The basaltic volcanism on the ESRP is localized in lava fields along volcanic rift zones. These zones are narrow belts, typically 3-12 miles (5-20 km) wide, composed of faults, grabens, eruptive and non-eruptive fissures, spatter cones, cinder cones, and low shield volcanoes (Kuntz, 1977a, 1977b; Kuntz, et al, 1992). Most volcanic rift zones are perpendicular to the long axis of the plain, and may be extensions of faults that bound basin-and-range mountains north and south of the plain (Kuntz, 1977b). Eight separate young basaltic lava fields (Craters of the Moon, Kings Bowl, Wapi, Shoshone, North Robber's, South Robber's, Hell's Half Acre, and Cerro Grande) can be identified in Landsat images of the ESRP (Lefebvre, 1977; Champion and Greeley, 1977; King, 1977). The largest of these is the Craters of the Moon (COM) lava field, which is also the largest basaltic lava field of dominantly Holocene age (less than 10,000 years) in the conterminous United States.

Figure 4. Great Rift at South End of Kings Bowl (Note- people at top for scale).

The COM lava field consists of more than 60 lava flows that cover an area of 618 mi² (1600 km²) and a volume of 7.2 mi³ (30 km³). The volcanic vents that supplied the lava flows of the COM lava field are aligned along the northern part of the Great Rift. The Great Rift is approximately 53 miles (85 km) long and extends from the southern Pioneer Mountains southeastward through the Monument to Pillar Butte, located about 18 miles (30 km) northwest of American Falls. The Great Rift is the best example of a volcanic rift zone on the ESRP and can be divided into four sections. The northern-most section lies beneath the COM lava field. Just south of the COM lava field, the rift changes to an inactive open fissure system. The southern portions of the rift gave rise to the Kings Bowl lava field and to the Wapi lava field. Figure 4 is a view down the axis of the Great Rift in Kings Bowl.

The variety of lava flows and volcanic vents in the COM lava field represent a nearly complete range of the types of volcanic features formed by basaltic eruptions. Lava flows and volcanic landforms, such as cinder cones and spatter cones, are highly concentrated in the northern end of the COM lava field. Based on observations of active basaltic eruptions, geologists believe that these volcanic landforms are the result of combinations of distinct eruptive phases (Kuntz, et al, 1982).

Ejection of lava by both curtains of fire and fire fountains can produce spatter ramparts, cinder cones, mounds of cinder, and generate several kinds of volcanic bombs. The fire fountains that produced many of the Monument's cinder cones were probably >1,000 feet (300 m) high. Big Cinder Butte, the tallest cinder cone in the Monument is over 700 feet (210 m) high and may have had a fire fountain >1,500 feet (450 m) high.

Figure 5. Spindle Bomb Figure 6. Breadcrust Bomb

Four kinds of bombs are found in the Monument, all of which started off as globs of molten rock thrown or ejected into the air. If the glob got twisted during its flight it is called a spindle bomb (Fig. 5) and typically range from a few inches to several feet in length. If the bomb was very tiny and twisted it is called a ribbon bomb. When a glob of molten rock forms a crust as it flies through the air and the gases inside continue to expand and crack that crust, it is called a breadcrust bomb, because of its similarity to bread rising in an oven (Fig. 6). If the bomb did not completely solidify during flight and flattened on landing, it is called a cow-pie bomb or is sometimes also referred to as a pancake bomb. Some cow-pie bombs in the Monument are over 10 feet long.

Many eruptions in the COM lava field are believed to have begun with a phase characterized by a "curtain of fire". Curtains of fire are long lines of gas-charged lava erupting in low fountains. These curtains can extend for up to several miles, are generally 100-200 feet (30-60 m) tall, and can be sustained for hours and up to several days. As a curtain of fire continues to erupt, segments of the fissure begin to clog. This results in the same amount of lava being forced to erupt from a limited number of vents producing fire fountains.

