Monitoring and detecting marine volcanic activities are key for scientific understanding and disaster prevention. However, this is difficult because they are hidden under water. Near Torishima Island in the Izu Islands, Japan, intensified seismic activity was observed during October 2023, including a mysterious tsunami-triggering earthquake on October 8 (UTC), which was considered to be linked to a volcanic activity. On October 20, 2023, aerial surveys confirmed an 80-km stretch of floating pumice near Torishima Island. This study conducted a Lagrangian back-tracking drift simulation using the ocean current data and surface wind data to trace the origin of the pumice while clarifying the theoretical basis of Lagrangian back-tracking from the Bayesian perspective. Results indicate that the pumice drifted southward from around extensional back-arc basins near Myojinsho Reef and Sumisujima Island approximately 3–5 days before its discovery. These findings are consistent with independent observations such as biological traces and the geochemical characteristics of sampled floating pumice, which is considered identical to that found on October 20 by an airplane. This indicates the presence of unknown volcanic activity around back-arc basins west of the major active volcanic zone. This study demonstrates the utility of combining drift simulations with geochemical and biological data to identify the sources of marine volcanic events, particularly in regions where direct observations are limited. The results of this study contribute to our understanding of volcanic mechanisms and their potential hazards.