Region 2: The Columbia Plateau
The interior areas of eastern Washington and central and northeastern Oregon are divided into three major areas: the Blue Mountains, the High Lava Plains, and the Columbia Basin (Figure 4.6).
The Blue Mountains include the Wallowas, which reach a height of 2999 meters (9838 feet). This area is formed of ancient microcontinents, small pieces of continental crust that collided with North America in the Jurassic.
The High Lava Plains are the result of extensive volcanism during the Neogene. Newberry Volcano, just south of Bend, Oregon, is the westernmost point of eruption for a hot spot that is now responsible for the spectacular volcanic features found in Yellowstone. This hot spot, which has existed for over 16 million years, left its trail across southeastern Oregon and southern Idaho.
See Chapter 1: Geologic History to learn more about hot spot volcanism.
The Columbia Plateau, 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, flowing westward. The basalt was so voluminous and fluid that it completely filled the preexisting topography (remnants of which can be seen in hills such as Steptoe Butte [Figure 4.7] and Kamiak Butte in easternmost Washington), forming a broad, flat plain that tilts downward to the west. The lava flowed all the way to the Cascades, and even to the ocean, along what is now the Columbia River drainage. In western parts of the plateau, the basalt has been gently folded and faulted by mountain building associated with the uplift of the Cascade Mountains.
Figure 4.7: Steptoe Butte, a 400-million-year-old quartzite mound protruding from the Columbia Plateau and Palouse Hills in Whitman County, Washington. Elevation: 1101 m (3612 feet).
The Missoula Floods swept periodically across eastern Washington at the end of the last ice age. The floods were a result of a rupture in the ice dam that contained Glacial Lake Missoula, a massive glacial lake originally holding 2100 cubic kilometers (500 cubic miles) of water.
The youngest topographic features in the Northern Interior of Washington formed during the most recent ice age when glacial outwash and loess was deposited and later cut by the Missoula Floods, which also carved the Grand Coulee (an ancient river bed) and many of the lakes and channels of the central part of the state. In the Palouse area of Washington and Oregon, glacial outwash formed a series of steeply-sloped silt dunes, which are agriculturally important today due to their highly fertile soil (Figure 4.8).