Collaborative study between Japan and Philippine on volcanic and geothermal history of Irosin caldera, south Luzon, Philippine

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Irosin caldera is located at the southeastern end of Luzon Island, Philippines. Bulusan volcano is a post-caldera cone. To reveal the sequence of the Irosin caldera eruption and the eruptive history of post-caldera volcanism, we conducted a collaborative project with PHIVOLCS. A small lava dome and ash-fall deposits were erupted as forerunners of the main caldera-forming eruption. The first stage of the caldera eruption was the plinian, which occurred shortly before the main ignimbrite eruption. The second stage of the eruption was the formation of the Irosin ignimbrite which can be divided into two units ; the lower fine unit and the upper coarse unit. The fine rhyolitic ash-fall deposit exposed in the summit crater of Inascan cone 80 km from the caldera is regarded as a co-ignimbrite ash-fall associated with the Irosin ignimbrite. Cummingtonite phenocrysts were identified in the Irosin ignimbrite and related deposits. Cored sediments obtained from the caldera are composed mainly of lahar deposits. Rhyolitic to andesitic ashes are intercalated with the sediments. One of them has similar petrographic characteristics with the Irosin ignimbrite. Recent volcanic ash from Bulusan volcano is composed of accessory materials, which involves gypsum. Chemistry and isotopic composition of hot springs distributed around Bulusan volcano suggests that all of the hot springs are dominant in HCO_3 suggesting a steam-heated type of water.