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October 24, 2022

Young volcanic rocks in Wyoming are not limited to the area in and immediately surrounding Yellowstone Caldera. Eruptions have also occurred in the past few million years along the leading edges of the Yellowstone hotspot track!

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. 

Lava Mountain, Wyoming
Lava Mountain, Wyoming.  (A) View from Dubois, WY, in the Wind River basin looking northwest ~30 km toward Lava Mountain. Yellow line is approximate boundary between Lava Mountain eruptive units (above line) and older Eocene (34-56 million years old) rocks.  Notice how the overall shape of 8-km-wide Lava Mountain resembles a shield volcano, like those in Hawaiʻi.  Arrows mark the top and bottom of well-exposed glacially truncated 230-m-high cliff, where at least 20 lavas crop out in a flow-on-flow stratigraphic context. Photo by Matthew Brueseke, Kansas State University, June 2006, used with permission.  (B) Close-up view of Lava Mountain illustrating the capping scoria cone at the summit and the underlying lava units. These show the volcano underwent at least two phases (effusive and then explosive) of volcanism.  Photo by Matthew Brueseke, Kansas State University, August 2014, used with permission.

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. 

Map showing volcanoes that formed at the leading edge of the Yellowstone hotspot in the past several million years
Map showing volcanoes that formed at the leading edge of the Yellowstone hotspot in the past several million years.  (A) Map of northwest Wyoming, eastern Idaho, and southern Montana (modified from Brueseke et al., 2017,  Upper Wind River Basin (UWRB) is depicted by dashed blue rectangle. Black dots denote igneous rocks that are less than 10 million years old on the periphery of the Snake River Plain (dashed ovals where HS=Heise volcanic field, YS=Yellowstone Plateau volcanic field; numbers are ages in millions of years when these volcanic fields were active).  Red lines denote the width of the ‘seismic parabola’ (an area of high seismicity) associated with the Yellowstone hotspot’s current location and the Yellowstone Crescent of High Terrain; JH=Jackson Hole, WRR=Wind River range, TR=Teton Range, CV=Centennial Valley.  (B) Google Earth satellite image of UWRB study area depicting identified volcanoes and the extent of studied UWRB igneous rocks. P=Pilot Knob, LM=Lava Mountain, CM=Crescent Mountain, RT=Raggedtop Mountain, HB=Hey Bear volcano, SM=Spring Mountain.

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!  

“Hey Bear” volcano, Wyoming, with Raggedtop Mountain in the background
“Hey Bear” volcano, Wyoming, with Raggedtop Mountain in the background. Hey Bear volcano is a small scoria cone that is about 1.3 million years old and that erupted over Eocene (~50 million-year-old) rocks (noted by yellow dotted line). Photo by Matthew Brueseke, Kansas State University, August 2021, used with permission.

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.

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