Craters of the Moon National Wilderness is best describe as a flood basalt lava field. As the North American Plate Past over a hot spot it created magma which came up through a series of fissure systems along what is known as the Great Rift. Repeated lava flows (which ended approximately 2,000 years ago) have shaped the way the landscape looks today. Although there is diversity in the type of lava found in Craters of the Moon, the dominant lava type is pahoehoe lava. Pahoehoe has a smooth ropy texture to it and is created from long lava tubes which allows for a slower cooling process. This cooling process, along with low viscosity, helps to produce the smooth ropy texture.

Aerial view of the Great Rift. via http://www.geocities.ws/ka7eii/desert.html