Mid-Miocene explosive super-eruptions from the Yellowstone hotspot track: the rhyolitic ignimbrite record in south central Snake River Plain, Idaho, USA

Explosive super-eruptions (≥450 km³) are amongst the most catastrophic events at the Earth's surface, with immediate and devastating regional environmental consequences. Recent catastrophic super-eruptions at Yellowstone are well-known, but the previous (Miocene) history of large explosive eruptions from the Yellowstone hotspot is less-well understood, even though some in the central Snake River Plain (cSRP) may have been similar in size, or larger. To test this, local successions of rhyolitic welded ignimbrites in the southern cSRP have been studied to distinguish and characterise individual eruption-units using a combination of fieldwork, whole-rock and mineral chemistry, rock magnetism and geochronology data to correlate them regionally. In the Rogerson Graben, the revised Rogerson Formation comprises five eruption-units, each designated as a member. In the Cassia Hills ~20 km further east, the revised Cassia Formation comprises thirteen eruptionunits, and a new deep drill-hole near Kimberly, reveals three rhyolitic eruption-units. Robust correlations between these sites and to the north of the Snake River Plain have revealed the presence of three new regionally widespread ignimbrite sheets: (1) the Brown’s View Ignimbrite (10.3±0.2 Ma; 2700 km²; 40 km³ DRE); (2) the McMullen Creek Ignimbrite (9.0 ±0.1 Ma; 12,000 km²; 589 km³ DRE); and (3) the Grey’s Landing Ignimbrite (~9.0 Ma; 18,000 km²; 708 km³ DRE).Using standard techniques to calculate eruption magnitudes, two of these record super-eruptions: the McMullen Creek eruption (magnitude 8.4, 1179 km3 DRE) and the Grey’s Landing eruption (magnitude 8.5, 1416 km³ DRE). These represent the largest super-eruptions within the cSRP and are also among the largest eruptions of the entire Snake River-Yellowstone volcanic province. The addition of the three correlations presented here has further reduced the total number of previously inferred eruption-units from 42 to 29. Therefore, mid-Miocene rhyolitic explosive eruptions in the central Snake River Plain were less numerous but significantly larger than previously thought.