Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Jennifer Lewicki, research geologist with the U.S. Geological Survey in Menlo Park, California.

Ever wonder what makes the Norris Geyser Basin in Yellowstone National Park so special? It turns out that the basin not only boasts the hottest temperatures and largest changes in hydrothermal (hot water) activity in the national park, but is also home to Steamboat Geyser, which erupts to unmatched heights of up to 100 meters (328 feet).

Norris Geyser Basin is also changing all the time, which makes it one of the most interesting thermal areas in the park. These changes can involve unusual boiling, chemical changes to water, variations in spring activity, and increases in acidity and cloudiness of certain hot springs and geysers. A recent example is activity of Echinus Geyser, which, for a few weeks during October-November 2017, erupted every two to three hours, instead of its more usual intermittent activity.

A more noteworthy change occurred in 2003 when the southern part of Norris Geyser Basin, known as the Back Basin, attracted the attention of scientists and visitors. Dramatic increases in hydrothermal activity occurred over a period of several months. Ground and water temperatures rose, water in some thermal pools boiled away and left hissing steam vents, new mud pots (depressions filled with boiling acidic water and mud) formed, and vegetation died and burned. Porkchop Geyser, inactive since 1989, increased markedly in temperature and then sprang back to life in an eruption. These changes motivated increased monitoring of seismicity and deformation by University of Utah scientists, and Yellowstone National Park ordered the temporary closure of the Back Basin to visitors to protect the public and park staff from hazards that could result from unusually high ground and water temperatures. It is still a mystery as to why the 2003 increase in hydrothermal activity was so extreme.

Norris Geyser Basin also hosts several craters that reach up to 100 meters (328 feet) in diameter. These craters were formed during hydrothermal explosions that ejected boiling water, mud and rocks. The most recent of these events happened when Porkchop Geyser exploded unexpectedly in 1989, throwing boulders up to 3 feet in diameter more than 216 feet from the geyser. Although no one was injured in the explosion, it served as a reminder of the sudden and unpredictable hazards these events can pose.

Since Norris Geyser Basin is such an active and hot area of the park, Yellowstone Volcano Observatory scientists use an array of equipment to monitor hydrothermal activity. Data from these instruments help to identify changes that occur before geyser eruptions, boiling episodes, or hydrothermal explosions, which will aid in minimizing impacts due to future hazardous situations. As related in previous columns, seismic instruments and Global Positioning System receivers are used to measure ground movements that occur when water and gas flow in the subsurface, but the unique nature of the geyser basin requires more detailed work.

The Norris Geyser Basin is host to an array of temperature sensors that monitor changes in the flow of hot water out of the ground. These data can be accessed on YVO's monitoring webpage (zoom in to the Norris area and click on any of the thermometer icons to see plots of temperature at those stations over time). YVO also measures water chemistry of samples taken from hot springs and geysers to track their origins and underground flow paths, and heat flow is tracked by thermal infrared instruments on aircraft and satellites. Many of these datasets are described in USGS data releases or publications about gas and water samples, gas and thermal samples from a 2016 temporary experiment, and satellite and ground-based thermal mapping. In the summer of 2018, new instruments are planned for continuous measurement of heat and gas emissions from Norris Geyser Basin.

Together, the array of information obtained from this diverse monitoring toolset should shed light on the potential causes of hydrothermal variations in Norris Geyser Basin and mitigate related hazards in the future. More information on Yellowstone's fascinating Norris Geyser Basin can be found at: https://pubs.usgs.gov/pp/1456/.