espite its highly extended and thinned crust, much of the western Cordillera in the United States is elevated more than 1 km above sea level. Therefore, this region cannot be thought of as thick crust floating isostatically in a uniform mantle; rather, the lithospheric mantle and/or the upper asthenosphere must vary in thickness or density across the region. Utilizing crustal thickness and density constraints, we modeled the residual mass deficit that must occur in the mantle lithosphere and asthenosphere beneath the western Cordillera. A major hot spot broke out during a complex series of Cenozoic tectonic events that included lithospheric thickening, backarc extension, and transition from a subduction to a transform plate boundary. We suggest that many of the characteristics that make the western Cordillera unique among extensional provinces can be ttributed to the mantle plume that created the Yellowstone hot spot.