Accretionary lapilli are tightly bound, spherical collections of particles. They are common in phreatomagmatic eruptions, which have been associated with magma-water interaction.
In 1949 Macdonald noted two types of accretionary lapilli at Kilauea volcano: those composed entirely of fine-grained ash, and those with a coarse nucleus surrounded by fine ash.
Accretionary lapilli have been found in fall deposits and, in pyroclastic flow and surge deposits have been made.
Fallout of accretionary lapilli was observed on 26th April 1979 during the eruption of
Soufriere St. Vincent, most of which disintegrated on impact.