Buried calderas on the track of the Yellowstone hotspot
The track of the Yellowstone hotspot is defined by a series of old caldera systems that get older the farther to the southwest you get from Yellowstone. Because they are mostly buried, it took decades of geologic investigations to identify these features.
Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Lisa Morgan, emeritus research geologist with the U.S. Geological Survey.
The youngest part of the Yellowstone hotspot left a trail of volcanic products that begins in northern Nevada and southern Oregon about 17 million years ago, with the volcanic rocks becoming younger as the track stretches across southern Idaho to Yellowstone National Park. The volcanic rocks are the products of ancient eruptions from systems that looked very much like Yellowstone Caldera does today. These “ancient Yellowstones” are now buried, so how were they recognized by geologists? How was the Yellowstone hotspot track identified?
Like most scientific advances, the answer builds on a long history of previous investigations—in this case, geologic mapping along the margins of the Snake River Plain (SRP), a volcanic province that is covered by a relatively thin veneer of basalt (which usually erupts as effusive lava flows, like those in Hawaiʻi, and is low in silica content) above a thick sequence of rhyolite (commonly an explosive volcanic product with high silica content). Geologists who worked in the area during the early part of the 20th century mapped extensive rhyolite deposits along the margins of the SRP but did not suggest an origin for these high-silica rocks. In fact, early explorers to Yellowstone itself in the 1870s, as well as the indigenous people that lived in the area for thousands of years, recognized that the area was a great volcanic system. But the questions of how and whether Yellowstone and the eastern SRP were related and what the relationship was between the basaltic and rhyolitic rocks were not pursued until the latter part of the 20th century.
In the early and mid-1970’s, landmark scientific studies suggested the existence of “hotspots”—mostly stationary regions of melting within the Earth that leave trails of volcanoes as the tectonic plates that make up Earth’s surface move over a thermal anomaly that melts through the plate. Hawaiʻi is perhaps the best-known example. These studies suggested that Yellowstone was a hotspot due to the general northeast-trending age progression of volcanic rocks extending along the SRP to Yellowstone. The first real physical proof of large rhyolitic calderas buried in the SRP, however, was from the 1979 INEL-1 borehole, at 3,160 m (10,365 ft), the deepest borehole drilled to date in the entire SRP. The young geologists (including the author of this Caldera Chronicles article!) who were logging the core from the borehole were told to expect a little basalt, some lake sediments, some rhyolite, and then a thick section of Paleozoic sediments that were hundreds of millions of years old. This sequence was assumed to exist given the proximity of the borehole to the front of the Lost River and Lemhi Ranges on the northeastern SRP.
In fact, the upper 756 m (2,480 ft) of INEL-1 contained a sequence of basaltic lava flows along with river, lake, and volcanic sediments, but below that sequence the borehole stayed in only rhyolitic rock units all the way to the bottom. Paleozoic rocks were never encountered. This was a totally unexpected and significant result, and the borehole demonstrated that rhyolite was a significant geologic unit making up the SRP. Some of the rhyolite rocks in the INEL-1 borehole later were correlated to rhyolites along the northern and southern margins of the eastern SRP, ultimately leading to the recognition of caldera-forming eruptions as the source of the rocks.
By the late 1980’s, much work on rhyolites along the length of the SRP had been completed and the story had taken shape. But geologists continued to build on the story into the 1990s, recognizing that faulting and uplift on the SRP also were related to the age progression of volcanism. Volcanic fields containing large, nested calderas, like those that had been mapped in Yellowstone National Park in the 1960s and 1970s, also were recognized. Volcanism, faulting, and regional uplift were identified as resulting from the deep-seated mantle plume that feeds the Yellowstone hotspot.
Identification of the mostly buried calderas was not based on aerial photographs, but rather on detailed geologic mapping, gravity anomalies that indicate the density of rocks beneath the surface, and, especially, on detailed analyses of the rock units that make up the SRP. The rock analyses were more than just physical examinations. Geologists measured the detailed chemistry of the rocks, their mineralogy, their ages, and even their magnetic properties to be able to correlate the rock units across the 90–100 km (55-60 mi) wide SRP and to determine their point of origin. The results demonstrated that these are some of Earth’s largest explosive rhyolitic deposits. And that work continues to this day, with improved understanding of individual caldera eruptions!
Even with all the technological advances that have occurred over recent decades, including photos not just from the air, but from space, geologic mapping and a broad variety of analyses of rocks remain the best tools for understanding the geologic history of a region. And geologists remain hard at work refining our understanding of the Yellowstone system, including the trail it left as it crossed southern Idaho over the past 17 million years!
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If you would like to learn more about the recognition of the calderas of the Snake River Plain and the Yellowstone hotspot system, check out the following scientific articles:
Armstrong, R.L., Leeman, W.P., and Malde, H.E., 1975, K-Ar dating, Quaternary and Neogene volcanic rocks of the Snake River Plain, Idaho: American Journal of Science, v. 275, 225–251, https://doi.org/10.2475/ajs.275.3.225.
Camp, V.E. and Wells, R.E., 2021, The case for a long-lived and robust hotspot: GSA Today, v. 31, 4–10, https://doi.org/10.1130/GSATG477A.1.
Christiansen, R.L., 2001, The Quaternary and Pliocene Yellowstone Plateau volcanic field of Wyoming, Idaho, and Montana: U.S. Geological Survey Professional Paper 729-G, 145 pp.
Morgan, W. Jason, 1972, Plate motions and deep mantle convection, Geological Society of America Memoir 132, 7–22, https://doi.org/10.1130/MEM132-p7.
Morgan, L.A. and McIntosh, W.C., 2005, Timing and development of the Heise volcanic field, Snake River Plain, Idaho, western US: Geological Bulletin of America, v. 117, 288–306, https://doi.org/10.1130/B25519.1.
Morgan, L.A., Doherty, D.J., and Leeman, W.P., 1984, Ignimbrites of the eastern Snake River Plain: Evidence for major caldera-forming eruptions: Journal of Geophysical Research, v. 89, 8665–8678, https://doi.org/10.1029/JB089iB10p08665.
Myers, M.L., Wallace, P.J., Wilson, C.J.N., Morter, B.K., and Swallow, E.J., 2016, Prolonged ascent and episodic venting of discrete magma batches at the onset of the Huckleberry Ridge supereruption, Yellowstone: Earth and Planetary Science Letters, v. 451, 285–297, https://doi.org/10.1016/j.epsl.2016.07.023.
Nelson, P.L. and Grand, S.P., 2018, Lower-mantle plume beneath the Yellowstone hotspot revealed by core waves: Nature Geoscience, v. 11, 280–284, https://doi.org/10.1038/s41561-018-0075-y.
Pierce, K.L. and Morgan, L.A., 1990, The track of the Yellowstone hotspot: Volcanism, Faulting, and Uplift: U.S. Geological Survey Open-File Rpeort 90-415, 70 p., https://doi.org/10.3133/ofr90415.
Pierce, K.L. and Morgan, L.A., 2009, Is the track of the Yellowstone hotspot driven by a deep mantle plume? — Review of volcanism, faulting, and uplift in light of new data: Journal of Volcanology and Geothermal Research, v. 188, 1–25, https://doi.org/10.1016/j.jvolgeores.2009.07.009.
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