Morphology of large-scale debris avalanche deposits in Cianjur Plains, West Java, Indonesia

Large-scale debris avalanche deposit (DAD) processes rarely occur, but the impact is catastrophic. Large-scale flank collapse from ancestral Gede volcano resulted in the deposition of debris avalanche deposits that manifested as prominent hummocky hill landforms covering 192 km2 of the Cianjur Plains, West Java, Indonesia. Although the deposit covers large areas and shows unique volcanic landscape morphology, studies on debris avalanche deposits are limited. Here, we show the results of field reconnaissance, satellite photography and digital elevation model analysis to understand the morphology of large-scale debris avalanche deposits, including their distribution and morphological characteristics. The interpreted thickness of the deposit ranges from 200-500 m, although the hummocky hill with a well-rounded texture is only 50-100 m high due to intense erosion in the tropical region with the remaining portion of the megaclast blocks buried. The distribution of hummocky hills indicates that the flow was constrained by the pre-existing basin configuration. The debris avalanche deposit shows fan-like morphology that characterizes block facies located near the fan mouth, while the matrix facies has matrix-supported smaller blocks in the distal part. The mouth of the debris avalanche deposit is located just south of Cianjur township, with the highest concentration of megaclast block as block facies, which is interpreted as the main depositional axis. The block facies apparently stopped by lineament of the SW-NE-oriented Cimandiri valley to the southeast and NNE-SSW-oriented Cisokan River lineament to the east, which was interpreted as a buried normal fault that was considered a barrier to confine the flow of block facies deposition.

The morphology of large-scale debris avalanche deposit in Cianjur plains, West Java, Indonesia

The large-scale debris avalanche deposit (DAD) process are quite rarely occurring but the impact is catastrophic. Large-scale flank collapse from ancestral Gede volcano resulted in the deposition of debris avalanche deposit that manifested as prominent hummocky hill landform covering 192 km2 area in Cianjur plains, West Java, Indonesia. Although the deposit covers large areas and showing unique volcanic landscape morphology, the study on the debris avalanche deposit is limited. Here we show the result of field reconnaissance, satellite photo and digital elevation model analysis to understand the morphology of large-scale debris avalanche deposit including distribution and morphological characteristics. The interpreted thickness of the deposit ranging from 200-500m, although the hummocky hill with well-rounded texture is only 50–100 m height due to intense erosion in the tropical region with the remaining portion of the megaclast blocks buried. The distribution of hummocky hills indicate that the flow constrained by pre-existing basin configuration. The debris avalanche deposit showing fan-like morphology that characterized block facies located near the fan mouth, while the matrix facies with matrix-supported smaller blocks in the distal part. The mouth of debris avalanche deposit located just south of Cianjur township with the highest concentration of megaclast block as block facies that interpreted as the main depositional axis. The block facies apparently stopped by lineament of SW-NE oriented Cimandiri valley to the southeast and NNE-SSW oriented Cisokan river lineament to the east which interpreted as buried normal fault that considered as the barrier to confine flow of block facies deposition.

A channelised debris-avalanche deposit from Pirongia basaltic stratovolcano, New Zealand

A newly discovered, large volume (3.3 km3) volcanic debris-avalanche is described from the Pirongia Volcano in North Island, New Zealand. Mapping, field surveys and drill core data were used to reconstruct the distribution and facies of the deposit (the Oparau breccia). The debris avalanche was channelised into a lowland graben structure resulting in a prolonged runout distance of ≥20 km and substantial thickness of >200 m in medial areas. The deposit contains block and matrix facies dominated by ankaramite basalt sampled from the oldest parts of the volcanic edifice. The age of deposition of the Oparau breccia is constrained to the period 2.2-1.75 Ma. The collapse source zone is marked by a prominent unconformity on the southwestern flank of the mountain. Movement on faults within the graben is identified as the most likely cause of sector collapse. The collapse scarp is infilled by 5 km3 of post-collapse volcanic material.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5505549

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

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.