Text Box: Columbia Plateaus Introduction | Columbia River Flood Basalts | Physical Features | More Physical Features | Snake River Plain | Resources                                                       The Columbia Plateaus province is mostly in southeastern Washington, eastern Oregon and southern Idaho.  It is sometimes referred to as the Columbia Intermontane province because it is bounded by mountainous regions on three sides:  the Cascades on the west and Rocky Mountains on the north and east.  Province boundaries on these three sides are distinct and often abrupt.  The southern boundary is somewhat arbitrary, as here some lava that was produced by the same source as the lava of the Columbia Plateaus is included in the neighboring Basin and Range province because there it has been subjected to the same block faulting that so strongly characterizes that province.  This has produced a 50-mile-wide transition zone where structures of the Columbia Plateaus province and the Basin and Range province, although very different than one another, grade into each other.  Structurally the Columbia Plateaus province is primarily a basin, much of which is several thousand feet below the crests of the neighboring Cascades and Rocky Mountains.  The main part of this province is one of the approximately 22 major flood basalt regions of the world that has been active within the last 250 million years.  The terms flood basalt, or fissure flow basalt or plateau basalt, are applied to large lava-covered regions composed mostly of horizontal layers of basalt that were erupted in relatively rapid succession and spread like floods over vast areas.  Most of the rest of the province has been covered with non-flood lava from volcanoes and fault rifts.  Together these two parts that form the province have produced more than 42,000 cubic miles of lava covering an area of more than 100,000 square miles–one of the largest such areas in the Western Hemisphere.  The Columbia Plateaus province has been divided in various ways by different researchers.  One scheme divides the province into 13 sections; another uses five sections.  The most fundamental differences in the province are represented by the flood basalts of Washington and Oregon, referred to as the Columbia River flood basalts, and the primarily non-flood lavas of the Snake River Plain in southern Idaho.  These two sections will be the main focus of the differentiation within the province discussed here.     Columbia Plateaus Introduction | Columbia River Flood Basalts | Physical Features | More Physical Features | Snake River Plain | Resources