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Explosive super eruptions are among the most extreme events to affect the Earth’s surface. Thankfully, humans have not experienced such an event in recorded history (the last massive volcanic explosion was 26,500 years ago). The only clues to help us better understand super eruptions and their impacts are hiding within the geological record—including along the track of the Yellowstone

Map and stratigraphy of ashflow sheets from Yellowstone Hotspot
Newly identified ash flow sheets within the Yellowstone–Snake River Plain volcanic province (Y-SRP).  The small inset in the center shows the track of the Yellowstone hotspot and its associated eruptive centers in light gray (M—McDermitt; OH—Owyhee-Humboldt; BJ—Bruneau-Jarbidge; TF—Twin Falls; P—Picabo; H—Heise; Y—Yellowstone). On the left are geological logs through the McMullen (red) and Grey’s Landing (blue) super eruption deposits from Twin Falls eruptive center. On the right are distribution maps and the deposit thicknesses (in meters from over 50 logged sites.  The areas covered by the distribution maps are given by the black box in the inset map of the Y-SRP volcanic province.  Adapted from Knott et al., 2020. (Public domain.)

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Dr. Thomas Knott, geochemist at the University of Leicester, England.

The Yellowstone hotspot is well known to have produced super-eruptions, such as the colossal Huckleberry Ridge Tuff eruption 2.1 million years ago and the more recent formation of Yellowstone Caldera 631,000 years ago. However, previously undiscovered Yellowstone hotspot super-eruptions are emerging from the rock record in southern Idaho, where the hotspot ‘burned’ its way through the crust several million years ago.

Researchers at the University of Leicester, in England, have announced the discovery of two newly identified Yellowstone hotspot super eruptions, including what they believe was the largest and most cataclysmic event of the volcanic province’s history. The results suggest the hotspot, which today fuels the famous geysers, mudpots, and fumaroles in Yellowstone National Park, may be decreasing in intensity.

In the study published in the journal Geology, the team explained how a combination of analytical techniques, including chemistry, magnetic data, and radio-isotopic dates, was used to “fingerprint” and correlate volcanic deposits scattered across tens of thousands of square kilometers. Deposits previously believed to belong to multiple, smaller eruptions were, in fact, correlated. These colossal sheets of volcanic material formed from two previously unknown super eruptions that occurred about 9.0 and 8.7 million years ago.

Grey’s Landing Ignimbrite in Idaho part of Yellowstone Hotspot
The Grey’s Landing Ignimbrite in Idaho (Dr. Thomas Knott, of the University of Leicester, England, gives the scale of the cliff). The entire cliff (and more not seen!) would have been deposited in a matter of moments as it welded to the land surface during a super eruption about 8.7 million years ago. The bright orange layer at the base of the cliff represents the upper land surface soil at the time of the eruption, which was burnt and baked to terracotta by the heat of the ash flow. (Credit: Professor Michael Branney, University of Leicester. Public domain.)

The younger of the two, the Grey’s Landing super eruption, is now the largest recorded event of the entire Yellowstone volcanic province, erupting more than 2800 km3 of volcanic material in a single event. This eruption was larger than the previous record-holder (the aforementioned Huckleberry Ridge Tuff) and would have had devastating local and global effects. The eruption enamelled an area the size of New Jersey in searing-hot volcanic glass that would have abruptly sterilized the land surface. In addition, particulates would have choked the stratosphere, raining fine ash over much of the United States and gradually encompassing the globe. Interestingly, the far-flung ash records of these eruptions are yet to be found, and once identified would increase their volumes of the eruptions even more!

The discovery of these two new super-eruptions brings the total number to 11 throughout the history of Yellowstone hotspot volcanic province, from about 16.5 million years ago to the present, and there may still be others waiting to be identified. Based on this record, researchers were able to demonstrate that the recurrence rate of Yellowstone hotspot super eruptions several million years ago was, on average, once every 500,000 years.  In comparison, three large-scale eruptions have originated from Yellowstone National Park in the past three million years, but only two were super eruptions. This equates to a recurrence rate of one super eruption every 1.5 million years—a three-fold decrease in the frequency of these colossal events.

Could this decline suggest that the Yellowstone hotpot is losing steam? This remains an intriguing possibility, and perhaps not unexpected as the hotspot starts to encounter colder and thicker continental crust as it migrates to the northeast due to the motion of the North American plate. But there may also be other super eruptions in Yellowstone’s geologic past that have yet to be discovered. Only time and further research will tell, and the work in Idaho provides a guide for how that further research might be done. Regardless, we must remain vigilant, since even if Yellowstone’s magmatic system is in decline, it may still erupt lava flows or experience smaller explosive eruptions in the future.

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