Lunar pyroclastic beads are interpreted to represent primitive magmas derived from great depths and rapidly erupted to the surface in explosive events. However, a detailed mechanism for gas generation at great depth and rapid magma transport to the surface has not yet been described. Furthermore, the pyroclastic beads are not petrogenetically related to basalts erupted near the sampling sites. We propose a model in which these conundrums are resolved through gas build-up in a low-pressure micro-environment near the tip of a magma-filled crack (dike) propagating rapidly from the magma source depth to the surface. The gas rich region consists of a free gas cavity overlying a foam extending vertically for ~20 km. Eruption of the foam results in the widespread emplacement of unfractionated pyroclastic beads. Subsequent ascent of the underlying gas-free picritic magma is unlikely to occur, perhaps accounting for the lack of sampled eruptive equivalents.