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Rock sampling

Montana State University students at an outcrop along Highway 20 in Idaho sample the Mesa Falls Tuff fall deposit exposed just beneath the ignimbrite.

During the spring term in 2019, geology students from Montana State University participated in a reading group focused on understanding the geology of the Yellowstone hotspot. The culmination of the class was a field trip into Yellowstone National Park to see first-hand the deposits that had been discussed throughout the semester. The goal of this field trip was to gain a better understanding of the three largest caldera-forming eruptions and the overall progression of the Yellowstone hotspot, which has shaped the park over the past two million years.

The trip focused on ignimbrites, lava flows and fall deposits — the major types of volcanic deposits found in and around Yellowstone National Park. An ignimbrite is formed by a pyroclastic flow, which is a mixture of hot gas, ash and rocks. In contrast to these quick moving flows (which can move at speeds of several hundred miles per hour), rhyolite lava deposits are thick, slow moving flows that make up many of the hillslopes near Grand Prismatic Spring. Finally, a fall deposit is a mix of pumice, ash and crystals that is typically laid down during the earliest phase of an explosive eruption, prior to pyroclastic flows.

When exploring the Yellowstone area, you may not realize there are several calderas (depressions created through the evacuation of magma) forming the complex geology within the region. An eruption that occurred 2.1 million years ago deposited the Huckleberry Ridge Tuff and formed a giant 58 by 37 mile depression, which is the largest of the three calderas. Subsequent eruptions produced the Mesa Falls Tuff around 1.3 million years ago, creating the Henry’s Fork Caldera (west of the park), and the Lava Creek Tuff around 0.63 million years ago, which formed the Yellowstone Caldera; this is the caldera which currently makes up the central portion of the Yellowstone National Park.

Upon arrival in Mammoth, the Montana State University students summited Mt. Everts, where a spectacular outcrop of Huckleberry Ridge Tuff is exposed. Students distinguished two types of deposits—an ignimbrite and a fall deposit—by mineral composition, grain size, color, hardness and depositional structures. The depositional structures preserved in the fall deposit provide evidence of reworking by wind and water, whilst the contrasting colors throughout the unit indicate differences in grain size. There are also signs of post-depositional heating from above caused by the emplacement of the high-temperature ignimbrite on top of the fall deposit.

The next stop was Sheepeater Cliff—an outstanding example of columnar basalt from an eruption about 108,000 years ago. As students traveled further down the trail along the Gardner River they encountered the Lava Creek Tuff, a thick ignimbrite that is distributed throughout the park. The Lava Creek Tuff is easily identifiable around Gibbon Falls and Tuff Cliff by its pinkish-grey color.

Leaving Yellowstone National Park, the group entered Idaho and explored the Henry’s Fork Caldera—one of the smallest caldera-forming eruptions of the Yellowstone hotspot. Along the Mesa Falls Scenic Bypass, the rim of the caldera can be observed to the west, and outcrops exposing the pink-colored Mesa Falls Tuff and fall deposit are found along Highway 20.

The diversity of the volcanic features of Yellowstone National Park provides a rich landscape of educational benefits for students. Through identifying the three major ignimbrites in and around the park, the students were able to develop observational skills for recognizing different types of volcanic rocks. Comparing the deposits from different time periods highlighted both the similarities between the deposits from each caldera forming eruption, as well as their unique characteristics. Yellowstone is truly a volcanological wonderland and outstanding outdoor classroom for amateur and professional geologists alike!

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Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory.

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