page_title: Craters of the Moon National Monument and Preserve
section_title: Geology
hierarchical_section_title: Craters of the Moon National Monument and Preserve / Geology
context_page_description: Craters of the Moon National Monument and Preserve is a U.S. national monument and national preserve in the Snake River Plain in central Idaho. It is along US 20, between the small towns of Arco and Carey, at an average elevation of 5,900 feet above sea level. The protected area's features are volcanic and represent one of the best-preserved flood basalt areas in the continental United States.
  The Monument was established on May 2, 1924. In November 2000, a presidential proclamation by President Clinton greatly expanded the Monument area. The 410,000-acre National Park Service portions of the expanded Monument were designated as Craters of the Moon National Preserve in August 2002. It spreads across Blaine, Butte, Lincoln, Minidoka, and Power counties. The area is managed cooperatively by the National Park Service and the Bureau of Land Management.
  The Monument and Preserve encompass three major lava fields and about 400 square miles of sagebrush steppe grasslands to cover a total area of 1,117 square miles. The Monument alone covers 343,000 acres.
context_section_description: The Snake River Plain is a volcanic province that was created by a series of cataclysmic caldera-forming eruptions which started about 15 million years ago. A migrating hotspot thought to now exist under Yellowstone Caldera in Yellowstone National Park has been implicated. This hot spot was under the Craters of the Moon area some 10 to 11 million years ago but 'moved' as the North American Plate migrated southwestward. Pressure from the hot spot heaves the land surface up, creating fault-block mountains. After the hot spot passes the pressure is released and the land subsides.
  Leftover heat from this hot spot was later liberated by Basin and Range-associated rifting and created the many overlapping lava flows that make up the Lava Beds of Idaho. The largest rift zone is the Great Rift; it is from this 'Great Rift fissure system' that Craters of the Moon, Kings Bowl, and Wapi lava fields were created. The Great Rift is a National Natural Landmark.
  In spite of their fresh appearance, the oldest flows in the Craters of the Moon Lava Field are 15,000 years old and the youngest erupted about 2000 years ago, according to Mel Kuntz and other USGS geologists. Nevertheless, the volcanic fissures at Craters of the Moon are considered to be dormant, not extinct, and are expected to erupt in less than a thousand years. There are eight major eruptive periods recognized in the Craters of the Moon Lava Field. Each period lasted about 1000 years or less and were separated by relatively quiet periods that lasted between 500 and as long as 3000 years. Individual lava flows were up to 30 miles (50 km) long with the Blue Dragon Flow being the longest.
  Kings Bowl Lava Field erupted during a single fissure eruption on the southern part of the Great Rift about 2,250 years ago. This eruption probably lasted only a few hours to a few days. The field preserves explosion pits, lava lakes, squeeze-ups, basalt mounds, and an ash blanket. The Wapi Lava Field probably formed from a fissure eruption at the same time as the Kings Bowl eruption. More prolonged activity over a period of months to a few years led to the formation of low shield volcanoes in the Wapi field. The Bear Trap lava tube, between the Craters of the Moon and the Wapi lava fields, is a cave system more than 15 miles (24 km) long. The lava tube is remarkable for its length and for the number of well-preserved lava cave features, such as lava stalactites and curbs, the latter marking high stands of the flowing lava frozen on the lava tube walls. The lava tubes and pit craters of the monument are known for their unusual preservation of winter ice and snow into the hot summer months, due to shielding from the sun and the insulating properties of basalt.
  A typical eruption along the Great Rift and similar basaltic rift systems starts with a curtain of very fluid lava shooting up to 1,000 feet (300 m) high along a segment of the rift up to 1 mile (1.6 km) long. As the eruption continues, pressure and heat decrease and the chemistry of the lava becomes slightly more silica rich. The curtain of lava responds by breaking apart into separate vents. Various types of volcanoes may form at these vents: gas-rich pulverized lava creates cinder cones (such as Inferno Cone – stop 4), and pasty lava blobs form spatter cones (such as Spatter Cones – stop 5). Later stages of an eruption push lava streams out through the side or base of cinder cones, which usually ends the life of the cinder cone (North Crater, Watchmen, and Sheep Trail Butte are notable exceptions). This will sometimes breach part of the cone and carry it away as large and craggy blocks of cinder (as seen at North Crater Flow – stop 2 – and Devils Orchard – stop 3). Solid crust forms over lava streams, and lava tubes (a type of cave) are created when lava vacates its course (examples can be seen at the Cave Area – stop 7).
  Geologists feared that a large earthquake that shook Borah Peak, Idaho's tallest mountain, in 1983 would restart volcanic activity at Craters of the Moon, though this proved not to be the case. Geologists predict that
caption: Kings Bowl and Great Rift from air. King's Bowl is a phreatic explosion pit 280 feet (90 m) long, 100 feet (30 m) wide, and 100 feet (30 m) deep, caused by lava meeting groundwater and producing a steam explosion 2,200 years ago. (NPS photo)
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