Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Stanley Mordensky, geologist with the U.S. Geological Survey.

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Before any volcanic material could erupt at what is now Yellowstone National Park, the Yellowstone hotspot had to melt through a diverse sequence of rocks including Absaroka volcanic deposits (53 to 43 million years old), Mesozoic (251 to 65 million years old) and Paleozoic (542 to 251 million years old) sedimentary rocks, and crystalline granites and gneisses (4 to 2 billion years old). These older rocks, which were brought to the Earth’s surface by the flat-slab subduction during the Laramide Orogeny—a mountain building event about 70 to 40 million years ago—are visible throughout the region, including in the Beartooth Mountains. 

The Beartooth Mountains are renowned for allowing visitors to drive amidst alpine tundra and towering peaks soaring above 3,650 meters (12,000 feet), including Montana's highest point, Granite Peak, which reaches an elevation of 3,900 meters (12,799 feet) along the Beartooth Highway (US 212). Although the high elevation elicits many to celebrate the Beartooth Mountains as the "Top of the World", the age of the rocks in these mountains creates even greater distinction—some of these crystalline rocks constitute not just the oldest rocks of the Greater Yellowstone Ecosystem, but the second oldest rocks yet found in North America and the third oldest in the world.

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The Beartooth Mountains consist of four distinct blocks of rock: 1) the South Snowy Block; 2) the North Snowy Block; 3) the Plateau Block; and 4) the Stillwater Block. The South Snowy Block is found along the northern boundary of Yellowstone National Park. The North Snowy Block defines the northeastern side of Paradise Valley and extends east to, and is separated from, the Plateau Block by the Mill Creek-Stillwater Fault System. The Stillwater Block shares its southern boundary with the northern extent of the North Snowy and Plateau blocks. The North Snowy, Plateau, and especially South Snowy blocks are probably buried in part by younger Absaroka volcanism. 

The South Snowy and North Snowy blocks are composed of metamorphosed oceanic sedimentary deposits and igneous rocks ranging from 2.8 to 2.5 billion years old and indicate the presence of an ancient ocean basin. Much like the South Snowy and North Snowy blocks, the Plateau Block is composed of metamorphosed sedimentary rock, but the Plateau Block appears to be older in age than the Snowy blocks. At Quad Creek, where the Beartooth Highway climbs up to the Beartooth Mountains from the east, the process of radioisotope dating from zircon crystals finds that some of the zircon crystals are as old as 3.96 billion years. Only two other places in the world (western Australia and northern Canada) have ages older than this. By way of comparison, the youngest Yellowstone caldera-forming eruption is 0.016 % the age of the oldest dated zircon crystals in the Beartooth Mountains.

The Stillwater Block shares some qualities with the South Snowy, North Snowy, and Plateau Blocks, like having > 2.5 billion year old crystalline rock, but the Stillwater Block also has the notable distinction of hosting an exceptionally large volume of crystalized magma called the Stillwater Complex.  The complex intruded into the crystalline rock around 2.7 billion years ago. The magma crystallized as layers, earning a special classification as a geologic formation  known as a layered mafic intrusion. Large layered mafic intrusions often host important ore types. The Stillwater Block contains the nation’s largest reserves of platinum, palladium, and chromium. Because of its distinction of having some of the highest-grade metal ore deposits in the world, as well as containing some remarkable layered igneous features, the International Union of Geological Sciences (IUGS) designated the Stillwater Complex as a ‘key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history’, a distinction held by only 99 other locations around the world.

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That was a lot of information to take in, so let us summarize the geologic story of the Beartooth Mountains. 

1. Between about 3.96 and 3 billion years ago, erosion from ancient granites deposited sediment into ancient basins.
2. Between about 3 to 2.5 billion years ago, those sedimentary deposits were buried to great depths (> 20 km, or 12 miles), where most of the buried rock recrystallized and metamorphosed as the gneisses and granites we see today. Some of the ancient granite, like the zircon crystals, survived the metamorphism intact.
3. Over the next 2+ billion years, tectonic forces moved the recrystallized gneisses and granites as blocks of rock within the Earth’s subsurface. During this time, these blocks were subject to varying degrees of magmatic intrusions, the most significant of which created the Stillwater Complex and its high-grade ore.
4. Uplift, most recently during the Laramide Orogeny about 70 to 40 million years ago, eventually raised and repositioned these blocks of crystalline rock to their present configuration.

Most recently, the Bull Lake (about 150,000 years ago) and Pinedale (20,000–15,000 years ago) glaciation events sculpted the terrain, giving us the Beartooth Mountains we recognize today.

So if you want to take a trip back in time—wayyyyyy back in time!—check out the Beartooth Mountains!

For more information on the Beartooth Plateau, check out:

• https://serc.carleton.edu/research_education/mt_geoheritage/sites/Beartooth_Mountains.html 
• https://iugs-geoheritage.org/publications-dl/IUGS-FIRST-100-SITES-WEB-BOOK.pdf (see sites 45 and 46 in this book)
• https://www.nps.gov/yell/learn/upload/YS_20_2.pdf ("Origins of a Continent" article)
• https://guidebooks.americangeosciences.org/vufind/Record/2025017787