Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Michael Poland, geophysicist with the U.S. Geological Survey and Scientist-in-Charge of the Yellowstone Volcano Observatory, and Lisa Morgan, emeritus geologist with the U.S. Geological Survey.

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Visitors to Yellowstone who enter or leave the park using the north entrance, near Mammoth Hot Springs, have probably passed by Roaring Mountain.  It is located about 5 miles north of Norris Geyser Basin and is home to a number of steaming, hissing fumaroles (gas vents) on the side of a treeless, barren hill right along the eastern edge of the Norris-Mammoth road.

Roaring Mountain is a classic example of an acid-sulfate thermal area.  No geysers are present on Roaring Mountain; rather, the surface is dotted with numerous steam and gas vents.  The gases that are emitted are at or above boiling temperatures for that elevation, and their acidic nature has caused much of the rock in the area, which is densely welded ash erupted during the caldera-forming event that created Yellowstone caldera, to become altered into clay minerals, especially kaolinite and smectite.  That alteration gives the thermal area its chalky, soft appearance.  The area of intensely hot ground—easily viewable from space!—makes Roaring Mountain one of the most intense thermal areas in the park despite its overall small size, with more than 100 megawatts of geothermal radiative power output.

Although most thermal areas in Yellowstone are found within or along the perimeter of the Yellowstone Caldera, Roaring Mountain is outside and north of the caldera along an important north-trending lineation that is tectonically and thermally active.  The area between Norris Geyser Basin and Mammoth Hot Springs, informally called the Norris-Mammoth Corridor, hosts abundant thermal and tectonic activity and is an area of fluid transport, allowing water heated by the Yellowstone magmatic system to migrate out of the caldera to the north.  The corridor is defined by a series of large, active faults that provide the pathway for these fluids, which feed hot spring activity that extends well north of the park, for example, to Chico Hot Springs in Paradise Valley, Montana.

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On a calm day, when traffic and wind are light, the sounds from Roaring Mountain are obvious.  But it hardly “roars.”  This is because, like most of the thermal areas in Yellowstone, Roaring Mountain has changed noticeably over the years.

At times during the latter portion of the 1800s, reports of gas escaping from the Roaring Mountain area described the sound as being audible some distance away—perhaps a mile or more.  By the late 1890s, activity had diminished, but in 1902 a thermal event occurred.  The sound could be heard far away once again, and increased heat emissions killed trees over an area of up to a half a square mile (about 1.3 square kilometers).

Within a few years, the sound and thermal emissions had returned to their quieter, more localized state, where they have remained to this day.  The 1902 thermal event at Roaring Mountain might have resembled a smaller version of a similar episode that occurred at Norris Geyser Basin in 2003, when temporary heating killed a number of trees and formed new thermal vents.  This sort of thermal unrest is common in Yellowstone and is related to changes in hydrothermal circulation and fluid conduits—for example, like that which occurred near Old Faithful in 2018 and 2023.

In addition to being the site of present-day thermal activity, the area near Roaring Mountain hosts a few hydrothermal explosion craters, including Semi-Centennial—a feature that erupted explosively, sending hot water and debris up to 300 feet (91 meters) high multiple times on August 14, 1922, but that has been quiet ever since  On the high plateau just east of Roaring Mountain are a few more craters; most are small.  Their age is uncertain, but they must have formed in the past 14,000 years, after the end of last glacial period in Yellowstone.  The largest crater, which is about 850 feet (260 meters) across, spread ashy debris to a distance of up to 0.5 mile (800 meters).

If you happen to find yourself driving between Norris Geyser Basin and Mammoth Hot Springs during your next visit to Yellowstone National Park, Roaring Mountain is worth a stop. While staring at a hillside of hissing fumaroles, imagine how it might have sounded when it really did roar. Those conditions, related to changes in the hydrothermal chemistry or circulation patterns, have occurred multiple times in the past and may again in the future.