Neogene Snake River Plain-Yellowstone volcanic province

(Above) 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).

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).

(Above) 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).

Evidence for a hotspot origin of the Snake River Plain The hotspot origin model of the Snake River Plain is the com

The hotspot origin model of the Snake River Plain is the commonly accepted model. Pierce and Morgan (1992; 2009) summarized several lines of evidence that support the hotspot model. The first line of evidence is the voluminous phase of mafic volcanism (Columbia River basalts) associated with the earliest apparent phase of hotspot activity. This was followed by a time transgressive record of silicic volcanism interpreted to be the result of the movement of the North American Plate over a stationary mantle plume (e.g., Anders et al., 2014).

A second line of evidence is the presence of a topographically high region centered near the Yellowstone caldera, called the Yellowstone crescent of high terrain. This crescent of high terrain flares out in a bow wave-like pattern about the volcanic track and heads in front of the volcanic progression and favors a sub-lithosphere driver. As the hotspot migrated to the northeast with respect to the North American Plate, the highlands subsided due to cooling of the underlying crust and crustal loading from the eruption of post-hotspot basalts. This changed drainage patterns in river systems within and adjacent to the ESRP.

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 (Beranek et al., 2006).

(Above) Topographic and drainage evolution of the ESRP-Yellowstone system (modified from Link et al., 1999 and Beranek et al., 2006).

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.

An additional line of evidence is the apparent match in timing of high rates of Basin and Range faulting and the locations of silicic volcanism (Rodgers et al., 2002). This suggests that weakening of the lithosphere during heating led to enhanced rates of faulting. The four zones of increasing seismic activity form the Intermountain seismic belt around the current location of hotspot activity.

Further information about the Yellowstone-eastern Snake River Plain can be found in the Digital Atlas of Idaho: Snake River Plain page and Rocks, Rails, and Trails page.