International Ocean Drilling Programme (IODP³) Expedition 502 explores the nature of the acoustic basement in the outer-rise area of the northwest Pacific subduction system, where the sediment layer above it is unusually thin. The acoustically thin sediment cover is thought to be attributed to basalt sill intrusions or sheet lava extrusions in and on the pelagic sediment package by basalts fed by petit-spot magmatism. Petit spots are a recently discovered volcanic system characterized by highly alkalic and CO₂-rich basalt that is generated by a low degree of partial melting of the asthenosphere.

The goal of IODP³ Expedition 502 is to determine if this volcanic activity is more widespread than previously thought. If confirmed, it could significantly influence how subduction zones work, including earthquake processes, volcanic activity, and global chemical cycles. Moreover, it will help understand the role of petit-spot magmatism and its impact on the Earth's systems.

This present volume of the Proceedings of the International Ocean Drilling Programme includes the Scientific Prospectus and scientific and engineering results of IODP³ Expedition 502, as well as post-expedition scientific outcomes after completion of the operations.