Region 2: 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. 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 2.7). The thickness of the lava flows reached 1800 meters (6000 feet), burying almost all of the older rock in the area.
Figure 2.7: Extent of Columbia Basin Flood Basalt.
Basaltic magmas are produced by partial melting of the upper mantle. Materials melt when we increase their temperature, but a second way to melt a solid is to decrease the pressure. In the interior of the Earth this second mechanism—decompression—is far more important. When pressure on the mantle is released as it is forced up through the crust due to subduction, it becomes basaltic magma.
The Columbia River has eroded deep into the lava flows of the Columbia Plateau, revealing layers of columnar jointed basalt. Layers of these jointed lava flows have been exposed at the Moses Coulee in Douglas County, Washington (Figure 2.8). The rhyolitic ash flows of the Bishop Tuff in California and Devil’s Tower (an igneous intrusion) in Wyoming (Figure 2.9) also exhibit columnar jointing.
Figure 2.8: Moses Coulee exposure of lava flows in Douglas County, Washington.
Figure 2.9: Devil’s Tower National Monument in Wyoming.
Columnar Jointing
As a lava flow cools, it contracts, and the resulting force may cause the rock to crack. These cracks continue down to the bottom of the flow, resulting in five- or six-sided columns. Columnar joints are not restricted to basalt flows and can form in ashflow tuffs as well as shallow intrusions. The columns are generally vertical, but may also be slightly curved.
