Journal of Geography (Chigaku Zasshi)
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
Original Articles
Eruptive Sequence and Characteristics of the Irosin Ignimbrite, Southern Luzon, Philippines
Tetsuo KOBAYASHIT. MIRABUENO Ma. HannahV. BORNAS Ma. AntoniaMasayuki TORIIP. LAGUERTA EduardoS. DAAG ArturoB. BARISO EricsonToshio NAKAMURAMitsuru OKUNO
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2014 Volume 123 Issue 1 Pages 123-132


 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.
 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 total bulk volume of the Irosin ignimbrite is estimated to be 25 km3.
 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.

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