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September 20, 2020

Thanks to technology like GPS (Global Positioning System), scientists know that the ground at Yellowstone moves up and down at a rate of a few centimeters—about an inch—per year. But what was happening to the caldera before scientists had the ability to make these measurements?  Yellowstone Lake holds the key, with a record of deformation that extends back thousands of years!

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Mike Poland, geophysicist with the U.S. Geological Survey and Scientist-in-Charge of the Yellowstone Volcano Observatory.

Photo looking north from the north shore of Yellowstone Lake at lake terraces.
Photo looking north from the north shore of Yellowstone Lake. The photo was taken from a level bench, or terrace, which marks a previous high stand of the lake.  In the middle distance (between the two red lines), the ground slopes up to second, higher-level terrace that indicates an even higher past lake level.

In the 1970s, leveling measurements revealed that the caldera had uplifted by more than 72 cm (28 in) since the 1920s, a rate of 1–2 cm (less than an inch) per year.  Continued measurements showed that the ground also could subside, as it did at about the same rate of 1–2 cm/yr during 1985–1995, and several additional cycles of caldera uplift and subsidence have occurred in the years since.  Starting in the late 1990s, space-based technologies like GPS and InSAR (interferometric synthetic aperture radar) took over as measurement techniques, revealing even more detail about the ups and downs of Yellowstone.

But what was happening before the 1920s, when the first measurements were made?  Was the caldera deforming?  And if so, were the deformation rates and style similar to what we have observed in the past 100 years?  Fortunately, there is a record of this past deformation preserved along the shores of Yellowstone Lake!

The Yellowstone River drains from Yellowstone Lake to the north, crossing the axis of the caldera and the center of historical deformation near LeHardy Rapids.  This means that whenever the caldera uplifts, the lake outlet rises, and water backs up into the lake.  When the caldera subsides, the lake outlet drops and water drains from the lake.

For all large bodies of water—lakes, seas, and ocean—when the level of the water remains at a set level waves cut a terrace, or bench, in the shoreline.  These terraces therefore record past shorelines, which indicate the level of the water at past times.

The north side of Yellowstone Lake has numerous terraces, easily seen in high-resolution topographic maps and also on the ground, that show high water levels from years past.  There are also terraces below the current water level that indicate times when the lake level was lower than it is today.

But how old are these terraces?  All are clearly younger than the end of the last ice age, about 14,000 years ago—anything older would have been obliterated by glacial ice—and there are several tools available to date individual terraces more precisely.  For example, archeological evidence, like projectile points, on terrace surfaces can constrain ages.  Also, ash from distant eruptions—like that of Glacier Peak, Washington, about 13,700 years ago, and Mount Mazama (Crater Lake, Oregon) 7600 years ago—and local hydrothermal explosion deposits within Yellowstone National Park provide “marker beds” for determining terrace ages.

Shaded relief map showing subtle terraces on the north side of Yellowstone Lake
Shaded relief map based on high-resolution topographic data from lidar and showing subtle terraces on the north side of Yellowstone Lake (a few example terraces are marked by yellow arrows and traced by dashed and dotted white lines). Each terrace represents a high-stand of lake water, which is caused by water backing up into the lake because caldera uplift raised the level of the lake outlet in the past.  The presence of these terraces, and the fact that they are tilted slightly towards the caldera, implies that the caldera has subsided since they formed several thousand years ago. 

When the terraces are mapped and dated, a story of deformation emerges.  Since ice receded from the Yellowstone Lake basin about 14,000 years ago, several cycles of uplift and subsidence have occurred that lasted thousands of years.  These cycles may have involved caldera uplift and subsidence of about 30 meters (100 feet)!  Overall, the level of the lake is lower compared to when the ice age ended, indicating that the caldera has subsided since the ice retreated.  This is apparent because some of the older lake terraces actually slope down to the north, toward the center of the caldera.  The terraces were initially level when they formed, so the fact that they tilt towards the caldera center means overall caldera subsidence has occurred since their formation.

The lesson from the lake is that cycles of deformation happen on multiple scales—over years to decades, as seen from historical measurements, and over thousands of years based on Yellowstone Lake terraces.

But what could cause such variation?  Magma intrusion is certainly a possibility, but that does a poor job of explaining why the caldera also subsides in these cycles and is overall down relative to thousands of years ago.  A more likely possibility is the hydrothermal activity that drives Yellowstone’s geysers and hot springs.  We know from multiple types of data sets that the hydrothermal fluids in Yellowstone migrate laterally.  Hot water migrating from one area to another in the subsurface could cause the surface to move both up and down, much as it seems to have done recently at Norris Geyser Basin.

So, the variations in ground deformation we see today are part of a larger cycle of ups (inflation) and downs (deflation) of the Yellowstone Caldera related to hydrothermal activity—a distinctive characteristic of the system!

If you would like to know more about the Yellowstone Lake terraces, check out USGS Open-File Report 2002-0142, and chapter E of USGS Professional Paper 1717, both of which contain detailed descriptions of the geology of these important markers of Yellowstone’s deformation history!

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