Support for the Colorado high topography is resolved using seismic data from the Colorado Rocky Mountain (CRM) Experiment and Seismic Transects. The average crustal thickness, derived from P wave receiver function imaging, is 48 km. However, a negative correlation between Moho depth and elevation is observed, which negates Airy-Heiskanen isostasy. Shallow Moho (<45 km depth) is found beneath some of the highest elevations, and therefore, the CRM are rootless. Deep Moho (45–51 km) regions indicate structure inherited from the Proterozoic assembly of the continent. Shear wave velocities from surface wave tomography are mapped to density employing empirical velocity-to-density relations in the crust and mantle temperature modeling. Predicted elastic plate flexure and gravity fields derived from the density model agree with observed long-wavelength topography and Bouguer gravity. Therefore, low-density crust and mantle are sufficient to support much of the CRM topography. Centers of Oligocene volcanism, e.g., the San Juan Mountains, display reduced crustal thicknesses and lowest average crustal velocities, suggesting that magmatic modification strongly influenced modern lithospheric structure and topography. Mantle velocities span 4.02–4.64 km/s at 73–123 km depth, and peak lateral temperature variations of 600°C are inferred. S wave receiver function imaging suggests that the lithosphere-asthenosphere boundary is 100–150 km deep beneath the Colorado Plateau, and 150–200 km deep beneath the High Plains. A region of negative arrivals beneath the CRM above 100 km depth correlates with low mantle velocities and is interpreted as thermally modified/metasomatized lithosphere resulting from Cenozoic volcanism.