Neogene Snake River Plain
The Snake River Plain is a broad arcuate topographic depression that extends across southern Idaho (figure 1 to the right; click on image for a larger image or click here for PDF version.) The western Snake River Plain sits in a fault-bounded graben while the eastern Snake River Plain (ESRP) is a large structural downwarp that formed due to the weight of the overlying volcanic rocks. The Owyhee Plateau is genetically related to the Snake River Plain, although it has remained a highland region. The map below shows the geography of the Snake River Plain and surrounding areas.
Figure 1. Geology of the Snake River Plain
Current studies suggest that the Snake River Plain resulted from the passage of the North American Plate over a stationary mantle plume or “hotspot” that is currently located beneath Yellowstone National Park (Figure 2) or what is often referred to as the Yellowstone Caldera or Yellowstone Plateau. The North American plate migrates at a rate of approximately 4.5 mm/year over the "hotspot". (Rodgers et al., 1990, Pierce and Morgan, 1992).
Figure 2. Igneous features of southern Idaho. After Kuntz et al., 1982.
Figure 3. Map showing the northeastward apparent motion of hotspot migration and the ages of the various calderas. From Link and Phoenix (1996), as simplified from Pierce and Morgan (1992).
Hotspot volcanism commenced ~17 Ma in northeastern Nevada and continues to the present (Figure 3). Ignimbrites and tuff deposits marginal to the Snake River Plain record the path of the movement. Basaltic lava fields, polygenetic eruptive centers, and rhyolite domes that spatially and temporally overlie Yellowstone Hotspot volcanism comprise the upper 1-2 km of ESRP stratigraphy. Many of the basalts of the ESRP erupted along volcanic rift zones (VRZ) that are oriented parallel to the direction of regional basin and range extension.
Current silicic volcanic activity is centered in the Yellowstone area. Previous silicic volcanic centers produced a topographic bulge that coincided with the continental divide. As the North American plate migrated to the southeast and volcanic activity shifted to new regions, the extinct calderas gradually subsided to their present elevations due to thermal and gravitational effects.
Figure 4: After Pierce and Morgan, 1992.
Four zones of seismic activity are associated with the migration of the hotspot (Figure 4 after Pierce and Morgan, 1992). Zone II contains active Holocene faults that are thought to be connected with current volcanic activity of the Yellowstone Hotspot. Zone III contains late Pleistocene faults that are decreasing in activity. Zone IV contains faults that are no longer active. The zones form belts which curve around the hotspot track.
Figure 5. Map showing the locations of Late Pleistocene to Holocene basaltic lava flows as well as older rhyolites associated with the Snake River Plain volcanism. Lava field abbreviations: SH= Shoshone, COM= Crators of the moon, W= Wapi, KB= Kings bowl, R= North and South Robbers, CG= Cerro Grande, HHA= Hells Half Acre. Figure from Hughes et al., 1999. Click on map for enlarged version.
Geology of the Snake River Plain
The western and eastern Snake River Plains are topographically continuous and seem similar; however, they are structurally quite different. The western Snake River Plain (WSRP) is a NW trending graben; both the land surface and the rock layers dip towards the axis of the plain (Shervais, et al., 2005; Bonnichsen and Godchaux, 2002). The rocks that occupy the WSRP are rhyolitic tuffs and ash flows of the Idavada Volcanic Group (15 to 11 Ma in age), and fluvial and lacustrine sediments with interbedded basalt flows of the Idaho Group (Pierce and Morgan, 1992; Bonnichsen and Godchaux, 2002). Lake Idaho occupied the WSRP during the Pliocene epoch, as the WSRP subsided and the hotspot continued to the northeast (see Figure 2).
The eastern Snake River Plain is underlain by silicic and mafic volcanic rocks with local interbeds of continental sediments. Quaternary basalt flows cover ~95% of the surface of the ESRP (Kuntz et al., 1992). The Idavada silicic volcanics of the ESRP are lithologically similar to those of the WSRP but are younger in age (10 Ma to 6.2 Ma). The tuffs at Yellowstone (0.6 to 2 Ma) represent the youngest pulse of silicic volcanic activity associated with the hotspot (Pierce and Morgan, 1992).
Figure 6. Typical cross-section through volcanic rocks on the eastern Snake River Plain. Figure from Hughes et al., 1999.
There is no evidence of faulting along the margins of the ESRP even though the Basin and Range province borders its northern and southern margins. Basin and Range faults are oriented perpendicular to the axis the eastern Snake River Plain (Rodgers et al., 1990).
Figure 7. Axis perpendicular to the Snake River Plain from Bestland 1998.
Basaltic lava flows erupted from northwest-trending volcanic rift zones. Regional extension across the plain allowed for the propagation of mafic dikes from the middle and lower crust. The VRZ's are oriented parallel to Basin and Range extension but they do not appear to connect to Basin and Range faults marginal to the plain (see Figure 7) (Kuntz et al., 1992; Hughes et al., 2002).
Figure 8. Relative location of the five buttes. From Hughes et al., 1999.
Five major rhyolite domes are present on the ESRP; Big Southern Butte, Cedar Butte, Middle Butte, Unnamed Butte, and East Butte (Figure 8). They are located near the extinct Picabo and Heise volcanic centers but are not directly associated with Yellowstone Hotspot volcanism.
