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.

Yellowstone is a caldera system—a large volcanic area that was never home to a single towering volcano, but that has experienced eruptions so large that the ground collapsed into the emptied magma chamber.  While these caldera-forming eruptions grab most of the attention, it is far more common that caldera systems see smaller eruptions.  Between eruptions, the ground deforms—rising and falling—as magma, water, and gas accumulate and withdraw from storage areas beneath the surface.

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At Yellowstone, ground deformation was first documented in the 1970s.  An initial leveling survey—a method of measuring surface elevation—was completed in 1923 and repeated in the mid-1970s. The results indicated that the ground had uplifted by a maximum of 72 cm (28 in) in the 50+ years between surveys—a rate of about 1.4 cm (0.5 in) per year.

Since the 1970s, leveling surveys, and later GPS data and satellite radar imagery, showed that Yellowstone did not just go up, but at times it also subsided.  Whether going up or down, the rate was always a few cm (about 1 in) per year. 

This sounds impressive, and it is—that huge areas of ground rise and fall by an inch per year is a testament to the powerful forces acting beneath the surface at Yellowstone.  But how does this compare to deformation measured at other calderas?  A logical comparison would be with Long Valley, a caldera system located in eastern California.  Uplift there was first documented starting in the late 1970s.  Over the nearly 50 years between 1975 and 2022, the overall maximum uplift of Long Valley caldera was 66 cm (26 in), similar in terms of rate to Yellowstone.

But Long Valley and Yellowstone pale in comparison to deformation measured at Campi Flegrei—a caldera system near Naples, in Italy.  Leveling surveys started at Campi Flegrei in 1905 and were repeated frequently to assess how the ground moved, since deformation was a well-known phenomenon in the region.  Between 1905 and 1950, the caldera subsided by over 1 meter (3 feet).  The sense of movement then reversed, and several episodes of uplift occurred.  The largest were in 1969–1972 and 1982–1984, when the ground rose by 1.75 meters (nearly 6 feet) during each episode!  These uplift crises were accompanied by strong seismicity that prompted evacuations due to building damage.  When you plot the vertical deformation at Campi Flegrei at the same scale as that from Long Valley and Yellowstone, the lines for the calderas in the USA look almost flat!

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But that doesn’t even tell the full story of Campi Flegrei deformation.  Near the center of the caldera is a 2000-year-old Roman ruin that has three columns, all of which have evidence of mollusk borings 7 meters (23 feet) above their bases.  The columns are above sea level now, and they were built above sea level.  But between the time they were built and now, the ground subsided below sea level by several meters (tens of feet) and then rose out of the sea once again.  It was study of this site in the 1820s that convinced geologists that the ground at volcanoes could move up and down by dramatic amounts due to subsurface magmatic activity.

The long-term record at Yellowstone also includes impressive amounts of surface movement. Studies of wave-cut terraces on the north shore of Yellowstone Lake have shown that the central part of the caldera today is about 30 meters (100 feet) feet lower than it was at the end of the last ice age 13,000 years ago—even in spite of overall uplift during the past century.

At Campi Flegrei, extreme rates of deformation are known to have occurred prior to the most recent eruption, which occurred in 1538 when a cinder cone and lava flow formed.  In the decades before that eruption, the ground uplifted so much that royal pronouncements were needed to establish ownership of the new land that had risen out of the sea.  And in the hours before the start of the eruption in 1538, the shoreline rose so quickly that fish were trapped, and the coastline moved seaward by about 370 meters (1200 feet)!

Similar rapid changes have been observed at other caldera systems prior to eruptions.  For example, at Rabaul, in Papua New Guinea, a tide gauge was lifted completely out of the water in the hours before the start of a mild explosive eruption in 1994.

Deformation at Yellowstone is certainly impressive, but it pales in comparison to what has occurred at Campi Flegrei—not just over the past century, but over the past two millennia.