https://doi.org/10.1130/G47384.1 Manuscript received 13 January 2020 Revised manuscript received 16 April 2020 Manuscript accepted 16 April 2020 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 1 June 2020 Discovery of two new super-eruptions from the Yellowstone hotspot track (USA): Is the Yellowstone hotspot waning? Thomas R. Knott1, Michael J. Branney1, Marc K. Reichow1, David R. Finn1, Simon Tapster2 and Robert S. Coe3 1 School of Geography, Geology and the Environment, University of Leicester, Leicester LE1 7RH, UK 2 British Geological Survey, Nottingham NG12 5GG, UK 3 Earth and Planetary Science Department, University of California–Santa Cruz, Santa Cruz, California 95064, USA ABSTRACT Super-Eruption Recognition Super-eruptions are amongst the most extreme events to affect Earth’s surface, but too few Recognizing a super-eruption requires quan- examples are known to assess their global role in crustal processes and environmental impact. tification of the dense rock equivalent (DRE) We demonstrate a robust approach to recognize them at one of the best-preserved intraplate volume of the erupted deposit (Pyle, 2000). large igneous provinces, leading to the discovery of two new super-eruptions. Each generated However, several similar deposits may coexist huge and unusually hot pyroclastic density currents that sterilized extensive tracts of Idaho in a succession, presenting a challenge to dis- and Nevada in the United States. The ca. 8.99 Ma McMullen Creek eruption was magnitude tinguish and correlate individual deposits. Suc- 8.6, larger than the last two major eruptions at Yellowstone (Wyoming). Its volume exceeds cessions of similar-looking ignimbrites occur 1700 km3, covering ≥12,000 km2. The ca. 8.72 Ma Grey’s Landing eruption was even larger, at throughout southern Idaho in the United States magnitude of 8.8 and volume of ≥2800 km3. It covers ≥23,000 km2 and is the largest and hottest (Fig. 1; Branney et al., 2008), so we developed documented eruption from the Yellowstone hotspot. The discoveries show the effectiveness of a robust approach to distinguish and regionally distinguishing and tracing vast deposit sheets by combining trace-element chemistry and min- correlate individual units by combining trace- eral compositions with field and paleomagnetic characterization. This approach should lead to element and mineral chemistry, paleomagnetic more discoveries and size estimates, here and at other provinces. It has increased the number data, and detailed field characterization. Criti- of known super-eruptions from the Yellowstone hotspot, shows that the temporal framework cally, any one correlation technique proved of the magmatic province needs revision, and suggests that the hotspot may be waning. insufficient in isolation. INTRODUCTION Anders et al., 2019) allows study of the tem- MCMULLEN CREEK IGNIMBRITE Explosive super-eruptions (≥450 km3; poral relationships among magma production, The McMullen Creek super-eruption is magnitude ≥8; Mason et al., 2004) are land- residence, recycling, and crustal response recorded by an extensive rhyolitic ignimbrite scape-changing extreme events that perturb (Leeman et al., 2008). hitherto known only locally in the Cassia Hills global climate and devastate environments Yellowstone has produced super-eruptions (Ellis et al., 2010; Knott et al., 2016a). We (Self, 2006). They have occurred through (e.g., magnitude 8.7 Huckleberry Ridge Tuff; now correlate it widely across southern Idaho, much of Earth history, but few robustly doc- Christiansen, 2001), but the number gener- where it overlies members of the Cassia For- umented examples are known (e.g., Rougier ated as the hotspot tracked across the central mation (Knott et al., 2016a), and for the first et al., 2018). Further recognition from the Snake River Plain (SRP; Fig. 1) is not known. time across to the north of the SRP, where it geologic record is essential to quantify global A Miocene ignimbrite flare-up has been pro- overlies the Challis Volcanic Group (Fig. 1; frequencies, the range of eruption styles, and posed (Nash et al., 2006), and evidence for very for previous local names, see Table S1 in the impacts (Robock, 2002). One approach is large Miocene eruptions is emerging (Finn et al., Supplemental Material1). It is widely overlain to assess their frequency in particular tec- 2016; Ellis et al., 2019), but, until now, none by the Grey’s Landing Ignimbrite (see below), tonic settings. Several examples are known exceeded the magnitude of the Yellowstone aiding the recognition of both units in tandem. in continental arcs (Lipman and McIntosh, super-eruptions. A ≥12,000 km2 distribution as estimated using 2006; de Silva, 2008), but