Evidence for the Geologic History of the Snake River Plain
The hotspot origin model of the Snake River Plain is the commonly accepted model. Pierce and Morgan (1992) suggest three main lines of evidence that support the hotspot model. The first line of evidence is the time transgressive record of silicic volcanism interpreted to be the result of the movement of the North American Plate over a stationary mantle plume. The second line of evidence is the four zones of increasing seismic activity that form the intermountain seismic belt around the current location of hotspot activity.
The third line of evidence is topographic changes resulting from the passage of North America over the hotspot. The land rose due to thermal uplift. As the hotspot migrated to the northeast, the highlands subsided due to cooling of the underlying crust and crustal loading from the eruption of post-hotspot basalts.
The location of the Continental Divide locally corresponds to the migration of the hotspot. The topographic high produced by the hotspot produced radial drainage systems that flowed away from the volcanic center. In the Miocene epoch, when Snake River Plain volcanism began, the continental divide was located west of its current position and local streams drained toward the Atlantic Ocean; sediments were transported eastward, northward and southward away from the location of the volcanic high.
As the continental divide moved eastward across southern Idaho, rivers south of the Snake River Plain began to flow south and east. Subsidence in the wake of hotspot migration caused a drop in the base level of the Snake River Plain which initiated headward erosion toward the modern Snake River channel. This eventually led to stream capture and the shift of drainage to the Pacific Ocean.
Further Reading -- plus links in References
Beranek, L.P., Link, P.K., Fanning, C.M., 2006, Miocene to Holocene Landscape Evolution of the Western Snake River Plain Region, Idaho: Using the SHRIMP detrital zircon provenance record to track eastward migration of the Yellowstone Hotspot. Geological Society of America Bulletin, September/October 2006, p. 1027-1050.
Bonnichsen, B., and Godchaux, M.M., 2002, Late Miocene, Pliocene, and Pleistocene Geology of Southwestern Idaho With Emphasis on Basalts in the Bruneau-Jarbidge, Twin Falls, and Western Snake River Plain Regions, in Bill Bonnichsen, C.M. White, and Michael McCurry, eds., Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin 30, p. 233-312.
Hackett, W.R. and Smith, R.P., 1992, Quaternary volcanism, tectonics, and sedimentation in the Idaho National Engineering Laboratory area, in Wilson, J.R., ed., Field Guide to Geologic Excursions in Utah and Adjacent areas of Nevada, Idaho, and Wyoming: Utah Geological Survey, Miscellaneous Publication 92-3, p. 1-18.
Hughes, S. S., Smith, R. P., Hackett, W. R., and Anderson, S. R., 1999, Mafic Volcanism and Environmental Geology of the Eastern Snake River Plain, Idaho, in Hughes, S. S. and Thackray, G. D., eds., Guidebook to the Geology of Eastern Idaho: Idaho Museum of Natural History, p. 143-168.
Hughes, S.S., Wetmore, P.H., Casper , J.L., 2002, Evolution of Quaternary Tholeiitic Basalt Eruptive Centers on the Eastern Snake River Plain, Idaho, in Bonnichsen, B., White, C.M., and McCurry, M., eds., Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin 30, p. 363-385.
Kuntz, M.A., Champion, D.E., Spiker, E.C., Lefebvre, R.H., and McBroome, L.A., 1982, The Great Rift and the evolution of the Craters of the Moon lava field, Idaho, in Bonnichsen, B. and Breckenridge, R.M., eds., Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 423 - 438.
Kuntz, M.A., Covington, H. R., and Schorr, L. J., 1992, An overview of basaltic volcanism of the eastern Snake River Plain, Idaho, in Link, P. K., Kuntz, M. A., and Platt, L. P., eds., Regional geology of eastern Idaho and western Wyoming: Geological Society of America Memoir 179, p. 227-267.
Link, P.K., Fanning, C.M., Beranek, L.P., 2005. Reliability and longitudinal change of detrital-zircon age spectra in the Snake River system, Idaho and Wyoming: An example of reproducing the bumpy barcode. Sedimentary Geology 182, 101-142.
Link, P.K., Kaufman, D.S., and Thackray, G.D., 1999, Field Guide to Pleistocene Lakes Thatcher and Bonneville and the Bonneville Flood, southeastern Idaho, in Hughes, S.S. and Thackray, G.D., eds., Guidebook to the Geology of Eastern Idaho: Idaho Museum of Natural History, p. 251-266.
Link, P.K. and Phoenix , E.C., 1996, Rocks Rails & Trails, 2nd Edition: Idaho Museum of Natural History, 194 p.
Pierce, K. L. and Morgan, L. A., 1992, The track of the Yellowstone hot spot: Volcanism, faulting, and uplift, in Link, P. K., Kuntz, M. A., and Platt, L. B., eds., Regional Geology of Eastern Idaho and Western Wyoming: Geological Society of America Memoir 179, p. 1-53.
Rubin, A.M. and Pollard, D.D., 1987, Origins of blade-like dikes in volcanic rift zones: U.S. Geological Survey Professional Paper 1350, Chapter 53, p. 1449-1470.
Shervais, J.W, Kauffman, J.D., Gillerman, V.S., Othberg, K.L., Vetter, S.K., Hobson, V.R., Zarnetske, M., Cooke, M.F., Mathews, S.H., and Hanan, B.B., 2005, Basaltic Volcanism of the Central and Western Snake River Plain: A Guide to Field Relations Between Twin Falls and Mountain Home, Idaho, in Pederson, J., and Dehler, C.M., eds., Interior Western United States: Geological Society of America Field Guide 6, 26 p.
Continue on to Module 12 - Mountain Glaciation in Idaho