Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Matt Brueseke, Professor in the Department of Geology at Kansas State University.

Many of us who visit Yellowstone and Grand Teton National Parks drive through (and stay in) Dubois, Wyoming, on U.S. Highway 26/287.  Given the ongoing impacts from the June 2022 flooding in Yellowstone, it is likely that even more people than in a typical summer drove this route.  The highway cuts through the Wind River basin west of Dubois and passes just north of Lava Mountain, which looms above the road.  Lava Mountain is aptly named—it is an ~400 thousand-year-old shield volcano, comprised of numerous lavas (scoured and exposed by younger glacial activity) and a capping scoria cone. The volcano is a great example of monogenetic volcanism—an eruption, or series of related eruptions from one volcano, characterized by small volumes of magma that ascend and erupt quickly, and once the eruption is over, the volcano does not erupt again.  Monogenetic volcanoes usually form as a cluster or group of many separate volcanoes scattered over an area. For example, many of the young basalt volcanoes on the eastern Snake River Plain in Idaho, like the Hells Half Acre lava field, are monogenetic. 

Sources/Usage: Some content may have restrictions. View Media Details

Even though many of the lavas on Lava Mountain are dark-colored, the volcano is not actually made of basalt—it erupted mostly andesite magma (which is slightly higher in SiO₂ than basalt).  

Lava Mountain is not alone.  Near Dubois, around the northern flank of the upper Wind River basin, are other monogenetic volcanoes, associated lavas, and intrusive bodies (magma that ascended toward the surface but solidified underground and did not erupt) that are also all less than 5 million years old.  Together, these lavas, intrusives, and volcanoes make up the Upper Wind River Basin volcanic field (UWRB).  These include Crescent Mountain, Spring Mountain, Pilot Knob, Raggedtop Mountain, and “Hey Bear” volcano.  In all of these locations, the relatively young volcanoes and lavas sit above 50–48 million-year-old (the Eocene geologic epoch) mudflows, lavas, and other strata from the Absaroka volcanic province, which extends from the UWRB northwest into Montana. 

Sources/Usage: Some content may have restrictions. View Media Details

Many of these UWRB volcanic rocks were identified by geologist J.D. Love and colleagues from the U.S. Geological Survey in the 1940–70’s during regional geological mapping. More detailed mapping by geologists from Kansas State University confirmed some of this earlier mapping, identified previously unmapped UWRB volcanics, and discovered that some of the rocks previously mapped as less than 10 million years old are actually Eocene!  

Sources/Usage: Some content may have restrictions. View Media Details

So what else do we know about UWRB magmatism? Well, the volcanoes are spatially and temporally associated with the current location of the Yellowstone Crescent of High Terrain, a zone of faulting, deformation, and doming uplift connected to the North American plate’s passage over the Yellowstone hotspot.  This zone of earthquakes and faulting formed as the hotspot interacted with the overlying North American plate, prior to and during, major caldera-forming eruptions along the Snake River Plain, creating the Yellowstone hotspot track.  In the UWRB, magmas erupted along mapped faults and fault zones in the Yellowstone Crescent of High Terrain. The chemical compositions of UWRB lavas and intrusives, as well as slightly older lavas around Jackson Hole, WY, however, resemble older Absaroka volcanic rocks and equally young volcanics of the Leucite Hills, WY (down near Rock Springs, WY) rather than similarly aged Snake River Plain-Yellowstone volcanism.  This difference probably reflects a different source in Earth’s mantle for UWRB magmas.  In this scenario, the Yellowstone hotspot provided the heat to melt the ancient mantle under northwest Wyoming, forming UWRB magmas. Then, the magmas ascended into the crust and sometimes erupted, forming monogenetic volcanoes like Lava Mountain and “Hey Bear” cone.  Faults associated with the Yellowstone Crescent of High Terrain aided the magma ascent.  The implication of this research is that geologically young volcanism in northwest Wyoming is not just limited to the Yellowstone Plateau Volcanic Field, as is often assumed, but also occurs on the margins of this area!

Geologists are continuing their exploration of these fascinating volcanoes, so stay tuned for more results.