We may think of mountains as immobile, but they’re not. The reason mountains exist is because Earth’s surface is in constant motion, with colliding tectonic plates raising the Himalayas, Andes and North America’s coastal ranges, and magma surfacing at or near these moving plate boundaries building up volcanoes. The surfaces of volcanoes also are moved by the magma and hydrothermal plumbing systems that feed them. Sometimes, these motions are small and sometimes they are really big.
The motion of a volcano resulting from pressure changes within its plumbing system is called deformation, and we can measure it with high-precision positioning instruments on the ground, such as GPS, and also from radar satellites. Sometimes, motions are so large we can see them with our eyes. In the spring of 1980, a bulge on the north flank of Mount St. Helens was growing by more than 3 feet per day, as magma pushed up into the mountain. While the exact magnitude of the motion was measured with precise ranging instruments, the growing bulge was clearly visible to the naked eye. Mount St. Helens erupted on May 18, 1980, beginning with the collapse of that bulge.
Rapid ground motion at volcanoes has been observed in many other places as well. At Rabaul caldera in Papua New Guinea, motion preceding an eruption in 1994 was so rapid that a reef rose out of the water quickly enough to strand fish. In 1927, uplift of the coast of Fernandina volcano, in the Galápagos, occurred so fast that a fishing boat became stranded above water while laying at anchor.
Most of the time, however, we need precise instrumentation to measure the deformation of volcanoes, like the GPS, tilt and strainmeter stations that measure the motions of Yellowstone. The magnitudes of these motions vary as much as the volcanoes themselves. Some volcanoes barely move, even when erupting; other volcanoes move a lot before, during and after eruptions. Yellowstone tends to move up and down without an eruption. Scientists have recognized that this up-and-down behavior is normal for many large calderas all around the world.
So how much does Yellowstone move, and how does it compare to these other calderas? Let’s look just in terms of how much the volcano has inflated or moved up. The largest uplift episode measured at Yellowstone during the past few decades averaged about 2 inches per year, between 2005 and 2009. By contrast the largest uplift episode, measured halfway around the world at Campi Flegrei, Italy, averaged about 25 inches per year between 1982 and 1984.
What’s more, the record at Campi Flegrei goes back far longer than at Yellowstone — back to Roman times. The caldera is partially submerged beneath the Tyrrhenian Sea, which means the port town of Pozzuoli is well within the caldera. Marble columns of a 2,000-year-old marketplace give us a remarkable record of Campi Flegrei’s uplift and subsidence. The columns are pocked with holes from burrowing sea mollusks, which means the marketplace was built when the land was above sea level, subsided by about 23 feet and was inundated by seawater, and then rose out of the water again.
So what is the source of these changes at Campi Flegrei, Yellowstone and elsewhere? Deformation can be caused by new intrusions of magma, cooling or release of fluids or gases from magma, changes in the volcano’s hydrothermal system and other factors. Geodesy, the study of these motions, is only one tool in the volcanologists’ toolkit. Combining geodesy with other measurements — like seismicity and changes in the composition of gases, rocks and water — can go a long way toward understanding how any particular volcanic system behaves and why.
Knowing the history of how a volcano has deformed is also important for defining “normal” for that volcano, since each volcano is different in terms of the activity it experiences. At Yellowstone and other calderas, changes in deformation patterns from uplift to subsidence and back again are a common occurrence. By studying these changes, we hope to learn more about the subsurface conditions that are causing the ground to move and also to monitor for changes that are not normal. Recognizing the normal from the unusual is key for identifying the potential for future hazardous volcanic activity.