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Today, YVO scientists monitor ground deformation at Yellowstone using the Global Positioning System (GPS) and interferometric synthetic aperture radar (InSAR). Both techniques rely on signals from Earth-orbiting satellites. But how did scientists study deformation in the "old days" (meaning, prior to the 1990s)? The answer isn't rocket science, but it requires a bit of expl

Leveling crew near South Sister volcano, Three Sisters Wilderness, Oregon USA. A surveying instrument called a level scans a barcode pattern on each of two staffs (inset) to measure the precise elevation difference between ground points. One staff is moved ahead and the process is repeated until a permanent benchmark is reached and its elevation relative to a starting benchmark is established. Leveling surveys helped scientists track an episode of surface uplift centered near South Sister that began in 1998 and continued through 2017 at a gradually declining rate. USGS photograph by Michael Poland.

Prior to GPS and InSAR, scientists used techniques from geodesy, a branch of Earth science that deals with Earth's shape. Thanks to numerous ancient Greek scholars from around 2,500 years ago, we know that Earth isn't flat (really!). And thanks to the work of countless geodesists since then, we know that Earth isn't exactly round, either. Our planet is actually a bumpy oblate spheroid—squashed at the poles and swollen at the equator as a result of its spin, and slightly irregular as a result of its uneven distribution of mass.

Earth's bumpy shape even changes over time in response to subsurface processes like magmatism and mountain building. Geodesists refer to the changes as deformation.

Leveling is a geodetic technique used to measure vertical deformation – surface uplift or subsidence. It's decidedly low-tech compared to GPS or InSAR, but it played a pivotal role in our understanding of Yellowstone.

Using a surveying instrument called a level and a pair of graduated staffs (imagine two very long rulers held vertically with the level positioned on a tripod between them), a leveling crew measures elevation differences between a starting benchmark and a series of temporary marks, keeping track of the total until the next benchmark is reached and its elevation is established. By repeating the process, elevations of an entire network of benchmarks are determined. At some later date, a second survey can detect any changes that occurred in the interim.

The first leveling survey in Yellowstone was done almost a century ago, in 1923. Some of the original benchmarks still survive, and in all there are now more than 500 benchmarks in Yellowstone National Park.

More than 50 years passed before the next leveling survey was done in response to the 1975 magnitude 6.1 Yellowstone Park earthquake. An astounding pattern emerged, much beyond anything that could be explained by the quake. The central part of the caldera had been uplifted more than 72 centimeters (28 inches) since the 1923 survey, at an average rate of 1.4 centimeters per year (0.5 inch per year). Yellowstone was rising!

Upper left: Contours of surface uplift that occurred in the Yellowstone caldera between leveling surveys in 1923 and 1975-77. Lower right: Locations of key benchmarks. The central part of the caldera floor rose more than 700 millimeters (28 inches), at an average rate of 14 millimeters per year (0.5 inch per year). The greatest uplift occurred between the Mallard Lake and Sour Creek resurgent domes, including near Old Faithful (OF, benchmark F10) and LeHardys Rapids (LH, benchmark DA3).

Annual leveling surveys starting in 1983 showed that uplift continued until 1985, when a strong earthquake swarm near the northwest rim of the caldera coincided with a reversal to subsidence. Scientists attributed the subsidence and swarm to magmatic or hydrothermal fluids leaking out of the caldera, relieving pressure that built during the preceding decades of uplift. Subsidence at rates of 9–35 millimeters per year (0.3–1.4 inches per year) continued for the next decade.

Starting in 1995, things got more complicated and a lot more interesting. Part of the caldera floor started rising again while another part continued to sink. A new area of uplift was discovered and for a time it rose faster than ever measured before or since. And scientists began to recognize connections between various sources of deformation, and between deformation episodes and earthquake swarms.

But those discoveries were made using GPS and InSAR, and will describe those revelations in future articles (so stay tuned!). The last Yellowstone leveling survey was completed in 2007. Leveling's heyday had passed, but not before advancing our understanding of one of Earth's most dynamic calderas.

Successive profiles across the Yellowstone caldera showing surface uplift and subsidence from repeated leveling surveys between Lake Butte and Mount Washburn. From 1976 to 1984, maximum uplift near LeHardys Rapids was nearly 180 millimeters (7 inches) at an average rate of 22 millimeters per year (0.9 inch per year). Uplift stopped during 1984-85, and from 1985 to 1998 the caldera floor subsided at rates of 9-35 millimeters per year (0.3-1.4 inch per year).

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