Figure 7. Big Cinder Butte

The first landforms to develop from the lava fountains are cinder cones, which are accumulations of cinders (light fragments riddled with gas holes), volcanic bombs, spatter and lava flows that collect around the vent forming a cone with a central crater. There are more than 25 major cinder cones within COM lava field. Prevailing winds from the west and southwest have caused a preponderance of downwind accumulation of cinders from many of the vents. This has resulted in an elongation of many cinder cones to the east or northeast, making them asymmetrical. The Great Rift volcanic rift zone is about 1.5 miles (2.5 km) wide in the COM lava field where many of the cinder cones are found. The cones are generally located along the outer margins of this zone. Cones on the western margin include (north to south) Grassy Cone, Silent Cone, Big Cinder Butte (Fig. 7), Echo Crater, the Sentinel and Fissure Butte. The eastern margin includes Sunset Cone, Paisley Cone, Half Cone, Broken Top, and the Watchman cinder cones.

After several hours, days, or weeks, a decrease in magma pressure and the amount of dissolved gases in the magma can produce a corresponding change in the output of lava. At this time, the amount of lava spraying from the vent(s) decreases and begins pouring out as lava flows. This phase may last many years and start and stop many times. The lava flows can vary greatly in temperature and composition from one vent to another and with time; therefore, they also vary in viscosity. This creates fields of pahoehoe, slabby pahoehoe, a'a, and block lava flows.

Figure 8. Block Lava Figure 9. Pahoehoe

The source of these lava flows can be from the same vent that formed a cinder cone, spatter rampart or spatter cone. When a cinder cone is the source of a flow, the lava burrowing through the side of the cone may breach the cone. This commonly results in a notch in the cone above the feeder fissure. The lava flow that breaches the cone may occur concurrently with the cinder cone development or it may occur long after the formation of the cone because of reactivation of the fissure system underlying it. In the COM lava field, reactivation of this sort probably occurred in vents of North Crater, Broken Top, and the Watchman cinder cones where the lava flows seem to be significantly younger than the cones. It is also common for lava flows to originate from portions of active rifts that have not previously undergone fountain-type eruptions. In this case, lava can flow directly out of the unobstructed vent and onto the landscape.

When the eruption of lava continues for a long time from an unobstructed vent, a large shield volcano can be produced. Shield volcanoes are gently sloping and have a flattened dome shape. Wapi lava field in the southern part of the Monument is an outstanding example of a shield volcano. Kuntz, et al, (1992) estimated from the calculated volume of basalt on ESRP and typical eruption volumes that the ESRP is made up of about 8,000 shield volcanoes. The largest shields are typically located near the center of the plain, suggesting that they overlie the central region of magma generation. Lava can travel great distances through lava tubes with very little loss of heat. Lava tubes and tube systems, therefore, facilitate the transport of lava over great distances. Some flows extend up to 30 miles or 48 km (Hughes, et al, 1999).

Lava is described by its physical appearance, which is largely determined by its composition, temperature, fluid and crystal content, and the influence the surface and slope it is flowing down exert on it. Block lava (Fig. 8) has a surface of angular blocks and forms from very dense lava. The typical composition of block lava in the Monument is trachyandesite (an extrusive rock, intermediate in composition between trachyte and andesite). A'a has a rough, jagged, or clinkery surface. Pahoehoe has a smooth, ropy, or billowy surface (Fig. 9). Pahoehoe can be further broken down into several types. Shelly pahoehoe forms from highly gas-charged lava, often near vents or tube skylights, and contains small open tubes, blisters and thin crusts. Spiny pahoehoe forms from very thick and pasty lava and contains elongated gas bubbles on the surface that form spines. Spiny pahoehoe is the dominant form found in the Monument. Slabby pahoehoe is made up of jumbled up plates or slabs of broken pahoehoe crust. Both slabby and spiny pahoehoe are transition phases to a'a.

Figure 10. Indian Tunnel lava tube.

Lava tubes, which are hollow spaces beneath the surface of solidified lava flows, are formed by the withdrawal of molten lava after the formation of the surface crusts. Indian Tunnel in the northern part of the Monument has a 40-foot (12 m) high ceiling and is 800 feet (240 m) long (Fig. 10). Bear Trap Cave, which lies between COM and Kings Bowl lava fields is >10 miles (16 km) long, but is not continuously passable. Inside lava tubes, one can see lava stalactites, lava curbs, stacked tubes, bifurcating and coalescing channels, skylights, tube linings, and other features. Remelt features include submetallic appearance caused by a lack of gas bubbles in remelted material, soda straw like formations on the ends of lava stalactites, flowstone appearing linings, and small slumps.

