Editor's note: Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Scott K. Johnson, science communication associate at EarthScope Consortium.

If you want to measure a climate trend, one thing you certainly have to account for is the seasons. It would be unwise to take one measurement in July, a second measurement in December, and declare that we are descending into an ice age. The same is true for measuring ground deformation. There is seasonal movement you have to consider, including in Yellowstone National Park.

A number of GPS stations continuously record ground motion in the park, and that continuous data shows that the ground can move up and down a by couple centimeters (nearly an inch) each year. Although the landscape of Yellowstone is unique in many ways, this isn’t one of them. This seasonal cycle is common for the Northern Rockies and regions beyond.

GPS monitoring station P709 is located on The Promontory between the South Arm and Southeast Arm of Yellowstone Lake. It was installed in 2005 as part of the Yellowstone component of the National Science Foundation's Plate Boundary Observatory (PBO) under permit YELL-SCI-5546. Photo from UNAVCO station overview page.

Ground motion is caused by different processes in different areas—sometimes it can even be caused by humans (usually when fluids like water and oil are removed from beneath the ground). The movement of tectonic plates, earthquakes, and volcanic activity are some of the processes that GPS stations have been used to measure. But Earth’s water cycle is also a significant cause of ground deformation due to the incredible amounts of water that move around over time.

Despite inspiring the phrase “rock solid”, rock actually behaves elastically. You just need huge forces and very precise measurements to see it. Just as you can squish rubber and watch it spring back when you stop pushing into it, seasonal changes in the mass of water on the landscape can temporarily depress the bedrock beneath it. Because of this, GPS stations can also be used to monitor declining groundwater due to drought or overuse by humans based on the resulting ground motion. On a shorter timescale, seasonal precipitation and weather patterns also lead to a varying burden on the landscape that causes the surface to move up and down very slightly.

Most GPS stations in the Yellowstone region record downward movement over the winter and upward movement from about June to October. That same pattern can be seen across the Northern Rockies. As snowpack accumulates through the winter, the mass of water in this region grows. Come spring, melting snow infiltrates into the ground or trickles directly into streams, feeding river systems that carry it away. The added weight of the snowpack depresses the land surface; the relief of that weight as snow melts away allows the land surface to spring back upward.

Vertical motion at GPS station P720, near the Slough Creek Campground in the northeast part of Yellowstone National Park, clearly shows a large annual cycle. Figure and data from EarthScope Consortium (https://www.unavco.org/instrumentation/networks/status/nota/overview/P720).

GPS measurements near the shores of Yellowstone Lake appear to depart from this pattern slightly. Because Yellowstone Lake swells with water from snowmelt through June, the mass burden continues to grow here even as it shrinks in the surrounding area. This further depresses the land beneath the lake—a miniature version of the regional snowpack response. But once the lake level begins to drop, nearby GPS stations also begin rising like the rest of the park.

While we can learn much about the water cycle and elastic properties of rock from studying these seasonal motions, the primary goal in Yellowstone is monitoring activity of the hydrothermal and volcanic systems. A simple way to do this is to use data spanning multiple years and average over seasonal cycles to see longer-term trends. This is one reason continuous, long-running GPS records are a valuable part of the Yellowstone monitoring system.

It is also possible to subtract out the seasonal motion based on snowpack measurements, the movement of other stations in the region, and other information to reveal subtler changes. In California, for example, research on the Sierra Nevada mountain range has had to account for snowpack, drought, and groundwater use to isolate the smaller tectonic motion happening in the background.

Vertical motion at GPS station P350, in Idaho, together with nearby snowpack measurements (given as snow water equivalent). The GPS station moves downward as snowpack grows, and moves upward after it melts away. Taken from Knappe, et al., 2018 (https://doi.org/10.1029/2018WR023289).

Portions of Yellowstone have been observed rising or falling 10 centimeters or more over periods of several years because of activity in the volcanic and hydrothermal systems, while cycling up and down a couple centimeters each year. With a more complete picture of ground deformation, it’s easier to identify exactly what is being measured at any given time and place—because sometimes what we’re measuring is the weight of a winter’s snowfall!