Large shield volcanoes and calderas are not observed on the Moon. Theoretical assessments show the melt ascending from the mantle will stall at rheological traps or at the shallower density trap at the base of the low-density lunar crust. Magma will then only reach the surface along dikes propagated from these deep reservoirs, and such events will be infrequent and characterized by very large-volume eruptions. These conditions favor the formation of sinuous rilles, single long flows, and widespread flow units, rather than abundant small eruptions from a central edifice. The absence of large shield volcanoes and calderas on the Moon is interpreted to be related to the fact that shallow neutral buoyancy zones and magma reservoirs are not favored on the Moon.