<p>Instability at volcanic edifices poses significant hazards, yet the processes driving rock weakening, particularly on steep, eroding flanks, remain poorly understood due to limited accessibility. Hydrothermal alteration is a key factor in weakening volcanic rocks, contributing to edifice destabilization and flank instability. La Fossa cone (Vulcano, Italy) provides an ideal setting for this study, with accessible hydrothermal alteration at the crater rim and similar alteration along inaccessible flanks that have a recent history of mass wasting. Here, we developed an integrated methodology combining drone photogrammetry with in situ Schmidt hammer testing to derive an empirical alteration-to-strength relationship for the crater rim and applied this knowledge to alteration sites on inaccessible flanks. An alteration map was generated using a Principal Component Analysis (PCA) to aid our classifications. This map was used to transpose over 1000 Schmidt hammer measurements (R-values ranging from 10.5 to 82), creating a thematic strength-alteration map. Results indicate a ~50% reduction in relative rock strength correlating with areas of degassing and hydrothermal activity, which coincides with past mass-wasting events. This integrated approach offers a transferable workflow for assessing volcanic slope instability, with direct applications to hazard monitoring and early warning systems.</p>