Region 4: The Columbia Plateau

The Columbia Plateau, also known as the Columbia Basin, is the site of one of the largest outpourings of lava that the world has ever seen. The Columbia Plateau flood basalts are a notable example of a “Large Igneous Province,” where vast volumes of basalt are erupted over a relatively short period of time. Such a high volume of basaltic lava is erupted that the lava flows flood the land’s surface. Between 15 and 6 million years ago, basaltic lava flooded approximately 163,000 square kilometers (63,000 square miles), covering large parts of Washington, Oregon, and Idaho (Figure 2.35). The thickness of the lava flows reached 1800 meters (6000 feet), burying almost all of the older rock in the area. Geological evidence suggests that many of these flows advanced over preexisting topography at a rate of five kilometers per hour (three miles per hour). This was made possible by the fact that basaltic lava erupts at a temperature of greater than 1100°C (2000°F), yielding a very hot and fluid form of lava that would have quickly inundated existing landforms. The Columbia Plateau in western Idaho is uniformly covered with basalt, although over geological time, a large degree of faulting and warping has altered once nearly uniform elevations to a range of 60 to 1500 meters (200 to 5000 feet). The basalt flows found in this region commonly exhibit spectacular examples of columnar jointing.

Figure 2.35: Extent of the Columbia Basin Flood Basalt.

Large areas of flood basalt are generally associated with mantle hot spots. In this case, they are associated with the Yellowstone hot spot, whose trail from Oregon to Wyoming has produced the Snake River Plain. As the North American plate passed over the hot mantle plume, melting the base of the crust and producing large volumes of magma, the hot spot’s eruptive center moved northeastward across Idaho.

The rocks of the Snake River Plain cut across both the older Rocky Mountain and Basin and Range regions of Idaho. The plain is deeply filled with 30 to over 300 meters (100 to over 1000 feet) of rhyolite and basalt. Thick layers of rhyolite are generally capped by basalt on the surface—smaller basaltic eruptions tended to continue long after a major rhyolitic caldera eruption. Deeper rocks are seen in drillhole cores, but there are many places where the surface basalts or lava fields can be seen. Craters of the Moon National Monument is a lava field where basalts have been erupted over the last 15,000 years; the youngest flow there is only 2000 years old (Figure 2.36). Visitors can see flows, lava tunnels, spatter cones, and other volcanic features. Hell’s Half Acre, Shoshone, Cerro Grande, and Wapi are also well known lava fields in the plain.

In addition to lava flows, eruptions from the Yellowstone hot spot often generated enormous clouds of ash created when rhyolite magma was erupted as tiny molten particles. The ash was buoyed through the air by hot gases and blanketed hundreds of kilometers (miles) of land. As it condensed, it solidified into thick layers of tuff (Figure 2.37).

Figure 2.36: A lava field at Craters of the Moon National Monument, Idaho.

Figure 2.37: The Owyhee Canyonlands of southwestern Idaho cut through the Snake River Plain’s volcanic units, revealing layers of basalt, rhyolite, and welded tuff.

Types of Volcanic Flows

Pahoehoe flows are fluid, fast flowing basaltic rivers of lava resulting in smooth, ropey surfaces. In contrast, ’a’a flows are blocky, rubbly, slow-moving basaltic flows of cooling lava. They advance as cooled fragments tumble down the steep front and are buried by the advancing flow, producing a rough, spiny surface. Pillow lavas are formed when lava enters water, such as a lake, river, or ocean. The surface of the lava mass entering the water is cooled instantaneously, insulating the inner mass, which cools more slowly to form an irregular ovoid with a glassy external surface and a fine crystalline core.

A pahoehoe lava flow at Craters of the Moon National Monument.

Not all features of Idaho’s Columbia Plateau are related to igneous activity. As the Cordilleran Ice Sheet retreated back into Canada at the end of the ice age, meltwater ponded in lakes of all sizes. One of the largest glacial lakes was Glacial Lake Missoula in Montana, which was dammed by the ice sheet. When the ice dam failed, the lake was released in a catastrophic flood. The resultant landforms from this violent flood event carved deep channels into the terrain and left giant ripple marks, potholes, and boulders in the Channeled Scablands of northern Idaho and western Washington.