Columnar basalt near Palisades Washington.

Giant lava flows covered much of Eastern Washington and parts of Idaho and Oregon 17 to 12 million years ago. Unlike volcanic eruptions such as Mt. St. Helens, lava erupted out of cracks in the earth's crust and flowed for long distances. This type of volcano is called a basalt flood. It is estimated that around 300 floods occurred on the Columbia Plateau. The basalt cooled from the top of the lava flow, and also from the bottom up. As the basalt cooled it shrunk 5 to 10 percent and cracked, forming columns. As the basalt crystallized the cracks grew, but the bottom cracks did not match those at the top and created an area between the cooling layers that is chaotic compared to the uniform top and bottom layers. The in-between layer is called the entablature. The long columns that are obvious in the photo to the left is the lower portion of a basalt flow; the upper columns of this flow have been subjected to significant erosion, and only the entablature between the layers remains.