During the main phase of Steens and Columbia River (United States) flood basalt eruptions between 16.7 and 15.0 Ma, ∼3900 km3 of silicic magma erupted from centers dispersed across ∼25,000 km2. The largest and oldest silicic centers, High Rock, McDermitt, and Lake Owyhee caldera complexes, lie along a narrow NNE trend at the transition between the craton and accreted oceanic crust, suggesting that silicic magma generation was greatest where mafic magma intruded more-felsic crust. Less voluminous and younger silicic centers track the spread of mafic dikes to the north. The High Rock caldera complex forms the western end of the Snake River Plain–Yellowstone trend of eastward-younging silicic centers. We attribute a regional lull in volcanism between ca. 23 and 7 Ma to lifting of the Juan de Fuca slab by a mantle plume beginning at ca. 25 Ma, and the distribution of main-stage flood basalts and coeval rhyolites to breakthrough of plume material at ca. 17 Ma to form a smaller plume head centered beneath Steens Mountain.