The design and interpretation of gravity-based Earth models require sufficient knowledge of the effect exerted on continental crustal density by chemical composition, pressure-temperature (PT) conditions, and water content. This need has motivated the development of a petrophysical modeling for 55 major element analyses compiled to characterize the geochemical differentiation trend of active continental margins. Equilibrium mineral assemblages and densities were computed using two independent thermodynamic approaches along conductive geotherms representing volcanic arcs and shields. Results under anhydrous conditions demonstrate that density is inversely correlated with the weight percent of SiO2 for all PT conditions. Empirical relationships with correlation factors commonly better than 0.9 allow the density under active continental margins to be estimated from silica content at critical conditions. Calculations for H2O-saturated systems and estimates of the effect produced by retained melt on the bulk density of lower crustal zones of melting, assimilation, storage, and homogenization (MASH zones) indicate that these empirical relationships should also hold for wet, melt-containing crustal columns of acidic to intermediate composition. They cannot be applied for basic compositions because mafic rocks absorb significant amounts of water via the formation of amphiboles, strongly reducing their density with respect to the dry granulites formed by garnet-pyroxene assemblages. This fact, plus the density reduction generated by the retention of some volume percent of melt, suggests that MASH zones underneath hydrated, subduction-related magmatic arcs thinner than 50 km could contain large amounts of basic to ultrabasic material (SiO2 as low as 43 wt%) in a gravitationally stable situation. This could have implications for estimates of global crustal composition and models of crustal growth. The results also suggest restricted thermal, compositional, and tectonic conditions for the removal of lower crust into the mantle.