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.

Earth’s surface might seem stable, but it is constantly moving in small ways.  Much of this motion is regular, in response to tidal forces.  In some places, there can be subsidence due to removal of groundwater, or uplift due to tectonic forces.  At volcanoes, ground motion can be especially dynamic, and the ups and downs provide clues as to what is happening beneath the surface.

At some volcanoes, deformation can be extreme.  For example, Campi Flegrei, in Italy, can see meters—many feet—of uplift over short periods of time.  There are records from that area, and in the Galápagos, and in other coastal volcanic areas of deformation occurring so quickly that fish were trapped as formerly submerged areas rose above sea level!

Sources/Usage: Public Domain. View Media Details

Deformation is not quite that dramatic at Yellowstone.  The ground does rise and fall but typically at rates of a few centimeters—1 to 2 inches—per year.  Leveling measurements collected between 1923 and 1977 demonstrated that the caldera rose by 72 centimeters (28 inches), but then switched to subsidence in the 1980s before going back to uplift in the 1990s.

Based on geologic studies of terraces on the north side of Yellowstone Lake, it is clear that the caldera has overall subsided by about 30 meters (100 feet) over the past 14,000 years, since the end of the last ice age.

During the past two decades, deformation monitoring at Yellowstone has been done using a satellite technique called InSAR, as well as continuously monitoring Global Positioning System (GPS) stations.  GPS is especially useful because it provides data year-round, regardless of the season (InSAR cannot be used in winter months because snow interferes with the satellite signal).

Since 2015, GPS data indicate that Yellowstone caldera has been subsiding by about 2–3 centimeters (about 1 inch) each year.  The subsidence is occurring across the caldera, seen at stations on both the east side, near the Mud Volcano area, and on the west side, near Old Faithful.  But the subsidence is not steady.  It is interrupted every summer by a pause, or even a small amount (0.5 to 1 cm, or just a fraction of an inch) of uplift.

The pause in subsidence and small amount of uplift occurs at about the same time each year—it starts in May–June and lasts until September–October—and in some years it is more pronounced than in others.  What is causing this seasonal signal?

Sources/Usage: Public Domain. View Media Details

The answer becomes clear when plotting the deformation data against the river levels.  The seasonal pause in subsidence begins right as river levels start to spike because of runoff from spring snowmelt.  This runoff not only causes the rivers to run higher and faster, but it also recharges the groundwater system, and the ground swells in response, like a wet sponge.

Groundwater is not the only cause of seasonal deformation in Yellowstone.  There are also signals seen near the margins of Yellowstone Lake.  Thanks to all that runoff, the lake level rises quickly in the spring and then decreases gradually over the course of the summer.  The change in lake level can be as much as 1.5 meters (5 feet)! 

All that extra water puts more weight on the surface and causes the ground to subside—like putting a bowling ball on a mattress.  GPS stations near the lake can “see” the weight of all this water.  In the spring, as the lake level rises, GPS stations near the lake shore show subsidence by up to 2 cm (less than an inch).  But that subsidence reverses as the lake level decreases—as the bowling ball is gradually lifted off the mattress.

Areas outside the caldera also see some of these seasonal changes, although they are usually not quite as dramatic (if you call a couple of centimeters dramatic!).  The current subsidence, however, is largely restricted to the caldera.  The area around the Norris Geyser Basin deforms independent of the caldera and has seen episodes of uplift and subsidence over the past few years that indicate water accumulating beneath the basin and then being released.

Ground motion in Yellowstone National Park is dynamic, changing from year to year and even season to season.  Although caldera subsidence has been dominant since 2016, at some point it will start to uplift once again—as it did most recently during 2004–2009 and 2014–2015.  It’s all part of the ups and downs of one of the largest magmatic and hydrothermal systems on Earth!