The Monument lies on the eastern portion of the Snake River Plain (SRP), which is a topographic low in southern Idaho that is 30 to 60 miles (50 to 100 km) wide and extends 360 miles (600 km) from Oregon to the Yellowstone Plateau. The plain is bounded by highlands to the north and south (Fig. 1). The Eastern Snake River Plain (ESRP) consists of broad flat basalt flows and thin discontinuous sedimentary deposits that together have a total thickness of ~0.6 to 1.2 miles or ~1 to 2 km (Doherty, et al, 1979). Magnetic polarity determinations and recent radiometric studies (Champion, et al, 1988; Kuntz, et al, 1992) indicated that most of the surface flows were erupted during the Brunhes Normal-Polarity Chron, and thus are younger than 780 ka (ka = thousand years ago). Data from wells that penetrate the ESRP to depths as great as 2 miles (3,500 m) show that the lava flow and sediment sequence is 0.6 to 1.2 miles (1 to 2 km) thick throughout most of the plain (Embree, et al, 1982). Drilling and field studies (Doherty, et al, 1979; Embree, et al, 1982; Morgan, et al, 1984) show that the basalt-sediment sequence is underlain by rhyolitic lava flows, ignimbrites (rock formed by the widespread deposition and consolidation of ash flows), and pyroclastic deposits (formed by volcanic explosion or aerial expulsion from a volcanic vent).

The structure of the SRP varies greatly from west to east. The northwestern plain is believed to be a graben, or fault-bounded depression (Malde, 1959). The ESRP, which includes the Monument, is less clearly defined. The earliest studies by Kirkham (1931) hypothesized that the plain was a downwarp, which had been filled with basalt both during and following the downwarping process. He believed that extrusion of lava was the major cause of subsidence and he supported this hypothesis with evidence of volcanic layers that gently dip toward the center of the plain.

Some have suggested that the volcanism and structure of the plain are the result of an eastward propagating rift (Myers and Hamilton, 1964; Hamilton, 1987), and transform fault boundaries across basin and range faults (Christiansen and McKee, 1978). Or the plain may simply be related to a preexisting crustal weakness, i.e., the structure of the Precambrian basement in southern Idaho (Eaton, et al, 1975). A more catastrophic explanation for the formation of the plain is related to a hypothesized meteorite impact in southwestern Idaho. The impact is conjectured to have caused deep fractures in the earth's crust that initiated the eruption of flood basalts, which were followed by lower-volume outpouring of lava from the fractures (Alt and Hyndman, 1988).

Some of the most popular recent explanations for the formation of the ESRP have involved theories incorporating a deep mantle plume. Thick rhyolitic rocks encountered during drilling in the ESRP suggested that some portions of the plain represent filled rhyolitic-calderas, similar to the Henrys Fork (Island Park) Caldera in Idaho (Doherty, et al, 1979; Embree, et al, 1982; Morgan, et al, 1984). This information, along with some geophysical data, led many geologists to believe that the ESRP is the site of a northeasterly propagating system of rhyolitic volcanic centers. It is thought that the southwesterly movement of the North American Continent, caused by plate tectonics, has passed southern Idaho over a stationary mantle plume or hotspot. In turn, the mantle plume has caused rhyolitic and associated basaltic volcanism to develop across southern Idaho.

This mantle plume is believed to be the same heat source for the volcanic and hydrothermal activity in Yellowstone National Park (e.g.: Armstrong, et al, 1975; Brott, et al, 1981; Maley, 1987; Pierce and Morgan, 1992). Some deep mantle-plume advocates believe that the plume ascended from the core-mantle boundary, was progressively overridden by the North American Plate starting about 60 million years ago, and may be responsible for the Carlin gold deposits found in Nevada (Oppliger, et al, 1997). Along the same line, others hypothesize that the mineralization in the Carmack Group, found in the Yukon, is related to the Yellowstone hotspot of some 70 million years ago (Johnston, et al, 1996).

Regardless of when the hotspot originated, the ESRP records a progressively younger trend of rhyolitic eruptions to the northeast. Henry Heasler with the Yellowstone Volcano Observatory reports that there have been 142 massive blasts, catastrophic eruptions of huge volumes of rhyolitic magma, in the last 17 million years along the ESRP (Sparrow, 2003). These eruptions typically produced calderas 10-40 miles wide. Many of the calderas overlapped and may be broken down into 7-13 volcanic centers. Although some of the mountain ranges that existed on the ESRP before the hotspot may have been blown away by the eruptions, it is more likely that they were swallowed up as the floor of the caldera sank during the violent explosions, thus producing the trough we see today (Smith and Siegel, 2,000). Kuntz, et al, (1992) believe that the source of the material for the ESRP eruptions is lithospheric mantle with the melting being driven by plume upwelling and decompression-melting. In contrast to a deep mantle plume, more recent teleseismic (utilizing distant seismic events) studies led Smith and Siegel (2000) to believe that the root of the hotspot is only at a depth of about 125 miles (200 km). Humphreys, et al, (2000) envision convective rolls within the athenosphere or local upper-mantle convection instead of a deep mantle plume.

This recent seismic data also suggests that the Yellowstone hotspot left behind a slab of basalt 6-10 miles thick in the mid-crust and that it contains partial melt. Smith and Siegel (2000) figuratively describe this slab as representing the slag left in the bottom of the numerous magma chambers spawned by the hotspot. The region surrounding the ESRP continues to experience basin-and-range type faulting, which is stretching or pulling apart the crust. This crustal extension continues to uplift the mountain ranges, such as the Lost River Range where a magnitude 7.3 earthquake occurred in 1983. On the ESRP in the wake of the Yellowstone hotspot, where all of these hot rocks have been left behind, instead of producing mountain ranges, the tensional forces help to create decompression melting, which results in dike emplacement and periodic eruption of molten rock onto the surface. As long as these forces continue to act, more eruptions will eventually occur. It is estimated that the ESRP is made of 8,000 shield volcanoes and the typical volume erupted is 1.2 mi³ or 5 km³ (Kuntz, et al, 1992). Coalesced shield volcanoes and lava cones constitute >95% of the total volume of basalt in the ESRP and the lava flows are dominantly of the tube-fed type (Kuntz, et al, 1992).

Several prominent rhyolitic domes lie along the ESRP in the Arco Desert and are visible from ten's of miles away. The tallest, Big Southern Butte, is a landmark and navigation aid visible from much of the Monument. It towers 2,500-ft (760m) over the ESRP and extends another 2,950-ft (900 m) into the subsurface. Big Southern Butte, one of the largest rhyolite domes in the world, has been dated at ~300 ka. Other buttes of the ESRP include East Butte (~600 ka), Middle Butte (~600 ka) and Cedar Butte (~400 ka). Current research indicates that these rhyolite domes formed through extreme fractionation of basaltic magma (McCurry, et al, 1999).