地域に強い地盤振動があり,プリニー式噴火による堆積物の上下面に撹乱構造を形成した。<br> イロシン火砕流の噴火は,グランドレーヤーを基底部に伴う下部ユニット火砕流の噴出ではじまった。そして,それに続く破局的な噴火によって上部ユニット火砕流が噴出し,現在のカルデラ地形を形成した。上部ユニットは,厚く粗粒な火砕流堆積物であり,基底部に粗粒な岩片濃集層(地点Dで厚さ約 2 m)が認められる。イロシン火砕流のバルク体積は,およそ 25 km<sup>3</sup>と見積もられる。<br> マヨン(Mayon)火山西麓のイナスカン(Inascan)スコリア丘の山頂部で,イロシン火砕流に伴う降下火山灰(co-ignimbrite ash-falls)が発見されている。ここでの火山灰層は,層厚約1mで細粒火山ガラスと少量の斑晶鉱物からなる。比較的粗粒の最下部層は,プリニー式降下軽石層の遠方相に対比される。これらの降下テフラも含めたイロシンカルデラから噴出した火砕物全体のバルク体積は,ほぼ 70 km<sup>3</sup>に達する。

 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 caldera-forming eruption.<br> 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.<br> 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 km<sup>3</sup>. 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.<br> 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 total bulk volume of the Irosin ignimbrite is estimated to be 25 km<sup>3</sup>.<br> 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 km<sup>3</sup>.

Journal

  • Journal of Geography (Chigaku Zasshi)

    Journal of Geography (Chigaku Zasshi) 123(1), 123-132, 2014

    Tokyo Geographical Society

Codes

  • NII Article ID (NAID)
    130003395487
  • NII NACSIS-CAT ID (NCID)
    AN00322536
  • Text Lang
    JPN
  • ISSN
    0022-135X
  • NDL Article ID
    025300668
  • NDL Call No.
    Z15-169
  • Data Source
    NDL  J-STAGE 
Page Top