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Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Patrick Muffler, emeritus geologist with the U.S. Geological Survey.

The U.S. Geological Survey has benefitted from the work of numerous outstanding scientists who have furthered our understanding of how Yellowstone works. For example, Bob Christiansen mapped the caldera structures in the region and defined much of the geologic history, while Ken Pierce worked out the glacial history of the region and studied the trends of geologic processes, like faulting and uplift, that extend across Idaho to Yellowstone National Park. USGS Geologist Robert (Bob) O. Fournier was another such pioneer—a scientist who contributed greatly to our current understanding of the hot springs and hydrothermal systems of Yellowstone National Park. Bob passed away in January 2023 in Portola Valley, CA, at the age of 91, leaving a legacy of discovery in Yellowstone that will inspire scientists for generations to come.

Bob received a B.A. from Harvard University in 1954 and a Ph.D. from the University of California at Berkeley in 1958 with a dissertation on the porphyry-copper deposit at Ely, Nevada—a deposit that was related to the hydrothermal system associated with ancient volcanism and presaged his work on modern hydrothermal activity in Yellowstone.  He then joined the U.S. Geological Survey in Washington, DC, studying the solubility of silica in water as a function of temperature, a theme that would persist throughout his long and productive career.

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Bob is best-known for his leadership in developing various methods for determining subsurface reservoir temperatures from the chemical analysis of hot-spring waters, based on both laboratory and field investigations, primarily in Yellowstone National Park.  His work in the park began in 1962 when he served as a geothermal consultant to the National Park Service during bridge construction and road relocation at Beryl Hot Spring.  After transferring to the USGS office in Menlo Park, California, in 1962 he joined a group led by Donald E. White to carry out research drilling in hot-spring areas of Yellowstone National Park.  These unique and seminal investigations provided ground truth for the subsurface temperature and pressure conditions beneath Yellowstone hot springs, with botttom-hole temperatures reaching 240 °C (464 °F). Bob developed equipment and procedures to measure pressures and to sample water and gas from these wells, he maintained and ultimately decommissioned the wells, and he used the resulting data in his exploration of the geochemistry and dynamics of the Yellowstone hot-spring systems. You can see a video describing how Bob helped to “tame” one of the research wells at Biscuit Basin, which had a blowout in 1992, at https://pubs.usgs.gov/of/2008/1014/. His Yellowstone investigations, which continued throughout his career, provided the basis for his achievements as a world-renown geothermal expert.  Bob considered Yellowstone his “cornerstone of reality.”

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Bobʻs field work at Yellowstone was closely integrated with his ground-breaking research into chemical techniques to estimate geothermal reservoir temperatures and other characteristics. Notable were his development of the Na-K (sodium-potassium) and Na-K-Ca (sodium-potassium-calcium) “geothermomenters,” which used the chemical composition of hydrothermal water measured at the surface to estimate the temperature of the reservoir of hydrothermal fluids beneath the ground. These tools, along with his silica geothermometer, have been adopted to better understand hydrothermal systems around the world.  He also pioneered the understanding of models to estimate the temperature of the hot-water component in mixed waters and to predict underground conditions in hot-spring systems—for example, whether a reservoir was steam or hot water. He even developed a better understanding of the processes related to movement of fluid from plastic into brittle rock in the transition zone between magma at depth and the overlying hydrothermal systems.

Bob was a leader in major international geothermal efforts as well. In 1975, he chaired the U.S. Organizing Committee for the 2nd United Nations Symposium on Geothermal Resources, a two-week meeting in San Francisco that drew some 1,100 participants from around the world.  He also led advisory panels of international experts overseeing development of the Miravales geothermal field in Costa Rica and several geothermal fields in El Salvador. Finally, he served on many committees overseeing continental scientific drilling activities within the United States and elsewhere—efforts that led to a better understanding of the subsurface conditions in active volcanic areas.

Bob’s passion throughout his career, though, was Yellowstone and the spectacular hydrothermal features found there. His work built a foundation upon which scientists today can better understand the conditions beneath the surface in these areas, which is important for understanding everything from geyser activity to potentially hazardous hydrothermal explosions.

For more about Bob’s career and perspective—in his own words!—check out the interview at https://www.usgs.gov/media/videos/inside-usgs-no-3-robert-fournier-yell….

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