During the Cenozoic Era, volcanism in the Basin and Range Province of the western United states changed from predominantly intermediate (andesitic and dacitic) to predominantly basaltic. Accompanying this shift was a general decrease in the amount of contamination suffered by basalts en route to the surface. Existing models related these progressive changes to plate tectonic events at the continental margin of western North America, and assume that extensional tectonism favors eruption of basalt. However, in many areas, synextensional rocks are predominantly intemediate or silicic in composition. Here we propose that the nature of volcanism in an area undergoing extension is a strong function of the density contrast between a magma and the overlying crust. Subtle variations in this density difference will determine the level of neutral buoyancy for a magma, and the height to which a column of magma may rise by hydrostatic pressure. Increases in crustal density favor eruption of dense magmas. Contamination of stagnated basalts by crustal material is more efficient than olivine fractionation in reducing the density of residual liquids, and a stagnated magma may ascend buoyantly after relatively small amounts of contamination. A progressive increase in mean crustal density, caused by crustal extension and consequent thining of the low-density upper crust coupled with emplacement of mafic plutons into the crust, may account for the observations that (1) the proportion of basalt erupted in the Basin and Range in the Cenozoic has increased with time, (2) younger basalts are generally less contaminated than older basalts, (3) major young silicic centers are apparently restricted to areas of low-density crust, and (4) midcrustal magma bodies are generally located at prominent P wave velocity discontinuities. ¿ American Geophysical Union 1989