Two types of spatial and temporal trends associated with Teritary and Quaternary High Lava Plains volcanism. Flood-basalt volcanism from 16.6 to 15.0 Ma began from a source area in southeastern Oregon and spread rapidly to the north, northeast, and south (A); while post-15 Ma, more silicic, bi-modal volcanism occurred along two age-progressive trends to the northeast along southern Idaho’s Snake River Plain, and to the west across the High Lava Plains of eastern Oregon (B).