Craters of The Moon National Monument and Preserve - Geography and Geologic Setting

Geography and Geologic Setting

Craters of the Moon is in south-central Idaho, midway between Boise and Yellowstone National Park. The lava field reaches southeastward from the Pioneer Mountains. Combined U.S. Highway 20-26-93 cuts through the northwestern part of the monument and provides access to it. However, the rugged landscape of the monument itself remains remote and undeveloped, with only one paved road across the northern end.

The Craters of the Moon Lava Field spreads across 618 square miles (1,601 km2) and is the largest mostly Holocene-aged basaltic lava field in the lower 48 U.S. states. The Monument and Preserve contain more than 25 volcanic cones, including outstanding examples of spatter cones. The 60 distinct lava flows that form the Craters of the Moon Lava Field range in age from 15,000 to just 2,000 years. The Kings Bowl and Wapi lava fields, both about 2,200 years old, are part of the National Preserve.

This lava field is the largest of several large beds of lava that erupted from the 53-mile (85 km) south-east to north-west trending Great Rift volcanic zone, a line of weakness in the Earth's crust. Together with fields from other fissures they make up the Lava Beds of Idaho, which in turn are in the much larger Snake River Plain volcanic province. The Great Rift extends across almost the entire Snake River Plain.

Elevation at the visitor center is 5,900 feet (1,800 m) above sea level.

Total average precipitation in the Craters of the Moon area is between 15–20 inches (380–510 mm) per year. Most of this is lost in cracks in the basalt, only to emerge later in springs and seeps in the walls of the Snake River Canyon. Older lava fields on the plain have been invaded by drought-resistant plants such as sagebrush, while younger fields, such as Craters of the Moon, only have a seasonal and very sparse cover of vegetation. From a distance this cover disappears almost entirely, giving an impression of utter black desolation. Repeated lava flows over the last 15,000 years have raised the land surface enough to expose it to the prevailing southwesterly winds, which help to keep the area dry. Together these conditions make life on the lava field difficult.