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November 20, 2023

Near the south boundary of Yellowstone National Park is a curious ash flow deposit of unknown origin. Geologists from Montana State University took on the challenge of solving the mystery.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Stacy Henderson, Ph.D. student in the Department of Earth Sciences at Montana State University.

Super eruptions, such as the two that have occurred in the Yellowstone Plateau Volcanic Field (YPVF) over the past ~2 million years, are now known to have much more complicated eruptive dynamics than previously thought. For example, the older 2.08 million year old Huckleberry Ridge eruption was not a single event, but had pauses in explosive activity that spanned years to decades, suggesting multiple eruptive events rather than a single large eruption. The Lava Creek Tuff (LCT), associated with the formation of Yellowstone Caldera 631,000 years ago, also has characteristics that indicate a complex eruptive event rather than a single large explosion. But how do we study these kinds of time breaks within an eruptive sequence?

Flagg Ranch exposure of the Lava Creek Tuff
Flagg Ranch exposure showing lower unknown ignimbrite (ash flow), the Lava Creek Tuff (LCT) ash fall deposit, and upper ignimbrite of the LCT. Thanks to work by Montana State University geologists, it is now known that the unknown ignimbrite was deposited immediately prior to the ash fall, with no significant time gap in between. Photo by Stacy Henderson (PhD student in Montana State University’s MOnSTER lab).

To explore this question, let’s take a trip to a well-studied exposure of the LCT that lies along the Snake River near Flagg Ranch in Grand Teton National Park, just south of the Yellowstone National Park boundary. This outcrop contains one of very few exposed ash-fall deposits of the LCT—that is a deposit formed by ash that settles from a tall eruptive plume, rather than one that sweeps across the landscape as a flow. The ash-fall unit is capped by an ash-flow deposit, called an ignimbrite, of the LCT eruption but overlies an ignimbrite of “unknown origin.” What is this lower “ignimbrite of unknown origin” and how do we determine where and when it came from?

To reconstruct the pace at which eruptive material was emplaced, volcanologists utilize the rock record, looking both at the physical properties of the deposit, as well as the minerals it contains. Two minerals geologists use to date the age of a deposit are sanidine (a high-temperature potassium feldspar) and zircon (a small mineral containing uranium). However, even the most precise of these dating techniques will have errors of 1,000–5,000 years. If we want to resolve anything less than this, we need to first look for clues within the deposit.

The contact between the lower ignimbrite and the ash fall can reveal important characteristics about the timing of their deposition. Some of the evidence geologists look for include:

  1. Erosion on the upper surface of this lower ignimbrite. Erosion can take the form of small gullies, preserved raindrops, or scouring of this surface, suggesting weeks of time between the emplacement of the ignimbrite and later ash-fall bed.
  2. Soil development between the upper ignimbrite and ash-fall deposit, which can indicate a time gap between the two deposits of hundreds of years.
  3. Evidence of water interaction with the still-cooling ignimbrite, suggesting a time gap of days to weeks.
  4. A chilled bottom surface of the ash-fall deposit overlaying lower ignimbrite, which would indicate that the ignimbrite had cooled completely before the ash fall occurred.
Flagg Ranch exposure of the Lava Creek Tuff with quartz-hosted melt inclusions
Photo of ash-fall deposit of the Lava Creek Tuff overlying the lower “unknown” ignimbrite (an ash-flow unit, denoted by light yellow box).  Images of quartz-hosted melt inclusions show fully “baked” (dark) melt inclusions in the lowest ash fall layers (1–2). Layers 3–6 contain glassy melt inclusions (clear in appearance), while layers 7 and 8 only have predominantly “baked” melt inclusions.  The baked nature of the melt inclusions in the lower layers indicate that the ash fall occurred while the underlying “unknown” ignimbrite was still hot and that there was no significant time gap between the two events. Photo of deposit by Stacy Henderson (Ph.D. student in Montana State University’s MOnSTER lab). Melt inclusion photos courtesy of Bryana McKay (M.S. student in Montana State University’s MOnSTER Lab).

In the case of the mysterious “ignimbrite of unknown origin” at Flagg Ranch, there is no erosional surface, no soil development, no evidence for water interaction, and no chilled base of the ash fall between the fall deposit and the lower tuff. This is a first indication of there being no significant time breaks between the two deposits and suggests that the lower ignimbrite was emplaced during an early phase of the LCT eruption.

Another way for addressing time breaks is to look at the cooling history of the deposit, which can be explored through investigation of little blobs of melt found encased within quartz minerals. During rapid cooling, these “melt inclusions” will turn to clear glass, but they are susceptible to heat and will “bake” and turn black if in contact with something hot.

Geologists from Montana State University sampled the ash-fall deposit from the base to the top every 10 cm, ending up with 8 samples representing the entire unit. Quartz grains were then picked from each layer, and the melt inclusions were investigated. Much to M.S. student Bry McKay’s surprise, the quartz-hosted melt inclusions in the fall deposit nearest the lower and upper ignimbrite contact were obviously baked (being dark in appearance), but the ones near the center were clear. This indicates that the fall was deposited over a still hot ignimbrite deposit, indicating no significant time breaks between deposition of the two. Thus, we now know that these units of the LCT were deposited in quick succession—quite a difference from the older Huckleberry Ridge Tuff, when there might have been decades between events that deposited ash across the landscape.

Another geological mystery solved! One can imagine the previously unknown ignimbrite exclaiming, “If it weren’t for those meddling geologists I would have remained unknown!”  All in a day’s work for a team of MSU volcanologists.

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