Morphometric classification and spatial distribution of Philippine volcanoes

The Philippine Island Arc has a large number of volcanoes with diverse morphologies, making it an ideal location to study the factors controlling the morphology and spatial distribution of island arc volcanoes. We have identified 731 volcanic edifices using the SRTM 30 m digital elevation model, and computed their quantitative morphology using the MORVOLC algorithm. Hierarchical classification by principal component (PC) analysis distinguishes four volcano types: small flat cones, small steep cones, large cones, and massifs, with mean volumes of 0.2 km3 (<6.2 km3), 0.4 km3 (<9 km3), 29 km3 (0.15–178 km3), 267 km3 (76–675 km3), mean heights of 125 m (16–721 m), 260 m (53–971 m), 842 m (59–2313 m), 1533 m (1012–2175 m), and mean slopes of 13° (3–21°), 22° (14–37°), 15° (3–28°), 15° (11–22°), respectively. This classification is based mainly on their size and irregularity (PC1) and steepness (mean slope and height/basal width ratio; PC2), and to a lesser extent on the size of the summit region and edifice truncation (PC3) and edifice elongation (PC4). These morphological volcano classes represent stages along an evolutionary trend. The small flat cones are mostly monogenetic, whereas the small steep cone class represents an early growth stage. Some can develop into large polygenetic cones while a few can further grow laterally into massifs, both of which are preferentially found on thickened crust. There is a trend towards more silicic compositions from small to large cones, perhaps due to larger edifice loads preventing mafic dykes from reaching the surface. The distribution and alignment of the edifices within volcanic fields seems to be influenced by both regional and local stress fields and pre-existing structures.