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Abstract The Colorado Plateau and its surroundings serve as an archetypal case to investigate the interaction of mantle melting processes and lithospheric structure. It has been hypothesized that widespread Cenozoic volcanism indicates the encroachment of the convective upwelling of asthenosphere toward the Plateau center. In this study, we generate a Common Conversion Point (CCP) stack of S‐to‐p (Sp) receiver functions to image the locations of lithospheric discontinuities in the southwestern United States. Our results are broadly similar to prior work, showing a strong and continuous Negative Velocity Gradient (NVG) consistent with the Lithosphere‐Asthenosphere Boundary (LAB) over much of the study area. However, with several methodological improvements, we are able to obtain more reliable NVG depth picks below the Colorado Plateau where the LAB becomes weaker, deeper, and broader. We compare the inferred topography of NVGs with the locations of volcanoes, and find that the majority of recent volcanoes are co‐located with lithosphere that is ∼80 km thick. This appears to be the critical depth at which partial melt from upwelling asthenosphere pooling at the base of (or within) the lithosphere may percolate to the surface. We compare our CCP profiles with magma equilibration conditions determined from petrologic analysis and find good agreement between the depth of NVGs and depth of magma equilibration. This analysis provides insight into the progression of magmatism and lithospheric loss toward the center of the Colorado Plateau, and demonstrates how small‐scale processes like melting influence lithosphere‐asthenosphere interactions that persist over large temporal and spatial scales.