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.

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Since 2017, YVO has produced annual reports that contain an abundance of information about a variety of topics—for example, ground deformation trends, earthquake activity, and upgrades to monitoring networks.  In addition, there is information about research results and new discoveries, like the recognition of the new thermal area near Tern Lake in 2018.

The 2022 YVO annual report was just published and is now available online at https://doi.org/10.3133/cir1508.  As in past years, the summary highlights monitoring data collected over the course of the year.  For example, 2,429 earthquakes were located in the Yellowstone region in 2022, the largest of which was a M4.2 event that occurred on May 11—the strongest earthquake in the area since a M4.4 in 2017.  The number of earthquakes is within the usual range of annual events and slightly less than occurred in 2021.  About 66 percent of the earthquakes took place as swarms, which are groupings of earthquakes that are clustered together in both space and time.  The most significant swarm, which had over 1,100 earthquakes and lasted for the entire second half of the year, occurred near Grizzly Lake, between Mammoth Hot Springs and Norris Geyser Basin.  Earthquake swarms with hundreds to thousands of events and lasting for months are not unusual in the Yellowstone region, occurring every few years.

As has been the trend since 2015, Yellowstone Caldera subsided by 1–2 inches (2–3 centimeters) over the course of the year, interrupted during the summer months by a pause in subsidence or minor amount of uplift as the ground swelled slightly due to groundwater recharge from snowmelt.  Little significant deformation has been detected in the area of Norris Geyser Basin since 2018.

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Speaking of water, there was a lot of it in Yellowstone in 2022—an atmospheric river event in mid-June dropped several inches of rain on late-season snowpack, causing catastrophic flooding in the Yellowstone region and leading to a temporary closure of the entire park.  The north and northeast entrance roads were inaccessible for the rest of the summer due to flood damage.  YVO monitoring stations, some of which actually recorded seismic noise associated with floodwaters, escaped largely unscathed, with the exception of one river monitoring station along the Gardner River in the north part of the park that was buried under several feet of debris.

All that water didn’t seem to have much of an impact on Yellowstone’s hydrothermal system—there were no significant changes in the behavior of most geysers.  Steamboat Geyser, the tallest active geyser in the world, erupted 11 times in 2022, continuing a trend of frequent activity that began in 2018.  The number of annual eruptions has been declining since reaching peaks of 48 eruptions in both 2019 and 2020, however, raising the possibility that the current period of numerous eruptions may be coming to an end.

Gas and water chemistry and emissions showed no major changes compared to previous years.  A new continuous sensor capable of measuring carbon dioxide emissions was installed in the Mud Volcano area, and expeditions were undertaken to measure gas and water chemistry at several areas in the park—including the new thermal area near Tern Lake.  There, geochemists found that much of the ground was at boiling temperatures just beneath the surface.  Carbon dioxide emissions were elevated relative to background but were minor compared to similar thermal areas—about half that of the Mud Volcano region when accounting for the difference in sizes of the two thermal areas, for example.

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Research into the causes and impacts of hydrothermal explosions in Yellowstone National Park focused on the north part of Yellowstone Lake and Lower Geyser Basin.  Sediment cores from the bottom of Yellowstone Lake revealed numerous thin layers of pulverized rock that are evidence of steam explosions of varying sizes that occurred over the past 14,000 years.  The largest explosions occurred from neutral-chloride hydrothermal areas, where the change from liquid water to steam was associated with rapid fluid expansion that drove significant explosions.  Vapor-dominated hydrothermal areas, in contrast, don’t experience this phase transition and so are not capable of large steam explosions.  The largest explosions in the area of Yellowstone Lake, from Mary Bay and Elliott’s Crater, were probably associated with earthquakes on the lake bottom that changed the pressure conditions in the lake-floor hydrothermal systems.  Similar triggers are thought to have driven steam explosions in Lower Geyser Basin as well.  There, sudden collapses of loose mounds of glacial debris deposited on hydrothermal areas by ice receding during the last glacial period would have changed the pressure conditions in the subsurface, causing water to flash to steam, expand rapidly, and result in explosions.

YVO scientists are also learning more about the complexity of the large volcanic explosion that formed Yellowstone Caldera 631,000 years ago.  The ash deposit that resulted from that explosion, called the Lava Creek Tuff, was originally thought to be made up of two geological subunits.  Montana State University geologists, however, identified several new units of the Lava Creek Tuff while mapping the rocks on the Sour Creek dome on the eastern side of Yellowstone Caldera.  This discovery implies that the eruption was much more complex than previously thought, and additional work is needed to sort out the details of the eruption’s history.

As usual, monitoring data and research results continue to open new paths for better understanding Yellowstone’s volcanic, earthquake, and hydrothermal behavior.  YVO will look to expand on these results in 2023—field work is already underway!