The West Antarctic rift system, marked by a 3-5-kilometer high shoulder from northern Victoria Land to the Ellsworth Mountains, extends through the Ross Embayment and the Byrd Subglacial Basin. Geophysical data suggest that the ice covered area beneath the rift zone is underlain by Cenozoic volcanic rocks (flood basalts?), and extended crust about 20 km thick. Exposed bimodal alkaline volcanic rocks (mostly basalts, indistinguishable from ocean island basalts that have been interpreted to be mantle plume derived) range in age from Oligocene to the present. We propose a plume (approximately ellipsoidal and coincident with the West Antarctic rift system) defined by the distribution of K/Ba ratios of basalts, the elevated tectonomagmatic dome of coastal Marie Byrd Land, the high topography marking the rift shoulder and the inferred flood basalts(?) beneath the ice covered Byrd Subglacial Basin. Although most extension in West Antarctica apparently occurred in the late Mesozoic with Gondwana rifting, all exposed rift related volcanic rocks are post early Oligocene; the time lag is possibly explained by the proposed mantle plume. As spreading centers surrounding the stationary Antarctic plate in the Cenozoic migrated away from Antarctica, continued rifting in West Antarctica to the present was focused by the mantle plume. The plume possibly caused reorganization of the existing ridge system through a ridge jump to the hot, extended, weakened lithosphere present in the Ross Embayment-Byrd Subglacial basin area at the end of the Cretaceous. The propagating rift may have been captured by the thermal plume.