In the 1970s, a resurvey of benchmarks discovered the unprecedented uplift of the Yellowstone Caldera of more than 28 inches (72 cm) over five decades. More recently, new and revolutionary satellite-based methods for tracking the Earth's shifting ground motions have enabled University of Utah, U.S. Geological Survey, and other scientists to assemble a more precise and detailed picture of Yellowstone's ground movements. [snip]

The 1975–77 survey showed that the area of recent uplift was located within the Yellowstone Caldera, a shallow, oval depression, 53 miles long and 28 miles across (85 by 45 km), in the middle of the park. This caldera was formed 640,000 years ago during the most recent of Yellowstone's great volcanic eruptions. In that eruption, 240 cubic miles (1,000 km3) of molten rock (magma) was blasted into the atmosphere and scattered on the Earth's surface-more than 1,000 times the volume erupted at Mount St. Helens in 1980! The ground then collapsed into the partly emptied magma reservoir, forming an enormous craterlike depression.

Later eruptions of many large lava flows, some as thick as 400 feet (120 m), buried the original caldera floor and most of the caldera walls. Mount Washburn, a prominent landmark in the park, is a section of the caldera rim that escaped burial. The most recent series of eruptions at Yellowstone, 160,000 to 70,000 years ago, covered much of the caldera floor with more than 20 thick lava flows, including the Elephant Back flow, which can be seen west of Fishing Bridge. [snip]

To learn more about the changing ground levels in the Yellowstone area, scientists conducted additional surveys across the eastern part of the caldera nearly every year from 1983 to 1998. In the 1990s, new and revolutionary satellite-based methods for tracking the Earth's changing ground surface—the Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR)—were applied by University of Utah, USGS, and other scientists to assemble a more detailed picture of how and when the ground moves above Yellowstone's magma reservoir.

These new data reveal that Yellowstone is in nearly continuous but frequently changing movement—the floor of the caldera continued to rise until 1984, stopped rising during 1984–85, and then subsided for the next 10 years. Parts of the central caldera began rising again in 1995, but a more complex pattern of uplift and subsidence has prevailed since 2000. InSAR data show that between 1995 and 1997 a large area along the northwest rim of the Yellowstone Caldera, centered near Norris Geyser Basin, started to rise. The picture that emerges from all these data is of a dynamic system in which the caldera floor is in almost constant motion—episodes of uplift and subsidence occur at various locations and over different time scales.