Bimodal volcanism and rhyolite migration along the High Lava Plains in central Oregon (United States) lie above a broader feature defined by low seismic velocity in the upper mantle that emanates from the Yellowstone hotspot (northwest United States) and extends westward across the northern Basin and Range. It was emplaced by a westward current, driven in part by rapid buoyancy-driven flow across the east-west cratonic boundary of North America. Geothermometry studies and geochemical considerations suggest that the low-velocity feature may be composed of moderately hot, low-density mantle derived from the Yellowstone plume but diluted by thermomechanical erosion and entrainment of colder mantle lithosphere. Finger-like conduits of plume-modified mantle beneath Quaternary eruption sites delineate flow-line channels that have developed across the broader mantle structure since 2 Ma. These channels have allowed low-density mantle to accumulate against the Cascades arc, thus providing a heated mantle source for mafic magmatism in the Newberry (Oregon) and Medicine Lake (California) volcanic fields.