Mascons, mare rock and isostasy

The nature of the large positive gravity anomalies discovered on the Moon (mascons) has been the subject of lively discussion. The high density (3.1 to 3.5 g cm⁻³) of mare rocks at Tranquillity Base supports one hypothesis—that mascons are surface sheets of high density volcanic rock. The non-isostatic conditions of the mascon areas shown by the free-air anomalies may have terrestrial analogues. in some regions of recent volcanism where breaks in the crust have allowed dense basalts to pile up on the surface faster than isostatic balance can be maintained.