Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week’s contribution is from Lauren Harrison, postdoctoral researcher with the U.S. Geological Survey.

Yellowstone is a place with a long and varied geologic history—a story told in the layers of rocks found throughout the Park. Although most attention focuses on Yellowstone’s most recent and better exposed Quaternary (<2.58 million years old) rocks associated with the current magmatic system, some rocks in Yellowstone are billions of years older. These old rocks record a time when Yellowstone was close to an ocean with an active subduction zone, where oceanic crust collides with and descends beneath the continental crust, much like what is happening off the coast of the Pacific Northwest USA today. Subduction zones create linear chains of volcanoes like the Cascade Range, which runs from northern California into British Columbia. Let's turn back the clock to have a closer look at these times long gone by in the Yellowstone region.

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Geologic time is split into a number of different subgroups.  The largest blocks of geologic time are the eons, of which there are four—from oldest to youngest, the Hadean, Archean, Proterozoic, and Phanerozoic. Each eon is then split into different eras. For example, the Phanerozoic eon is made up of, from oldest to youngest, the Paleozoic, Mesozoic, and Cenozoic eras. Eras are split into periods. The Cenozoic era includes, from oldest to youngest, the Paleogene, Neogene, and Quaternary periods. Finally, the periods are divided into epochs. The Quaternary period, for example, is made up of the older Pleistocene and younger Holocene epochs.

The Archean eon lasted from about 4 billion to 2.5 billion years ago. A very large global magmatic event took place from 2.9 to 2.7 billion years ago, with massive outpourings of lava creating many volcanic arcs and oceanic plateaus that are now preserved in the geologic record. One place where evidence of this very old magmatic event shows up is in northern Yellowstone National Park!

In Yellowstone, the Hellroaring and Crevice Plutons—large coherent bodies of intrusive rock—are Archean in age and located between Gardiner and Cooke City, MT, along the northern boundary of the park. Both plutons have been dated using the known rate of decay of radioactive uranium to a stable isotope of lead in a very resistant mineral, zircon. The age of the zircon reveals when the magma solidified—in this case, about 2.8 billion years ago!

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The geochemical compositions of the plutons also tell a story about the conditions at the time of their formation. They are felsic (in other words, light-colored) plutons with very high amounts of SiO₂ (silica), which indicates these rocks are the unerupted, crystalline equivalent of rhyolite, the same rock type that erupted during and after the formation of Yellowstone caldera about 631,000 years ago. However, the trace element chemistry of these old plutons is very different from recent rhyolites, even though both rocks were formed from melting of the continental crust. The plutons have depletions in elements such as zirconium, niobium, and phosphorous that indicate they formed from magmas generated by fluids derived from a subduction zone. Conversely, recent Yellowstone rhyolites lack this chemical fingerprint because there is no close subduction zone to contribute these distinctive fluids in recent geologic time.

The Hellroaring and Crevice Plutons formed in a chunk of old, cold crust called the Wyoming Province. These chunks of crust have a specific name-craton, which is the Greek word for "strength." This is an apt name, as these are long-lived and stable pieces of the continental crust that have survived many rounds of continent formation and breakup over Earth's history. They are also twice as thick as the surrounding crust and are composed of rocks that are the least dense and most viscous on Earth. These properties make it easy to visualize these cratons as "ships" with deep keels that "float" on the mass of surrounding rock and resist the tectonic stresses that may destroy them. Amazingly, the Hellroaring and Crevice Plutons were a part of one of the last crust-building episodes that constructed the northern Wyoming Province, contributing to the strength that has allowed this unique chunk of ancient rock to survive into modern time.

Next time you're hiking around northern Yellowstone, keep your eye out for coarse-grained crystalline igneous rocks--you may just be looking at a rock that formed billions of years ago.

Further Reading

Philbrick, Kate, Ware, Bryant, Henry, Darrell, Mogk, David, Mueller, Paul A., Foster, David A. (2011). Evolution of the Precambrian rocks of Yellowstone National Park (YNP): Late Archean Felsic Plutons. Joint Rocky Mountain and Cordilleran Meeting Geological Society of America,  Abstracts with Programs. Vol. 43, No. 4, p.62. 

Mogk, David, W., Frost, Carol D., Mueller, Paul A., Frost, B. Ronald, Henry, Darrell J. (2023) Crustal genesis and evolution of the Archean Wyoming Province: Continental growth through vertical magmatic and horizontal tectonic processes. In: Laurentia: Turning Points in the Evolution of a Continent, Eds:  Whitmeyer, Steven J., Williams, Michael L., Kellett, Dawn A., Tikoff, Basil. Geological Society of America Memoir vol 220, https://doi.org/10.1130/MWR220.&nbsp;