A quantitative petrologic model for the formation of the Tharsis rise of Mars has been developed that is consistent with global gravity and topography data. Partial melting of the mantle underlying the Tharsis plateau results in the depletion of basaltic chemical components in the source region. Iron is partitioned preferentially into the basaltic liquid, so that the density of the source region is lowered relative to surrounding unmodified mantle. Basalt is erupted to the surface and/or intruded at shallow levels in the crust, where it solidifies to low-pressure mineral assemblages or perhaps glass. These low-pressure materials are less dense than their mantle equivalents; hence the net volume of the Tharis crust-mantle system is increased, resulting in the prominent topographic rise with no net gain or loss of mass. The distribution of densities with depth is quantitatively consistent with the isostic models of Sleep and Phillips <1979, 1985>. Typical values of parameters that are constrained by the gravity and topograpy data are 150-km crustal thickness, 250-km elastic mantle thickness, 26% partial melting in the mantle source region, and 32% addition of intrusive material to the crust. This closed-system petrologic process is strictly constructional in nature, but shallow intrusion of magma into the crust can cause uplift of overlying strata, as advocated by R. J. Phillips and N. H. Sleep .