Lava plateaus are extensive areas of flood basalt which are formed by the extensive eruption of basaltic lava over a large area. Rifting of the continental crust is often responsible as the crust thins and allows magma from the mantle to erupt over an extensive area of fissures. The eruptions are often non explosive and continue for a long time building lava flow upon lava flow.
The Colombia river Flood Basalts are situated between the Casacades and Rocky Mountains in the NW USA. Over 10-15 million years during the Miocene ( 5-23 Ma ago) a 6000 feet thickness of basaltic lava was built up. As the lava erupted the crust sank into the cavity left by the eruption.