Geology of Craters of the Moon National Monument and Preserve, Idaho

Geodiversity is the full variety of a park's rocks, minerals, sediments, fossils, landforms, soils and the processes that shape them. This summary is from the NPS Geodiversity Atlas (Geologic Resources Inventory).

A "weird and scenic landscape"

Craters of the Moon National Monument and Preserve protects the "weird and scenic landscape" of three young lava fields — Craters of the Moon, Wapi and Kings Bowl — along the Great Rift System on Idaho's eastern Snake River Plain. The latest eruption was 2,100 years ago. The park holds a wide range of volcanic vents and landforms — cinder cones, spatter cones, fissure volcanoes, a shield volcano and lava flows — and its mostly basaltic flows show pāhoehoe and ʻaʻā lava, tree molds, lava tubes and many surface features.

• Craters of the Moon lava field is the largest on the eastern Snake River Plain — about 620 square miles (1,600 km²) and more than 7 cubic miles (30 km³) of lava — and the largest recent, mostly Holocene, lava field in the contiguous United States. It has erupted many times over the last 15,000 years.
• Wapi erupted 2,270 years ago; its flows form a low shield volcano with many lava tubes.
• Kings Bowl, the smallest, formed in a single short eruption — likely hours to days — about 2,130 years ago, fed by eruptive fissures.

The monument was established in 1924 and expanded in 2000, adding Wapi and Kings Bowl and creating Craters of the Moon National Preserve. Its name dates to the 1920s, when the landscape was likened to the Moon's surface. It is one of 19 National Park System units set aside mainly for volcanic resources.

Setting

• The Snake River Plain stretches about 400 miles across Idaho, a depression of low relief. The western plain formed by faulting; the eastern plain subsided after the continent passed over the Yellowstone thermal plume, under the weight of magma that collected in the middle crust and the volcanic and sedimentary rocks above it. Its surface is mostly young basalt flows with thin loess and wind-blown deposits.
• The Yellowstone hotspot track runs from the McDermitt Caldera on the Oregon–Nevada border to Yellowstone; its rhyolitic calderas on the eastern plain are mostly buried by younger basalts.
• A monogenetic volcanic field: most vents erupt during one period of activity. The eastern plain has eight lava fields of latest Pleistocene to Holocene age, fed mostly by cinder cones and fissure volcanoes.
• The Great Rift is a volcanic rift zone of eruptive and noneruptive fissures, cones and faults in a narrow belt about 53 miles (85 km) long and 1.3–5 miles (2–8 km) wide, with Craters of the Moon near its north end and Wapi and Kings Bowl near the south. The best example of its kind on the plain, unique in North America and with few counterparts worldwide, it became a National Natural Landmark in 1968.

Older rocks

Older rocks show in places: the Mississippian Copper Basin Group of sedimentary rocks in the north, the Eocene Challis Volcanic Group of the Pioneer Mountains, and weathered, loess-covered Pleistocene and Pliocene basalts of the Snake River Group.

Eight eruptive periods

Most of the park's volcanic rock erupted in the last 15,000 years. Researchers, chiefly Mel Kuntz of the USGS and colleagues, identified eight eruptive periods in the Craters of the Moon field, dated mostly by radiocarbon. Each lasted a thousand years or less, with gaps of a few hundred to more than 2,000 years.

The rocks are mostly mafic to intermediate — high in magnesium and iron, low in silica (45–52% SiO₂) — and richer in sodium and potassium than typical basalts and andesites: hawaiites (trachybasalts) and latites (trachyandesites). Such low-viscosity magma travels far; eruptions are effusive to mildly explosive (0 to 2 on the Volcanic Explosivity Index), from Hawaiian fire fountains to Strombolian "fireworks."

Part of the park's geologic map. Map: National Park Service, Geologic Resources Inventory.

