小特集「フィリピン・ルソン島のイロシンカルデラとブルサン火山の地質と最近の噴火活動(その1)」序論  [in Japanese] Introduction to the Special Issue on “Geology and Recent Eruptions of Irosin Caldera and Bulusan Volcano, Southern Luzon, Philippines (Part I)”  [in Japanese]

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Abstract

 The Irosin caldera located at the southeastern tip of Luzon Island in the Philippines was formed by the eruption of 41cal kBP Irosin ignimbrite. Bulusan, a post-caldera volcano, has repeated phreatic eruptions during historical times. The special issue on “Geology and Recent Eruptions of Irosin Caldera and Bulusan Volcano, Southern Luzon, Philippines (Part I)” provides various data and discussion mainly on the formation of Irosin caldera. First based on interpretations of volcanic landforms, the evolution of 84 volcanoes in the Philippines is outlined (Moriya, 2014). Fifty-six stratovolcanoes, three caldera volcanoes accompanying four post-caldera volcanoes, three lava domes, four scoria cones including two maars, four lava fields, and ten shield volcanoes are identified. The sequence of caldera-forming eruption at Irosin consists of a precursory fine ash eruption (Malobago lava dome), plinian pumice fallout, and intra-plinian flow deposits (Kobayashi <i>et al.</i>, 2014a). Evidence of ground shaking during the plinian phase was also found. The total DRE volume of erupted tephra is estimated to be 30 km<sup>3</sup> (VEI = 6). A gravity survey in February 1996 revealed a semi-circular feature with a steep gravity gradient in the Bouguer anomalies, which corresponds clearly to the southern rim of the Irosin caldera (Komazawa <i>et al.</i>, 2014). The funnel-shaped depression structure of the gravity basement, which is significantly smaller than that of the topographic depression, was recognized from a three-dimensional analysis of residual gravity anomalies. The mass deficiency was estimated to be 1.1 × 10<sup>10</sup> tons, corresponding to 40 km<sup>3</sup> of DRE volume. Four thermoluminescence (TL) ages (36 ± 8 ka, 38 ± 10 ka, 33 ± 8 ka and 45 ± 10 ka) are obtained from the matrix and lithic fragments of the ignimbrite and co-ignimbrite ash-falls, respectively (Takashima and Kobayashi, 2014). Of these, the first two ages are in good agreement with a radiocarbon age of 41 cal kBP. A pictorial of representative outcrops is offered to provide an understanding of the geology in and around the caldera (Kobayashi <i>et al.</i>, 2014b). Recent activity at the Bulusan volcano is described and discussed in the next issue (Part II).

Journal

  • Journal of Geography (Chigaku Zasshi)

    Journal of Geography (Chigaku Zasshi) 123(1), 82-88, 2014

    Tokyo Geographical Society

Codes

  • NII Article ID (NAID)
    130003395495
  • NII NACSIS-CAT ID (NCID)
    AN00322536
  • Text Lang
    JPN
  • ISSN
    0022-135X
  • NDL Article ID
    025300631
  • NDL Call No.
    Z15-169
  • Data Source
    NDL  J-STAGE 
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