The 1.95-km-thick Cassia Formation, defined in the Cassia Hills at the southern margin of the Snake River Plain, Idaho, consists of 12 refined and newly described rhyolitic members, each with distinctive field, geochemical, mineralogical, geochronological, and paleomagnetic characteristics. It records voluminous high-temperature, Snake River-type explosive eruptions between ca. 11.3 Ma and ca. 8.1 Ma that emplaced intensely welded rheomorphic ignimbrites and associated ash-fall layers. One ignimbrite records the ca. 8.1 Ma Castleford Crossing eruption, which was of supereruption magnitude (~1900 km 3 ). It covers 14,000 km 2 and exceeds 1.35 km thickness within a subsided, proximal caldera-like depocenter. Majorand trace-element data define three successive temporal trends toward less-evolved rhyolitic compositions, separated by abrupt returns to more-evolved compositions. These cycles are thought to reflect increasing mantle-derived basaltic intraplating and hybridization of a midcrustal region, coupled with shallower fractionation in upper-crustal magma reservoirs. The onset of each new cycle is thought to record renewed intraplating at an adjacent region of crust, possibly as the North American plate migrated westward over the Yellowstone hotspot. A regional NE-trending monocline, here termed the Cassia monocline, was formed by synvolcanic deformation and subsidence of the intracontinental Snake River basin. Its structural and topographic evolution is reconstructed using thickness variations, offlap relations, and rheomorphic transport indicators in the successive dated ignimbrites. The subsidence is thought to have occurred in response to incremental loading and modification of the crust by the mantle-derived basaltic magmas. During this time, the area also underwent NW-trending faulting related to opening of the western Snake River rift and E-W Basin and Range extension. The large eruptions probably had different source locations, all within the subsiding basin. The proximal Miocene topography was thus in marked contrast to the more elevated present-day Yellowstone plateau.

Rogerson Graben, USA, is critically placed at the intersection between the Yellowstone hotspot track and the southern projection of the west Snake River rift. Eleven rhyolitic members of the re-defined, ≥420-m-thick, Rogerson Formation record voluminous high-temperature explosive eruptions, emplacing extensive ashfall and rheomorphic ignimbrite sheets. Yet, each member has subtly distinct field, chemical and palaeomagnetic characteristics. New regional correlations reveal that the Brown's View ignimbrite covers ≥3300 km 2 , and the Wooden Shoe ignimbrite covers ≥4400 km 2 and extends into Nevada. Between 11.9 and ∼8 Ma, the average frequency of large explosive eruptions in this region was 1 per 354 ky, about twice that at Yellowstone. The chemistry and mineralogy of the early rhyolites show increasing maturity with time possibly by progressive fractional crystallisation. This was followed by a trend towards less-evolved rhyolites that may record melting and h y b r i d i s a t i o n o f a m i d -c r u s t a l s o u r c e r e g i o n . Contemporaneous magmatism-induced crustal subsidence of the central Snake River Basin is recorded by successive ignimbrites offlapping and thinning up the N-facing limb of a regional basin-margin monocline, which developed between 10.59 and 8 Ma. The syn-volcanic basin topography contrasted significantly with the present-day elevated Yellowstone hotspot plateau. Concurrent basin-and-range extension produced the N-trending Rogerson Graben: early uplift of the Shoshone Hills (≥10.34 Ma) was followed by initiation of the Shoshone Fault and an E-sloping half-graben (∼10.3-10.1 Ma). The graben asymmetry then reversed with initiation of the Brown's Bench Fault (≥8 Ma), which remained intermittently active until the Pliocene.

Super-eruptions are amongst the most extreme events to affect Earth’s surface, but too few examples are known to assess their global role in crustal processes and environmental impact. We demonstrate a robust approach to recognize them at one of the best-preserved intraplate large igneous provinces, leading to the discovery of two new super-eruptions. Each generated huge and unusually hot pyroclastic density currents that sterilized extensive tracts of Idaho and Nevada in the United States. The ca. 8.99 Ma McMullen Creek eruption was magnitude 8.6, larger than the last two major eruptions at Yellowstone (Wyoming). Its volume exceeds 1700 km3, covering ≥12,000 km2. The ca. 8.72 Ma Grey’s Landing eruption was even larger, at magnitude of 8.8 and volume of ≥2800 km3. It covers ≥23,000 km2 and is the largest and hottest documented eruption from the Yellowstone hotspot. The discoveries show the effectiveness of distinguishing and tracing vast deposit sheets by combining trace-element chemistry and mineral compositions with field and paleomagnetic characterization. This approach should lead to more discoveries and size estimates, here and at other provinces. It has increased the number of known super-eruptions from the Yellowstone hotspot, shows that the temporal framework of the magmatic province needs revision, and suggests that the hotspot may be waning.

Individual ignimbrite cooling units in southern Idaho display significant variation of magnetic remanence directions and other magnetic properties. This complicates paleomagnetic correlation. The ignimbrites are intensely welded and exhibit mylonite-like flow banding produced by rheomorphic ductile shear during emplacement, prior to cooling below magnetic blocking temperatures. Glassy vitrophyric lithologies commonly have discrepantly shallow remanence directions rotated closer to the orientation of the subhorizontal shear fabric when compared to the microcrystalline center of the same cooling unit. To investigate this problem, we conducted a detailed paleomagnetic and rock magnetic study of a vertical profile through a single ignimbrite cooling unit and its underlying baked soil. The results demonstrate that large anisotropy of thermal remanent magnetization correlates with large (up to 38°) deflections of the stable remanence direction. Anisotropy of magnetic susceptibility revealed no strong anisotropy. A strong lineation and deflection of the remanence declination suggest that rheomorphic shear above magnetic blocking temperatures is the dominant mechanism controlling the formation of the magnetic fabric, with compaction contributing to a lesser extent. Nucleation and growth of anisotropic fine-grained magnetite in volcanic glass at high temperatures after, and perhaps also during, emplacement is indicated by systematic variation of magnetic properties from the quickly chilled ignimbrite base to the interior. These properties include remanence directions, anisotropy, coercivity, susceptibility, strength of natural remanent magnetization, and dominant unblocking temperature. The microcrystalline ignimbrite center has a magnetic direction that is the same as the underlying baked soil and, therefore, is a more reliable recorder of the paleofield direction than the glassy margins of highly welded ignimbrites.

In this paper, we present paleomagnetic, geochemical, mineralogical, and geochronologic evidence for correlation of the mid‐Miocene Cougar Point Tuff (CPT) in southwest Snake River Plain (SRP) of Idaho. The new stratigraphy presented here significantly reduces the frequency and increases the scale of known SRP ignimbrite eruptions. The CPT section exposed at the Black Rock Escarpment along the Bruneau River has been correlated eastward to the Brown's Bench escarpment (six common eruption units) and Cassia Mountains (three common eruption units) regions of southern Idaho. The CPT records an unusual pattern of geomagnetic field directions that provides the basis for robust stratigraphic correlations. Paleomagnetic characterization of eruption units based on geomagnetic field variation has a resolution on the order of a few centuries, providing a strong test of whether two deposits could have been emplaced from the same eruption or from temporally separate events. To obtain reliable paleomagnetic directions, the anisotropy of anhysteretic remanence was measured to correct for magnetic anisotropy, and an efficient new method was used to remove gyroremanence acquired during alternating field demagnetization.