Volcanoes of every basaltic kind

• Cinder cones: 21 prominent cones in the Craters of the Moon field. The tallest, Big Cinder Butte, rises more than 700 feet (200 m). Big Craters is a complex of at least nine nested cones along a fissure. Slopes average about 30 degrees, near the angle of repose for cinders, and many cones are lopsided because prevailing winds carried more cinders to the east or northeast.
• Fissure volcanoes: long vents where magma-filled dikes reached the surface, aligned on the Great Rift — Fissure Butte, South Echo, Deadhorse, Vermillion Chasm, Big Craters, Trench Mortar Flat and Blue Dragon. They usually erupt effusively, building spatter cones and spatter ramparts, as on the Blue Dragon fissure and at Kings Bowl.
• Shield volcanoes: the only Holocene one is Wapi's. Laidlaw Butte, about 425,000 years old, stands as much as 900 feet (275 m) above the surrounding flows and forms the field's largest kīpuka.

Lava flows and tubes

The young flows are nearly bare and barely weathered, so delicate glassy rinds survive. Most are pāhoehoe, but rough, clinkery ʻaʻā and rare block lava occur too. Viscosity decides the form: pāhoehoe forms from more fluid lava, and a flow can turn to ʻaʻā far from its vent. Flows moved as sheets, in channels and through lava tubes, leaving squeeze-ups, tumuli, pāhoehoe toes, and rafted blocks of cinder cone carried off by moving lava.

Blue Dragon and some other flows shimmer blue or green: tiny titanium magnetite crystals in the glass reflect blue light.

Most flows were fed at least partly by lava tubes, which may drain to leave caverns with lava levees, linings and lava stalactites; collapsed stretches leave natural arches.

A hiker in Indian Tunnel, a large lava tube. Photo: National Park Service.

Kīpukas, caves and fossils

• Kīpukas — islands of older ground surrounded by younger lava — number more than 600. Several are research natural areas kept as scientific reference areas: Carey Kipuka, Brass Cap, Big Juniper and Sand Kipuka.
• Caves: more than 500, mostly lava tubes, but also fissure caves, lava blisters and tumuli. They hold ice, fossils, archeological sites and bat habitat. With volcanic bedrock almost everywhere, 99% of the park counts as pseudokarst. Caves are protected by the Federal Cave Resources Protection Act of 1988.
• Fossils: tree molds form when lava surrounds a tree before it burns away; the Tree Molds Trail, a 2-mile (3.2 km) round trip, leads to many at the edge of the Blue Dragon flow. The park is one of five National Park System units known to have them. Marine invertebrates occur in the Mississippian rocks, and Pleistocene and Holocene fossils in packrat middens in caves; all are protected by the Paleontological Resources Preservation Act of 2009.

Rare plants and Apollo astronauts

• Obscure phacelia (Phacelia inconspicua), nationally imperiled, grows on north- and east-facing volcanic slopes where snow lingers into spring; it is known only from a few places in Idaho and Nevada.
• Picabo milkvetch (Astragalus oniciformis) grows only in the north-central Snake River Plain, on deep, well-drained soils in basins and flats; it is imperiled range-wide or statewide.
• In 1969 the monument helped train Apollo 14 astronauts to collect lunar samples. Weight limits made every sample precious, so choosing the most scientifically valuable rocks mattered.

Hazards

• Volcanic: individual vents should not erupt again, but new eruptions in the lava field are possible. Eruptions have come about every 2,000 years since about 15,000 years ago, and the last was 2,100 years ago. The USGS 2018 national volcanic threat assessment rates the Craters of the Moon and Wapi fields low threat (scores 13.9 and 8.3) — an evaluation of possible impacts, not a forecast.
• Seismic: no Quaternary faults are known in the park, but the hazard is moderate: the USGS 2014 map gives a 2% chance in 50 years of shaking of 8–10% of gravity.
• Abandoned mineral lands: five sites with 53 features, none needing mitigation, among them the Martin Mine, which produced copper, gold, lead, silver and zinc in 1922–1929.

Maps and reports

The park's Geologic Resources Inventory report. National Park Service.

The park's geologic maps, report and posters, and its Soil Resources Inventory, are on the NPS DataStore.

Sources

Based on "NPS Geodiversity Atlas—Craters of the Moon National Monument & Preserve, Idaho," National Park Service; a work of the United States government in the public domain. The source spells the fissure both "Trench Mortal Flat" and "Trench Mortar Flat"; the second is used. Its "Astragalus onicifomis" is corrected to Astragalus oniciformis.