A novel mechanism is proposed for the emplacement of mare basalts and the formation of mascons that does not involve melting or remelting of the source region. The source magmas are the remnants of the primordial lunar magma ocean and are rich in incompatible elements, including radioactive isotopes; potassium, rare earth elements, and phosphorus (KREEP); and volatiles. Exsolution of volatiles as the magmas cooled and crystallized (a process called second boiling) produced overpressures which drove the dense basalts upward to the surface, creating the mascons. This process occurred preferentially beneath nearside mare basins, where the crust is thinnest and lithostatic pressure is lowest. The delay between impact and magmatic extrusion reflects the time needed for the volatile-rich magma to cool to its second boiling point. Radioactive heating buffered the magmas against cooling and solidification, allowing for an extended period of magmatism.