Influence of weathering grades on landslide occurrences in the tropical volcanic region of Kulon Progo, Indonesia

Abstract Landslides in tropical volcanic terrains are significantly affected by progressive rock weathering, which changes the physical, mechanical, mineralogical and geotechnical properties of slope materials. However, the impact of weathering on slope stability is often overlooked in regional landslide risk assessments, which often ignore underlying geological changes. This study investigated the influence of rock weathering on landslide occurrences in the Kulon Progo Mountains, Indonesia, using a combination of site investigations, laboratory tests and spatial analysis. A total of 110 sites were evaluated for weathering grade and mechanical strength through Schmidt hammer tests, while 56 X‐ray diffraction (XRD) samples were analysed for mineralogical changes. Additionally, 28 soil samples underwent geotechnical testing, including Atterberg limits, grain size analysis and direct shear tests. The results showed that weathering increased clay content, lowered friction angles and promoted the formation of expansive clays such as smectite and montmorillonite. Landslides were most concentrated in moderately weathered volcanic rocks, with densities of 2.60 landslides/km 2 in Grade III andesite and 3.35 landslides/km 2 in Grade III diorite. Conversely, more advanced weathering grades displayed lower densities, such as 1.70 landslides/km 2 in Grade IV andesite and 1.77 landslides/km 2 in Grade IV diorite. Less weathered (Grade II) and fluvial sand–clay deposits, treated separately due to their depositional origin, showed even lower landslide densities. These findings indicate that weathering exerts a dual control on landslide occurrence: it promotes instability at intermediate stages by weakening material, while reducing landslide activity at advanced stages through long‐term geomorphic processes. Therefore, landslide susceptibility assessments should explicitly account for the interactions among lithology, weathering state and slope conditions, rather than treating them as independent predictors, to improve the reliability of hazard mapping and risk mitigation in tropical volcanic regions.