Columbia River flood-basalt volcanism is the product of mid-Miocene distortion of the Yellowstone mantle plume head against the Precambrian margin of North America. The overall migration pattern of Columbia River basalt vents reflects northward propagation of the plume head beneath the west-southwest–migrating North American plate. Thermal uplift mimics the vent migration. Over the 1.5 m.y. of Grande Ronde basalt extrusion, the projected crest of the plume head migrated 105 km (7 cm/yr), from the Chief Joseph dike swarm east-oortheast into the Clearwater embayment, where uplift occurred over the same period at a rate of 0.67 mm/yr. Chemical variations reflect progressive melting of the plume and mantle lithosphere, followed by the mixing of these components with an enriched source from the overriding craton. Rapid thickening of the plume head against the cratonic margin led to decompressional melting and magma rise beneath a thin oceanic lithosphere of accreted terranes.