Magma fragmentation is a fundamental process of volcanism, and its mechanisms have important hazards implications for active volcanoes and monogenetic volcanic fields, since some eruptive styles are more dangerous than others to humans and infrastructure. Learning about magma fragmentation mechanisms for unwitnessed eruptions (and even witnessed ones) builds understanding of eruptive energy partitioning, and helps build a picture of “what this volcano (or volcanic field) can do”, supporting risk mitigation.This special session aims to bring together scientists working on magma fragmentation processes – and the state of the magma before fragmentation in all environments and from all different perspectives. This includes the study of primary volcaniclastic deposits on land or in water, with particular focus on juvenile clasts (particle density, shapes, surface features and internal textures),all relevant laboratory experiments, and physical models for fragmentation.