Craters of the Moon National Monument and Preserve lies on the Great Rift, a 60-mile-long volcanic rift zone on the Snake River Plain. Rift zones occur where the Earth's crust is pulled in opposite directions.
Lava Flow with stretched vesicles
The monument is one of the few continental hot spots in the world. What is now Yellowstone used to lie underneath the area that is now the Craters of the Moon. That area had eight major eruptive periods spanning 13 thousand years. As recently as 2,000 years ago, the Great Rift pulled apart and cracked. Molten lava shot up in a curtain of fire.
Read an interview with Professor Scott Hughes, Chair of the Department of Geosciences at Idaho State University, explaining more about the geology of Craters of the Moon.