Reconnaissance geology of Isarog volcano (Luzon, Philippines): Evidence for Holocene explosive eruptions and a major edifice collapse

A field reconnaissance study of the volcanic geology of Isarog volcano (Luzon, Philippines), complemented with radiocarbon dating, geochemical and petrological data, shows evidence for repeated explosive activity generating pyroclastic density currents and block-and-ash flows, including Holocene events. The chemical composition of juvenile material is dominantly andesitic. As there are several indications for magma mixing taking place, the andesitic composition possibly represents a hybrid composition between mixed magmas. In addition to explosive eruptions, unambiguous field evidence is provided for a sector collapse generating a debris avalanche deposited to the NW. The deposit shows characteristic hummocky and ridge topography, highly brecciated jigsaw-cracked blocks and relict internal stratigraphy in transported masses. In a first-order approximation the deposit volume is estimated to be ~ 6–8 km3. The run-out distance is 21–25 km — the distal parts of the deposit were transported into San Miguel Bay.