IODP Expedition 340 successfully drilled, for the first time, large and likely tsunamigenic volcanic island arc landslide deposits. These cores provide evidence and tests of previous hypotheses for the composition, origin, and mode of transport of those deposits. Sites in the medial to distal parts of the landslide deposits offshore Montserrat and Martinique recovered seafloor sediment, comprising turbidites and hemipelagic deposits, and lacked the coarse and chaotic subaerial volcanic debris avalanche material. This supports the concepts that (i) the volcanic debris avalanche component of these landslides is restricted to proximal areas and tends to stop at the slope break and (ii) emplacement of volcanic debris avalanches in marine settings can trigger widespread and voluminous failures of preexisting low-gradient seafloor sediment. The most likely mechanism for generating these large-scale seafloor sediment failures appears to be the propagation of a décollement, from proximal areas that are loaded and incised by a volcanic debris avalanche. These results have implications for the magnitude of tsunami generation by volcanic island landslides. Volcanic island landslides composed of mainly seafloor sediment may form smaller magnitude tsunamis than equivalent volumes of subaerial block-rich mass flows rapidly entering water.
