Workman, Rhea

Craters of the Moon Lava Field: a History of Volcanism

Introduction
The Eastern Snake River Plain (ESRP) in Southern Idaho is a 300 km long, northeast-southwest trending topographic trough bounded to the north and south by Basin-and-Range-type hills and valleys. The plain is covered with a 1-2 km thick veneer of basaltic lava of which over 95% of the surficially exposed lava flows are less than 730,000 years old. Most lava flows have originated from volcanic rift zones that are rough continuations of adjacent NW-SE trending structural lineaments of the Basin and Range. The elevation of the ESRP climbs from 700m in the west up to 2000m in the east near the Yellowstone plateau. Basaltic volcanism in the ESRP follows in the wake of rhyolitic, caldera forming volcanism thought to have originated from the passage of the North American plate over the Yellowstone hot spot. Ash flow tuffs from the ESRP show an age progression from about 15 Ma in the southwest to the most recent tuff, the Lava Creek Tuff, associated with the formation of the Yellowstone caldera, at 0.6 Ma.

Craters of the Moon Lava Field
In the ESPR, there are 8 lava fields less than 20,000 years old - all distributed within the southwestern 2/3 of the plain. Of these eight, the Craters of the Moon lava field (COM) displays the most recent eruptions within the ESRP, is the only polygenetic lava field , and shows the greatest range in whole-rock major element variability. COM is spatially (1600 square km) and volumetrically (30 cubic km) the largest of three recent lava fields all erupted from the Great Rift, a lineament of extension projected southward from the White Knob Mountains, which help define the northern boundary of the ESRP (Fig. 1, borrowed from the National Park Service web site). Manifestations of extension along the Great Rift include fissures, spatter cones, cinder cones, cracks, and pit craters. Radiocarbon studies have established eight eruptive periods for the COM lava field, starting at 15,100 years ago and continuing at approximately a 2,000 year interval. For COM, the volume of an eruption is a function of the time since the last eruption, implying a constant magma production rate from the COM source area. At around 7 ka, there was a step increase in the magma volume per time relationship accompanied by both a cluster of more frequent eruptions and an increase in compositional variability of the lavas toward more a siliceous endmember. Eruptions during this active period produced abundant a'a flows instead of the more common pahoehoe flows, possibly because an increase in silica content results in an increase in the lava's viscosity; all else being equal, higher viscosity lends to an a'a type flow morphology. All the geochemical evidence available (major element, trace element and isotope data) points to an substantial increase in crustal contamination starting at ~7 ka and continuing to the last COM eruptive event.

Native Inhabitants of the ESRP
According to archeological findings, humans have inhabited what is now Southern Idaho for at least 14,000 years. The most popular dwelling places from the start were along the Snake and Salmon River Valleys, where hunting and gathering were most prosperous, but the specific locations of settlements fluctuated along with climate and food sources. The most outstanding point in the context of Craters of the Moon is that the formation of the lava field has been almost completely witnessed by humans. Cinder cones of COM have no doubt produced enormous fountains of lava that would have been seen all across the Snake River Plain. Since most evidence of the native passerby has been buried by new flows, we have no way of knowing the exact human history of the land; however, we can be assured that eruptions at COM would not have been missed.

Must see spots at COM
The arid climate of the ESRP has allowed for spectacular preservation of 2-15 ka volcanic features. Much basalt erupted on the globe today spills out onto tropical ocean islands, if not underwater, where vegetation on new flows can thrive within a few hundred years and obscure details of lava flow emplacement. Craters of the Moon National Monument has established a few roads and trails that make some features, young and old, accessible to the wandering geologist; otherwise, since 80% of the lava flows are easily traversed pahoehoe, a short diversion off trail to look at flow contacts would instill a better understanding for the COM volcanic history (backcountry permits are available in the park). A few accessible "must see" spots include:
1. Big Craters area. These are the source vents (spatter cones and fissures) for the most recent Blue Dragon Flows. This is a great spot to get a feel for the orientation of the Great Rift, as the spatter cones, cinder cones and fissures are aligned and can be seen from the rim of Big Crater cinder cone.
2. The Caves Trail. Here you can immerse yourself in the Blue Dragon flow field and observe surficial lava transport mechanisms by walking through old lava tubes and among the ropy pahoehoe. Bring a flashlight.
3. Devil's Orchard. Although this is also an area covered with flows from the recent eruptive episode, lava flow morphology is quite different here. A'a and block flows are abundant in the Devil's Orchard flow field, which is uncommon for COM as a whole.

Reading List
Kuntz, Mel A., 1989, Geology of Craters of the Moon Lava Field, Idaho, Ruebelmann, K. L. (editor), Snake River Plain - Yellowstone Volcanic Province: Field Trip Guidebook T305, p. 51-61

Kuntz, Mel A., Covington, Harry R., Schorr, Linda J., 1992, An overview of basaltic volcanism of the eastern Snake River plain, Karl, P. (editor), Kuntz, Mel A. (editor), Platt, Lucian B. (editor), Regional geology of eastern Idaho and western Wyoming, Memoir - Geological Society of America, 179, p. 227-267

Reid, Mary R., 1995, Processes of mantle enrichment and magmatic differentiation in the eastern Snake River plain; Th isotope evidence, Earth and Planetary Science Letters, 131 (3-4), p. 239-254

Stout, M. Z., Nicholls, J., Kuntz, M. A., 1994, Petrological and mineralogical variations in 2500-2000 yr B.P. lava flows, Craters of the Moon lava field, Idaho, Journal of Petrology, 35 (6), p. 1681-1715