Based on textural differences in lava flows within the Monument, many separate lava flows have been recognized. Some flows can begin as one flow type and change to another over time. Therefore, textural differences cannot be used as the sole method of separating flows. Aerial photographs and analysis of Landsat images can also be used to distinguish flows from one another. Subtle differences in lava surfaces can often be detected on the images. These surface variances result from differing vegetation, weathering, and sediment coverage on individual flows (Lefebvre, 1977). But individual flows can also have uneven vegetation, differential weathering, and varying sediment coverage.

Figure 11. Squeeze-ups: Top--squeeze ups that came from the tension fracture in the top of a pressure ridge; Bottom–squeeze-up that looks like a candy kiss that oozed up through the a crack in the crust of a lava pond. Figure 12. Pressure Plateau on south side of Broken Top (lower right). (Also note slumping and open fissures on side of cone)

Most of the lava flows in the Monument are pahoehoe and were fed through tubes and tube systems, though there are some sheet flows. Structures representing both inflation and deflation of the lava surface can be seen along with both hot and cold collapses of lava tube roofs. Some lava flows produce tumuli (small mounds) or pressure ridges (elongate ridges) on their crusts. In some places squeeze-ups formed when pressure was sufficient to force molten lava up through tension fractures in the top of pressure ridges or cracks in the solidified crust of lava ponds (Fig.11). There are also pressure plateaus (Fig.12) that were produced by the sill-like injection of new lava beneath the crust of an earlier sheet flow that had not completely solidified.

Although it is not a convenient field method, geochemistry is the most accurate way to distinguish lava flows. The chemistry of the COM lava field has been studied in more detail than any other lava field in the ESRP. Geochemical examinations of the ESRP rocks have shown that the majority of the basalts to be olivine tholeiites. However, COM basalts are enriched in iron, phosphorus, titanium, and the alkali elements. Leeman, et al, (1976) believe the magma that fed the COM eruptions evolved from the SRP tholeiites (silica-oversaturated basalt). This evolution could be the result of fractionation of the source magma (separation through crystallization) or crustal assimilation (melting and incorporation of crustal rocks as the magma migrated toward the surface of the earth) (Leeman, et al, 1976; Leeman, 1982; Kuntz, et al, 1992). For evolved lava, crustal contamination produces lava with silica (SiO₂) ranges of ~49% to 64%, while crystal fractionation produces lava with silica ranges of ~44% to 54% (Kuntz, et al, 1986).

Other lava features include spatter cones (Fig. 13) that formed when fluid globs (spatter) were ejected short distances (generally<200 ft or <60 m) from the vents and accumulated immediately around the vent forming short steep-sided cones. Along eruptive fissures where a whole segment erupted, spatter can accumulate to produce low ridges called spatter ramparts. Hornitos, also known as rootless vents, are similar in appearance to spatter cones, but formed from spatter ejected from holes in the crust of a lava tube instead of directly from a feeding fissure. The Monument has collapse features known as sinks or pit craters (Fig. 14). During some eruptions, pieces of crater walls were carried off like icebergs by the lava flows. These wall chunks are known as rafted blocks (Fig. 15). Devils Orchard in northern part of the Monument is an entire field of rafted blocks that were carried off from the North Crater area. The Monument contains more than 580 Kipukas. Kipukas are older high areas that younger lava flowed around, but not over. They often appear as grassy hills surrounded by relatively barren lava.

Figure 13. Spatter Cone
Figure 14. Pit Craters

Figure 15. Rafted Blocks at Devil's Orchard

Besides shelly pahoehoe areas that contain many small open tubes and blisters and the numerous lava tubes associated with tube fed pahoehoe flows there are other kinds of caves found in the Monument. They include fissure caves associated with the Great Rift, many, such as Bear's Den waterhole, are ice floored. Flowing lava also can produce shallow caves and overhangs at flow fronts and as a result of the inflation process. Differential weathering of agglutinated cinders on some cinder cones has also generated a few shallow caves (Fig. 16); less firmly welded or sintered layers being more easily eroded. Some of these small caves are over 10 feet deep. Figure 16. Differential weathering cave.