Abstracts Geography

A study of the stratigraphy, lithofacies and geochemistry of Taal Caldera Volcano, Philippines, and its implications for the understanding of flooded caldera volcanoes

by Yannick Withoos

Explosive eruptions from flooded caldera volcanoes are some of the most violent on Earth. Taal Caldera Volcano, Philippines, is one of the most dangerous because it is poorly understood and in the middle of a densely populated area. A new stratigraphic framework for Taal’s prehistoric eruptions increases the number of major (>5 km3 DRE) explosive eruptions from 4 to 7, records 3 new minor eruption-units, and newly identifies the Sampaga Formation as the oldest pyroclastic eruption-unit. This study significantly increases the estimated eruption volume for the youngest major eruption-unit, the Buco Formation, from 3 km3 to 28 km3 DRE, and for the largest eruptionunit, the Pasong Formation, from 31 km3 to 90 km3 DRE. Ash aggregates, fluidal juvenile bomb-rich and poorly-lithified ignimbrites, the presence of fine ash-rich cross-bedded facies, and active and passive ash grain textures are more common in younger eruption-units, especially the Buco Formation, compared to older eruption-units like the Alitagtag Formation. This study therefore hypothesises that water had more access to erupting magma during recent major eruptions, although no major eruption is thought to have been triggered by the explosive interaction of water with magma. All major eruptionunits show evidence of plagioclase, pyroxene and magnetite fractionation, although magma evolution for the youngest three eruption-units was dominated by mixing processes. Over time, major eruption-units have been becoming more mafic (from 63 wt.% SiO2 for the Alitagtag Formation to 58 wt.% SiO2 for the Buco Formation); almost all major eruption-units can be chemically distinguished from each other using a combination of Y, Fe2O3 and Cu. Major eruptions are punctuated by smaller, but more frequent explosive eruptions, similar to Taal’s historical eruptions, which continue today and upon which hazard maps are based. However, this study has shown that Taal’s prehistoric eruptions were significantly larger and more frequent than previously thought and that major eruptions have occurred in the past, and therefore could in the future, following a phase of smaller-scale eruptions, similar to Taal’s current eruptive phase.