We document a systematic inverse relationship between the volume of erupted volcanic rocks assoicated with continental rifts and the seismic compressional wave velocity contrast across the Moho. The relationship is interpreted as evidence that magmatism significantly modifies the composition(s) of the lower crust (and upper mantle) during rifting processes. The relationship is not influenced by the current thermal state of the lithosphere, being observed for both modern rifts (Rio Grande, Kenya, Rhine) and paleorifts (Oslo), and may also characterize continental flood basalt provinces (Columbia River and Snake River plain). The trend is reconciled by igeneous and metamorphic processes that collectively modify the lower crust to more mafic bulk compositions, resulting in higher compressional velocities, and that decrease the density of the residual mantle, resulting in lower compressional velocities. These results support the view that most mantle-derived magma in continental rift settings is trapped in the crust. ¿American Geophysical Union 1991