Geomorphic Evolution of Karthala’s Summit Caldera: Insights from Photogrammetry, Satellite Imagery, and Historical Aerial Photographs

Karthala (Ngazidja Island, Comoros archipelago), an active basaltic volcano in the Indian Ocean, provides an excellent natural laboratory for studying the geomorphic evolution of a rapidly evolving caldera complex. Eruptive events in 2005–2006 reached a VEI 3 and emplaced fresh tephra and lava across the summit area, covering the cratered region and creating a time-zero surface for tracking post-eruptive erosion and drainage network development. Karthala’s craters are also shaped by mass-wasting processes, evidenced by landslide deposits in the craters that are visible in satellite and aerial imagery.

In this study, we construct a geomorphic chronology that spans 76 years using a combination of photogrammetry from a 2025 Unoccupied Aerial System (UAS) survey, Pléiades satellite imagery (2015, 2024), and orthorectified historical photographs (1949, 1961). This interval includes significant eruptions in 1952, 1965, 1972, 1991, and 2005-2007. We primarily focus on geomorphic change since the 2005–2006 eruptions, measuring erosion within the tephra-mantled summit region and mapping the temporal evolution of fluvial channel networks. By tracking the development of the drainage network, we can precisely constrain landscape response times and quantify the timescales at which volcaniclastic material is mobilized and redistributed in the landscape. In addition, we evaluate crater rim retreat and map collapse structures through time to explore how mass wasting interacts and competes with fluvial processes. Together, this work provides constraints on the timescales and relative importance of erosional processes that shape Karthala’s summit region between eruptive events, while placing its recent evolution in the context of crater changes that have occurred over decadal timescales.