Hummocky terrain of the Kalibabak debris avalanche deposit, Lombok Island, Indonesia

Abstract

The debris avalanche deposit (DAD) coverage can drastically modify the surrounding landscape of volcanoes. DAD can be distinguished by hummocky irregular surface, jigsaw fissure, and mixing horse-shoe shape. Due to its particular shape of a hummock, the topography can be easily identified using DEM (Digital Elevation Model) and satellite imagery. The aim of this study is to characterize hummocky terrain in Lombok Island, which is located in the Kalibabak formation on the Geological map through the geomorphic approach. Hummocky terrain in this study are analyzed using DEM data from DEMNAS (DEM Nasional: 0.27-arcsecond resolution). Our study of this DAD encompasses seven variables, namely H/L ratio, numbers hummocks, distance to source, slope, area (size), relative height, topographic section, and hummock-spreading shape. A minimum of 756 hills derived from this DEM are considered as hummocks from this DAD, which extends ~18 km (NS) and ~25 km (WE) in the central part of Lombok Island. With an area of ~200 km2 and a volume estimated around 8.8 km3, the Kalibabak DAD is more than three times larger than the one of the Mount St-Helens in 1980. The morphology of hummocky terrain is bounded by a sudden change of slope, which is indicated by a river confluence. Average hummock size is 2.7 ha and average distance between each hummock is 150 m to 300 m. The hummocks are characterized by steep slopes (25 % to 45 %) at the boundary to colluvium plain (debris deposit), which makes them easy to identify using DEM. Hummocks spreading distribution forms a conical-like shape with a H/L ratio of 0.13. The spreading distribution shape, the H/L and V/L ratios are useful as an input for reconstructing the mechanism of debris avalanche emplacement.