Craters of the Moon National Monument was established in 1924 to protect the geologic features of a small portion of southern Idaho's Great Rift volcanic rift zone. It was greatly expanded in November of 2000 and now covers approximately 750,000 acres (~300,000 hectares) or about 1,100 square miles (~2,800 square km). Craters of the Moon National Monument and Preserve now contains almost all of the Great Rift, the best-developed example of a volcanic rift zone on the Eastern Snake River Plain (ESRP). See figures 1 & 2 for setting and location. The Monument lies within the Snake River Basin-High Desert (Omernik, 1986) and is dominated by 3 geologically young (Late Pleistocene-Holocene) lava fields that lie along the Great Rift. The Great Rift varies in width between approximately 1 and 5 miles (1.6 and 8 km). It begins north of the Monument, about 6 miles (9.6 km) from the topographic edge of the Snake River Plain, in the vent area of the Lava Creek flows located in the southern Pioneer Mountains (Kuntz, et al, 1992). The Great Rift extends southeasterly from the Lava Creek vents for more than 50 miles (80 km) to at least as far as beneath Pillar Butte on the Wapi lava field (Kuntz, et al, 1982). The Great Rift volcanic rift zone is a belt of open cracks, eruptive fissures, shield volcanoes, and cinder cones.

The Craters of the Moon (COM) lava field is the northernmost and largest of the 3 young lava fields. Kings Bowl lava field is the smallest and lies between COM lava field and the Wapi lava field located on the southern end of the Great Rift. The other areas of the Monument, located either between these 3 young lava fields or surrounding them, are made up of Pleistocene age pahoehoe and a'a flows, near-vent tephra deposits, cinder cones, lava cones, and shield volcanoes (Kuntz, et al, 1988). These older areas are mantled with loess deposits (windblown silt) and in some places by eolian sand. Longitudinal sand dunes are prominent features surrounding the southern end of the Wapi lava field (Greeley and King, 1977). During the Holocene (last 10,000 years), the highest volcanic activity of any of the eastern Snake River Plain (ESRP) basaltic rift systems was exhibited by these 3 lava fields associated with the Great Rift (Hughes, et al, 1999).

Figure 1. Regional setting and location of Craters of the Moon National Monument and Preserve (click on image for an enlargement in a new window)

The COM lava field is the largest dominantly Holocene basaltic lava field in the lower 48 states (Kuntz, et al, 1992); it covers 618 mi² (1,600 km²). COM lava field is a composite field made up of at least 60 lava flows and 25 tephra cones. It has 8 eruptive fissure systems that are aligned along the northern part of the Great Rift (Kuntz, et al, 1992). The COM lava field has a tremendous diversity of volcanic features, with nearly every type of feature that is associated with basaltic systems (Hughes, et al, 1999). Unlike most of the ESRP, where the basalts are predominantly diktytaxitic olivine tholeiites (or more simply-- olivine basalts) associated with small monogenetic shield volcanoes (Hughes, et al, 1999), the basalt deposits in the COM lava field exhibit a wide range of chemical compositions. Though the COM lava flows are believed to have similar parent magma to the volcanoes in the rest of the Plain, their varied compositions are due to crustal contamination from assimilating older rocks or from crystal fractionation (Kuntz, et al, 1986). The COM lava field formed during at least 8 major eruptive periods over the past 15,000 years in contrast to most of the other lava fields on the ESRP that represent single eruptive events.

Figure 2.Craters of the Moon (CRMO)

Kings Bowl lava field formed about 2,200 years ago during a single burst of eruptive activity that may have lasted as little as six hours (Kuntz, et al, 1992). Kings Bowl has a central eruptive fissure set that is about 4 miles long, which is flanked by 2 subparallel sets of non-eruptive fissures. The bowl that the field takes its name from is a phreatic explosion pit 280 feet (85 m) long, 100 feet (30 m) wide, and 100 feet (30 m) deep, caused by lava coming in contact with groundwater producing a steam explosion (Fig. 3). Adjacent to the bowl is an outstanding example of a lava lake with well-developed levees. The crust of the lake was broken by many of the blocks ejected by the phreatic explosion. The interior of this lake was still molten and oozed up through the holes punched in its crust, resulting in a large number of squeeze-up mounds of gas-charged lava (Hughes, et al, 1999). Many of the squeeze-ups look like mushrooms or candy kisses. There is also a plume of ash or tephra blanket on the east side of the pit that resulted from the explosion. Fissure caves, such as Crystal Ice Cave and Creons Cave lie along the Great Rift at Kings Bowl. At South Grotto, the rift may be passable to a depth of 650 feet (198 m) below the surface (Earl, 2001). Feeding dikes drain back features, and spatter cones can all be seen along the Great Rift at Kings Bowl.

Wapi and Kings Bowl lava fields are identical in age (approximately 2,200 years old), i.e., within the limits of analytical error (Hughes, et al, 1999). The Wapi lava field is a classic shield volcano with a flattened dome shape. Kuntz, et al, (1992) believe that the Wapi lava field began as a fissure eruption, but with prolonged activity developed a sustained eruption from a central vent complex, which produced the low shield volcano seen today. The vent complex is made up of 5 major and 6 smaller vents, with the vents being steep-sided circular depressions typically about 300 feet (90 m) in diameter and 30 feet (9 m) deep (Kuntz, et al, 1992). Rising about 60 feet (18 m) above the south side of the largest vent is a mass of agglutinate and layered flows known as Pillar Butte. Medial and distal parts of the lava field are mostly composed of tube-fed pahoehoe flows. Pressure plateaus, flow ridges, and collapse depressions characterize the margins of the field where local relief can be over 30 feet or 9 meters (Kuntz, et al, 1992). Greeley, 1971, reported that the only known dribblet spires in the continental U.S. occur on the flows associated with Pillar Butte. Now however, dribblet spires are known to occur at least also in Diamond Craters in Oregon. The spires found in the Wapi lava field average 12 feet (3.6 m) high and 5 feet (1.5 m) in diameter. They consist of imbricated rounded slabs of lava that range from 5 inches by 5 inches (12.7 cm by 12.7 cm) by 1 inch (2.5 cm) thick to 12 inches by 9 inches (30.5 cm by 22.9 cm) by 4 inches (10.2 cm) thick (Greeley, 1971). These bizarre spires are a type of hornito.