fewer have been We report the discovery of two super-erup- field mapping, logging, and the contemporane- found in intraplate settings. Therefore, we tions revealed by meticulous correlation of ous topography (Fig. 1; Williams et al., 1990; targeted the Yellowstone hotspot track in the central SRP ignimbrites previously thought to Michalek, 2009). It erupted from the Twin Falls United States because it is one of the best- be smaller localized units. We show they were eruptive center, as inferred from the distribution, preserved intraplate large igneous provinces, larger and more frequent than those at Yellow- distally decreasing grain sizes and thicknesses, where time-transgressive magmatism (due to stone, and we propose that the hotspot was per- and rheomorphic lineations and kinematic data 2 cm/yr plate motion; Armstrong et al., 1975; haps more vigorous in the Miocene. (Fig. 1; Knott et al., 2016a). 1Supplemental Material. Unit summaries, previous nomenclature, methodologies, and all raw data. Please visit https://doi .org/10.1130/GEOL.S.12360149 to access the supplemental material, and contact [email protected] with any questions. CITATION: Knott, T.R., et al., 2020, Discovery of two new super-eruptions from the Yellowstone hotspot track (USA): Is the Yellowstone hotspot waning?: Geology, v. 48, p. 934–938, https://doi.org/10.1130/G47384.1 934 www.gsapubs.org | Volume 48 | Number 9 | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/9/934/5135163/934.pdf by guest on 24 September 2021 Figure 1. Field area (black square) within the Yellowstone–Snake River volcanic province (Y-SRP) in the northwest United States, showing rhyolitic eruptive centers: M—McDermitt; OH—Owyhee-Humboldt; BJ—Bruneau-Jarbidge; TF—Twin Falls; P—Picabo; H—Heise; Y—Yellow- stone. Other: wSRr—western Snake River Plain. State abbreviations: WA—Washington; ID—Idaho; MT—Montana; OR—Oregon; CA—California; NV—Nevada; UT—Utah; WY—Wyoming. (Left) Select logs through McMullen (red) and Grey’s Landing (blue) super-eruption deposits from Twin Falls eruptive center. Site abbreviations: 3C—Three Creek; RG—Rogerson graben; RC—Rock Creek, Cassia Hills; OH—Oakley Hills; LFC—Little Fish Creek, Lake Hills; MBH—Mount Bennett Hills. (Right) Distribution maps and isopachs given in meters with representative outcrop thickness (from >50 logged sites) shown for reference (inset). Deposit Distinction N The McMullen Creek Ignimbrite is distin- guished from others in the region using a com- Figure 2. Stereonet of site-mean thermorema- bination of seven characteristics: nent magnetization (TRM) (1) Broad color layering reflects a compound McMullen Creek directions showing tightly welding profile with two dark intensely welded Indian Springs ignimbrite clustered McMullen Creek Inset area (red) and Grey’s Land- zones and a pale, less-welded center (Fig. 1). ignimbrite (5 sites, n = >50) (9.0 ± 0.3 Ma) ing (blue) Ignimbrites Distinct lithophysal bands enclose the less- from both flanks of the welded center and persist distally beyond where Grey’s Landing Castleford Crossing Snake River Plain (SRP; the central zone pinches out. ignimbrite Ignimbrite northwest United States), (8.19 ± 0.19 Ma) (2) The center has a distal-fining concentra- (10 sites, n = >80) demonstrating a clear dis- tion of angular nonvesicular vitric lapilli sup- tinction from one another and from other units ported in devitrified tuff (Fig. 1; Knott et al., ‘Tuff of Thorn Creek’ Before tilt correction nearby (grays). Data were 2016a). (10.1 ± 0.3 Ma) corrected for postem- (3) The entire deposit has a normal paleo- placement tilting. Inset: Dry Gulch ignimbrite Uncorrected TRM direc- magnetic polarity, and thermoremanent mag- R (reversed polarity) RR tions, complicated by netic (TRM) directions are tightly clustered R postemplacement tilting, and indistinguishable at all sites and differ from Key: shown for comparison other units (Fig. 2). Filled symbol - lower hemisphere (see the method in the projection R = Rogerson graben Supplemental Material (4) It contains 5%–15% crystals of plagio- Open symbol - upper hemisphere reference sites clase, pigeonite, augite, magnetite, apatite, and projection [see footnote 1]). zircon, but no sanidine, a phase ubiquitous in (all with 95 error ellipses) Geological Society of America | GEOLOGY | Volume 48 | Number 9 | www.gsapubs.org 935 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/9/934/5135163/934.pdf by guest on 24 September 2021 central SRP ignimbrites older than ca. 10 Ma Volume and Magnitude west along the basin axis, where evidence is (Cathey and Nash, 2004). Sourceward thickening of the McMullen concealed; and (3) assuming a caldera of mod- (5) It has a single equilibrium pair