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November 8, 2020

Of the three enormous explosive eruptions from Yellowstone in the past 2.1 million years, the earliest and largest was the one that gave rise to the Huckleberry Ridge Tuff. The generalized perception of these colossal eruptions is that they are short-lived events lasting hours or days, but recent field observations indicate a more complicated story.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Colin Wilson, Professor in the School of Geography, Environment and Earth Sciences at Victoria University of Wellington in New Zealand.

Summary diagram of the geological record and timing of the Huckleberry Ridge Tuff eruption
Summary diagram of the geological record and timing of the Huckleberry Ridge Tuff eruption. See Swallow et al. (2019) for more details.

Over the past two decades, Colin Wilson, a volcanologist based in New Zealand, has been studying the products of the earliest and by far the largest of the huge caldera-forming eruptions that have helped shaped the history of Yellowstone. Occurring just over 2 million years ago, this eruption laid down vast deposits of ash from plumes that rose tens of kilometres into the atmosphere, called “ash fall” deposits. In addition, ground-hugging pyroclastic flows that reached over a hundred kilometres from source formed thick sheet-like deposits called “ignimbrite”.  Together, the fall deposits and ignimbrite from this eruption make up the Huckleberry Ridge Tuff (HRT). So much magma was evacuated that the ground surface around the eruption vents collapsed to form a 100 x 50 km caldera that is among the largest on Earth. The fall deposits from the eruption plume can still be found over most of the western half of the coterminous USA, and remnants of the ignimbrite extend from Big Sky, Montana, to Idaho Falls, Idaho.

Huckleberry Ridge Tuff fall deposits at Mount Everts, Yellowstone
View of ripples caused by wind winnowing of the Huckleberry Ridge Tuff fall deposits at Mount Everts. Other layers below show evidence for rain and hail landing with the falling ash. Scale in centimeters and inches.

Field studies of the HRT eruption deposits show that this eruption was much more complicated (and interesting) than previously thought. The first stage of the eruption produced fall deposits that are thickest in the Mount Everts area, near Mammoth Hot Springs on the northern edge of Yellowstone National Park. Changes in these deposit layers, including ripple structures, suggest that there were time breaks, long enough for snow fall to occur (although the event started when there was no snow on the ground) and weather systems to pass through, whipping up the ash and redistributing it.

The later deposits consist of three large ignimbrite units, labeled A (the biggest and oldest), B and C (the youngest). Although each unit consists of a single body of material and was probably deposited within days, the boundaries between the units show that there were time breaks between them.

Between units A and B, there was some cooling, and water that gained access into the hot deposits (700-850 °C) flashed to steam, causing explosions that laid down crystal-rich layers along the contact. This probably represents a period of some weeks to a few months. Between units B and C, there was more time for cooling, so that the base of the hot C flows chilled against the top of B. However, the underlying A plus B layers were still giving off hot gases—fumarolic activity—that altered the base of unit C. Based on historical observations from the 20th Century’s largest eruption (Novarupta, Alaska, in 1912), we can roughly estimate this time break. The ignimbrite from that eruption is similar in thickness and temperature to parts of the HRT, and hot, powerful fumarolic activity lasted only about 20 years (this is how the “Valley of 10,000 Smokes” got its name). In the HRT deposits, this time break is thus thought to have been some years to a few decades.

Geologic evidence therefore indicates that this enormous eruption took its time. The initial fall deposits represent activity that was hesitant, stopping and starting a number of times over probably several weeks. Even when the vast volumes of material were being erupted to generate the ignimbrite units and their associated widespread fall deposits, the eruption stopped twice, both times for periods that, although geologically negligible, would have been highly relevant to human interests, through impacts by repeated hazards and interruptions to recovery efforts. Indeed, if humans had been present to witness the HRT eruption, they likely would have considered it to be multiple smaller eruptions, given the months to decades between major eruptive events.

These findings change the way we think about the massive Yellowstone explosions—rather than single large events, they may be composed of multiple smaller events, and this would have significant implications for our understanding of these eruptions and their impact on the landscape. The Lava Creek Tuff (LCT), which formed during the most recent major caldera-forming eruption 631,000 years ago, also has multiple units. Like the HRT, the LCT may therefore have also have occurred over years, instead of all at once. Geologic studies are underway to investigate this fascinating and important question!

The contact between Huckleberry Ridge Tuff ignimbrite members A and B
The contact (red arrow) between Huckleberry Ridge Tuff ignimbrite members A and B is marked by a time break of probably weeks to a month or so.
Contact between Huckleberry Ridge Tuff ignimbrite members B and C
The contact (red arrow) between Huckleberry Ridge Tuff ignimbrite members B and C is marked by a time break of probably years to a few decades.

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