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Eruption unknown: The case of Yellowstone National Park's newly recognized geologic unit • Idaho Capital Sun [1]
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Date: 2025-07-12
Yellowstone’s geological history is one of the most intensely studied in North America.
From a map drawn on a bison pelt by Native American tribes to Bob Christiansen’s 1:125,000 scale map published in 2001, the resolution of Yellowstone maps only continues to evolve. Although these maps — especially those that show the geology — have created a solid foundation for tourists and researchers alike, many remote or poorly exposed areas of Yellowstone have been generalized at broad scales, now requiring geologists to revisit these regions.
Their goal? To improve our understanding of all rocks that make up Yellowstone’s geologic history. But what happens when a rock previously mapped as a younger eruption is dated using new techniques and returns an age that doesn’t match anything in the geologic record? This is exactly what happened recently, when Montana State University geologists identified a new geological unit in the west part of Yellowstone National Park.
Yellowstone’s landscape tells a story written in ash, lava and time. Scientists have worked to understand that story by mapping volcanic rocks and using tools like argon dating on individual minerals to determine when eruptions occurred.
Our current maps have identified three volumetrically major explosive eruptions separated by periods of basaltic lava flow activity and slower, thicker rhyolitic lava flows. One such group of lava flows, primarily located in the Madison Junction area, is known as the Mount Jackson Rhyolite Series, which is dated between 1.28 million and 700,000 years ago. This is the time period between the formation of the Henrys Fork Caldera (1.3 million years ago) and Yellowstone Caldera (631,000 years ago).
Until recently, the Mount Jackson Rhyolite Series was believed to consist of non-explosive, effusive rhyolite lava flows; however, remapping in the area revealed a previously unrecognized volcanic unit. This unit — a dense, black, glassy rock — had been mapped as part of the Lava Creek Tuff (LCT), Yellowstone’s youngest super eruption, which occurred around 631,000 years ago. Yet argon dating on this glassy rock returned an age of 780,000 years, making it older than the LCT and placing it within the Mount Jackson Rhyolite Series.
Although not surprising — geologists often find more eruptive diversity within known units — this specific rock suggests that the Mount Jackson Rhyolite Series is not just composed of oozy lava flows.
So, what exactly is this mystery rock? It is a vitrophyre — a volcanic rock with a glassy matrix formed by rapid cooling. When lava or pyroclastic (ashy) material erupts, it can cool so quickly that the molten material solidifies into volcanic glass before a crystalline matrix has time to grow. In vitrophyres, phenocrysts — larger, well-formed crystals — are suspended in the glassy groundmass. These crystals grew slowly in the magma chamber before eruption, but the glass preserves the story of a rapid temperature drop during and immediately after eruption.
This vitrophyre probably represents a smaller explosive eruption associated with the Mount Jackson Rhyolite Series, different from the massive caldera-forming events, but still geologically significant. Such events might have produced ash plumes or pyroclastic flows that did not spread too far beyond the eruption source.
These combined ash and lava eruptions could have occurred more frequently in Yellowstone in the past but have gone unrecognized due to the limited resolution of older maps, burial by later eruptions, and the difficulty of preserving small amounts of ashy material in the landscape.
As scientists continue to write the story of Yellowstone’s eruptive history, remapping and dating are crucial for finding more of these overlooked units. If one previously unrecognized volcanic unit can be uncovered, how many more remain hidden in Yellowstone’s geologic record?
Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory.
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