- Flood basalt volcanism occurred in the Pacific Northwest, USA, between 17.5 and 6 Ma when over 300 basaltic lavas of the Columbia River Basalt Group (CRBG) were erupted from fissures in eastern Washington, eastern Oregon, and western Idaho (Fig. 1) (Swanson et al., 1979a). These flood basalts cover over 200,000 km2 of the Pacific Northwest and have an estimated volume of more than 234,000 km3 (Camp et al., 2003). Concurrent with these massive basalt eruptions was the folding and faulting of the basalt in the western part of the Columbia Basin and development of generally east-west trending anticlinal ridges and synclinal valleys collectively known as the Yakima Fold Belt