Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is led by Kenneth Befus, professor in the Department of Earth and Planetary Sciences at the University of Texas at Austin.

When Yellowstone erupts, crystals (mineral grains) that grew below ground in the magma chamber are brought to the surface. Those crystals have been used for the past century to better understand how magmas form and then erupt at Yellowstone. Imperfections in those crystals may also be used to understand volcanic processes that happen at the surface. The faces of volcanic crystals are sometimes marked by almost-microscopic holes that penetrate into the crystal interior. These holes are called embayments, and they look a little like a map of a bay along a coastline. Although some embayments may be empty holes, they are more commonly filled with glass produced by rapidly cooling magma. 

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Glass-filled embayments have been used to estimate how rapidly magma rises to the surface during an eruption. As magma rises, the host crystal partially shields melt held in the embayment from creating bubbles. In this way, volatiles, like dissolved water and carbon dioxide, have concentrations that are elevated in the embayment but that decrease towards the embayment mouth at the outer edge of the crystal. When the crystal reaches the surface in an eruption, the melt in the embayment quenches to glass, and that preserves the volatiles. Measuring how the volatile concentration varies within an embayment can provide information about how fast magma ascended to feed an eruption. 

A team of geologists studied embayments that were found in  quartz crystals that erupted 1.3 million years ago in the Mesa Falls Tuff, which formed Henrys Fork Caldera, in southeast Idaho, just west of Yellowstone National Park. Samples were collected from a 1-meter (3-foot) deposit of ash fall that is directly overlain by about 10 meters (33 feet) of ash that formed from hot pyroclastic flows.  

The water content in glass embayments was determined using a technique called Fourier Transform Infrared spectroscopy (FTIR) with equipment at Lawrence-Berkeley National Lab in California. The exceptional resolution of the instrument resulted in maps of water content with one-of-a-kind detail—down to a few micrometers (about 0.0001 inch)! Unexpectedly, water concentrations in Mesa Falls embayments increase towards the embayment mouths—from 1.0 weight percent (wt.%) in the interior to about 2 wt.% close to the embayment mouth. 

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The pronounced increase in water and its molecular composition indicate that meteoric water—which comes from precipitation (rain and snow)—is somehow part of the embayment. This is unexpected, because the embayment should just host water that was dissolved in the magma, and an influence from rainwater had never previously been described. After some initial confusion, the geologists studying the embayments realized that they record something that happened after the crystals were erupted. 

Decades ago, archaeologists discovered that volcanic glass, like obsidian, hydrates when it is exposed to moisture. The thickness of ‘hydration rinds’ on obsidian artifacts can be used to determine the age that an obsidian artifact was buried. Using the same principles, geoscientists recognized that hydration of glasses from past volcanic eruptions can be used to reconstruct geologic processes related to climate, hydrology, topography, faulting, and even volcanoes.

In the case of the Mesa Falls eruption, geologists used the hydration rinds in the quartz crystal embayments to understand what the region looked like after the eruption occurred. Heat from the cooling Mesa Falls ash flow created a high-temperature hydrothermal (hot water) system that was active for a few decades. The hot water circulated through the ash flow and the underlying ash fall, and some of the water was absorbed into the crystal embayments. The temperature of the ash flow must have been around 500 °C (930 °F) for this to happen. 

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Similar year-to-decade hydrothermal systems produced by pyroclastic flow deposits have been scientifically observed and photographed at Pinatubo in the Philippines after its 1991 eruption, and at Mount St. Helens in Washington State following its 1980 eruption. The most analogous system, however, may be in Alaska. A scientific expedition first reached the area of the Novarupta in 1916, 4 years after its massive eruption caused the collapse of Katmai volcano and formed a caldera that looks much like Crater Lake in Oregon. The expedition christened a nearby valley as the Valley of Ten Thousand Smokes because "the whole valley as far as the eye could reach was full of hundreds, no thousands – literally tens of thousands – of smokes curling up from the fissured floor". The "smokes" that the expedition described were the surface manifestation of the hydrothermal system produced by depositing a hot pyroclastic flow across a wet, cold landscape.

The water enrichments preserved in the Mesa Falls quartz-hosted embayments allow us to look back 1.3 million years and see Yellowstone as a barren, steaming landscape, perhaps much like the Valley of Ten Thousand Smokes in Alaska appeared in 1916. Granted, Yellowstone is famous today for its modern hydrothermal system highlighted by Old Faithful and the geyser basins. Although magnificent, those systems are isolated and limited by comparison to what might have existed after the Mesa Falls eruption—a Yellowstone landscape blanketed with gray ash, with the only break in that monotonous gray landscape being thousands, tens of thousands, or maybe even millions of "smokes."

More information about this research can be found in the following publication: Befus, K. S., Thompson, J. O., Allison, C. M., Ruefer, A. C., and Manga, M. (2024). Rehydrated glass embayments record the cooling of a Yellowstone ignimbrite. Geology, v. 52, no. 7, p. 507–511, https://doi.org/10.1130/G51905.1.