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The Timber Hill basalt tells a story of bygone days, when southwestern Montana looked very different and was a site of vigorous volcanic eruptions.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Jesse Mosolf, geologist with the Montana Bureau of Mines and Geology.

Every summer, gaggles of geology students congregate in the Sweetwater Hills southeast of Dillon, Montana, to study and map the Timber Hill basalt as part of their college capstone courses. The Timber Hill basalt is an ancient lava flow that now forms a prominent mesa in the area, standing high above Sweetwater Canyon where the road passes beneath towering cliffs of dark basalt near the Springbrook Creek junction. Here, students are given the opportunity to investigate the volcanology of the Timber Hill basalt and to ponder its origin. After all, southwest Montana isn’t necessarily known for its volcanoes.

Map of Timber Hill basalt in the context of the Yellowstone hotspot track
Map of the northwestern U.S., showing locations of the Yellowstone hotspot volcanic fields (orange) and Columbia River Basalts (gray).  Boundary of Yellowstone National Park is yellow.  Inset shows physiographic map of southwest Montana and central Idaho.  Major extensional faults are shown by black lines with balls-and-bars on down dropped block.  The Heise Volcanic Field and outcrops of the Timber Hill basalt are shown in red.  The interpreted flow path of the basalt is shown by the orange arrow.  Base figure by Zack Lifton (Idaho Geological Survey), with physiographic map by Jesse Mosolf (Montana Bureau of Mines and Geology).

How do geologists know that the Timber Hill basalt was born from an ancient lava flow? Vesicles are the first clue—they give the rock a porous texture. Magma is full of gas (mostly carbon dioxide and water), and when it reaches the surface and erupts as a lava flow it experiences a tremendous pressure drop, which causes the dissolved gases to “exsolve” and form bubbles. Trapped bubbles are “frozen” in the lava as it cools. The basalt also contains columnar joints (cooling cracks) that indicate a hot rock that has cooled and contracted. When the basalt is broken open with a hammer, a fresh surface reveals tiny crystals of olivine, pyroxene, and plagioclase minerals set in a dark, fine-grained matrix. This crystal assemblage is common in basalt, and geochemical data from the lab confirm a composition that is relatively low in silica and high in magnesium and iron—also indicative of basalt. Collectively, these characteristics suggest the Timber Hill basalt was an extrusive lava that erupted and flowed downhill. The lava was much thinner than the thick, pasty rhyolite lava flows you can find within Yellowstone Caldera, but not so dissimilar from the basaltic lava flows that are found within the older calderas of the Yellowstone system.

But if the lava flowed downhill, why is it currently residing 500 feet above Sweetwater Creek? Geologists have used modern radiometric dating techniques to determine that the Timber Hill basalt erupted 6 million years ago. The landscape in southwest Montana looked very different at that time, and the lava likely flowed down an ancient river valley, called a paleovalley in geologist speak. The lava flowed over loose stream sediments of the Sixmile Creek Formation that had accumulated in the bottom of the paleovalley, which itself was cut into the basement rock. The basalt is much more resistant compared to the underlying sediments and basement rock, and so the basalt protected the underlying rock units from erosion. Eventually, the topography became inverted, creating a mesa that now stands high above the modern stream drainage.

Geologic map of the Timber Hill basalt Sweetwater Hills, Montana
Simplified geologic map of the Timber Hill basalt and underlying geology in the Sweetwater Hills. The Sweetwater Road is shown by red. Note the basalt flow generally rests on poorly cemented sediments of the Sixmile Creek Formation shown in orange.  Map by Jesse Mosolof (Montana Bureau of Mines and Geology).

Exactly how long was this paleovalley? And from which volcano did the Timber Hill basalt erupt?  Some geologists speculate that the basalt came from a nearby volcano whose edifice has long since eroded away. Others think that the basalt lava erupted from the Heise Volcanic Field, which is one of many large volcanic centers created as the Yellowstone hotspot tracked northeast along the Snake River Plain south of the Montana-Idaho border. The Timber Hill basalt is over 80 miles from Heise with several imposing mountain ranges in between, begging the question: was it even possible for a basalt flow to travel that far? Remnants of the Timber Hill basalt can be traced approximately 30 miles from the Sweetwater Hills to Lima, Montana. The path of the lava flow from Lima to the Snake River Plain is harder to pin down, but the lava may have traveled an additional 30–50 miles. This might seem implausible to some, but geologists can actually point to a number of examples of long basalt flows. Lava flows of the Columbia River Basalt Group are tens to hundreds of miles long and likely record the emergence of the Yellowstone hotspot after a period of relative dormancy. Another well-known hotspot serves as a modern analogue: basalt flows from Mauna Loa have traveled over 30 miles across the Island of Hawaiʻi on their journey from their source vents to the ocean.   

Ok, so basalt can flow long distances, but how did the Timber Hill basalt get around those imposing obstacles we call mountain ranges? Again, the landscape looked very different 6 million years ago, and the mountain ranges we see today had not yet formed when the lava flowed down a paleovalley connecting what is now the Sweetwater Hills and Snake River Plain. Thermal bulging of the Earth’s crust as the Yellowstone hotspot tracked northeast past Heise created many of the northwest-trending, fault-bounded mountain ranges we see in southwest Montana today, chopping up the Timber Hill basalt into many segments. The Sweetwater Fault appears to be one of these northwest-trending structures, vertically offsetting the Timber Hill basalt by over 700 feet. This fault remains seismically active with the potential to generate a strong earthquake in the Dillon area.  

Field photograph showing the plateau-like nature of Timber Hill basalt
Field photograph showing the plateau-like nature of Timber Hill basalt. The Sweetwater Fault (yellow) vertically offsets the Timber Hill basalt at least 700 feet at this location. Photograph by Jesse Mosolf, Montana Bureau of Mines and Geology, 2022.

If you ever find yourself driving the Sweetwater Road connecting the Beaverhead and upper Ruby Valleys in southwest Montana, be sure to stop and admire the natural beauty of the Timber Hill basalt, and to contemplate the awesome volcanic history that has sculpted the geology and landscape of southwest Montana over millions of years.

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