Region 4: The Columbia Plateau

The Columbia Plateau lies to the west of the Rocky Mountains in eastern Washington, Oregon, and Idaho. This region, also called the Columbia Basin, is a broad, volcanic plain composed of basalt. Basalt solidifies from lavas that are very fluid when hot, and the basalt lava in this area erupted along a series of fractures in eastern Oregon between 17 and 14 million years ago. The basalt was so voluminous and fluid that it completely filled the preexisting topography, forming a broad, flat plain that tilts downward to the west. Geologists believe that some of these lava flows were 30 meters (100 feet) high, and flowed at speeds of up to 5 kilometers (3 miles) per hour. The Columbia Plateau also includes an area of volcanic materials erupted from the Yellowstone hot spot onto the relatively flat Snake River Plain. In the Northwest Central, the Columbia Plateau can be divided into the Walla Walla Plateau, the Payette Section, and the Snake River Plain (Figure 4.30).

Figure 4.30: Physiographic subregions of the Columbia Plateau.

The Walla Walla Plateau is covered by flood basalt, and, in fact, one basalt flow in western Idaho, the Imnaha basalt, is 900 meters (2950 feet) thick. Some rivers were able to cut through the basalt, forming deep canyons (Figure 4.31). The presence and thickness of sediment varies on this plateau since some of the small rivers in this area were dammed by the basalt flows, forming lakes where sedimentation could occur. In some places, thick layers of wind-blown glacial sediment were deposited, eroding to form hills.

Figure 4.31: Hells Canyon, near Wallowa along the Oregon-Idaho border, cuts deeply through the Columbia Flood Basalt.

The Payette Section is a flat-lying area dominated by the drainage basin of the Payette River (a tributary of the Snake River with two major tributaries of its own: the North and South forks). The Snake River Plain is a low-lying, relatively flat area underlain by volcanic rocks. This low-lying area, formed from eruptions of the Yellowstone hot spot as the North American plate moved westward (see Figure 4.25), forms an obvious feature on maps and satellite imagery. The majority of the features on the plain’s surface are lava flows and cinder cones, with a few volcanic domes (Figure 4.32). As one moves toward the western edge of the Snake River Plain, ash flows and tuff become more common.

Figure 4.32: Craters of the Moon National Monument encompasses three major lava fields, spanning about 1000 square kilometers (400 square miles) along Idaho’s Snake River Plain. The area’s volcanic features include volcanic domes, basaltic flows, lava tubes, open rifts, and ash flows.