A mechanism for the creation of lunar mascons is proposed that requires no abnormal density materials or major density inversions. The mascons are produced by mantle plugs upwelling into giant impact basins punched through the lunar crust followed by volcanic filling of the remainder of the crater above the plug. It is explicitly shown that continued volcanic filling is not inhibited by the attainment of isostatic equilibrium. If the density contrast between the lunar crust and mantle is 0.5 g/cm3, the minimum depth of the crust is 45 km in the Imbrium region. The strength of the lunar mantle must be somewhat greater than the strength of the earth's mantle; however, this is not inconsistent with a composition essentially similar to the composition of the earth. The approximately linear relationship between the peak gravity anomaly and the mare basin diameter suggests that for craters less than 200 km in diameter, the strength of the moon coupled with its crustal thickness are adequate to preclude significant mantle plug upwelling and mascon formation. With better density data the mascons may well be seen to result from combination of several effects requiring some modification of present geometry based on plug and topographic effects alone. Nevertheless, given a lunar (or Martian) crust and. mantle capable of healing giant impact basins by normal isostatic and volcanic processes, then mantle plug mechanics should make a significant contribution to the geologic evolution of the basin and to the associated gravity anomaly.