Skip to content

Contrasting Emplacement Mechanisms of Two Debris Avalanches in Banahaw Volcano, Philippines revealed by Morphometric Analysis and Field Evidence

Sandra G. Catane , Aime C. Lamoste

Abstract

Volcanic flank collapses have resulted in the largest terrestrial landslides known as volcanic debris avalanches. Morphological signatures include a horseshoe-shaped breach in the source area and associated hummocky deposits, seen in many Quaternary volcanoes including Banahaw in the Philippines. The transport and depositional mechanisms of the Lucban and Lucena debris avalanches at Banahaw volcano were deduced using morphometric assessment and field evidence. A total of 801 and 370 hummocks were delineated for the Lucban and Lucena deposits, respectively. Lucban hummocks exponentially decrease in size with distance, while Lucena exhibits no trend. In the Lucban, clusters of hummocks are displaced at 60°-90° relative to the flow direction, but the majority are aligned parallel (0° to 30°) along the runout path. In contrast, Lucena hummocks are notably displaced 60° to 90°, where flow transitions from an ESE to a SSE direction, while distally, hummocks are displaced 0° to 30°, reflecting further lateral spreading and extension towards the coast. The behavior of the Lucban avalanche may be due to limited substrate entrainment, resulting in consistency of rheology and fragmentation patterns during transport, while competent avalanche material or high basal shear strength may have caused localized compression despite the dominantly extensional stress regime. The irregularity of spatial distributions in size and displacement of the Lucena hummocks may have been the cumulative effect of local topography and slope of the runout path, as well as a shift in flow character due to increasing amount of matrix in the collapsing mass during transport.