YELLOWSTONE NATIONAL PARK — In this week’s Caldera Chronicles, geologists with the Yellowstone Volcano Observatory in Yellowstone National Park (YNP) discuss the history of uplift and volcanism in mountains near the park, especially within the Gallatin Range.
According to the column’s author U.S. Geological Survey geologist Stanley Mordensky, the Gallatin Range stretches from just north of Norris Geyser Basin in YNP to Bozeman Pass between Bozeman and Livingston, Montana. The range has an average width of about 20 miles and contains 10 peaks over 10,000 feet, with the highest being Electric Peak at 10,969 feet.
“The Gallatin range is an assemblage of old (even by geological standards) metamorphic rock, younger sedimentary layers, and younger still volcanics,” writes Mordensky.
The metamorphic rocks formed between 3.6 and 2.7 billion years ago under high-temperature and high-pressure conditions at a great depth. After that, Mordensky explains a gap in the geologic record, which is consistent with the Great Unconformity — a substantial break in the geologic record described across the world, from 2.7 billion to 542 million years ago. After that, a shallow inland sea formed in the region, causing sedimentary deposits that formed limestone, shale and sandstone.
A mountain-forming event began about 160 million years ago, which raised parts of western North America. Between 80 and 50 million years ago, the metamorphic and sedimentary rocks began rising into the Gallatin Range. Part of this event coincided with the formation of other ranges, like the Wind River Range and the Beartooth Mountains. Next came a period of volcanism in the region.
“The Gallatin Range itself experienced some volcanic activity, and the distinction between Gallatin and Absaroka ranges becomes a bit blurred where Absaroka volcanics are found in the Gallatin Range — so much so that the two ranges have been collectively referred to as the Absaroka-Gallatin volcanic province,” writes Mordensky.
The volcanic activity buried the living forests, creating the Gallatin Petrified Forest, similar to how petrified trees were formed in YNP. Mordensky explains that although the ranges are not volcanically active now, the area still presents a potential for geohazards. It is believed that the Gallatin and Madison ranges are made up of about 20% mass wasting deposit from landslides, debris flows, mudflows, rockfall and more.
Researchers are looking to see how seismic activity in the region might be contributing to mass wasting events and ground deformation. Some faults near the Gallatin Range are being studied in order to inhibit the likelihood of strong earthquakes in the future.
