U.S. GEOLOGICAL SURVEY and the NATIONAL PARK SERVICE—OUR VOLCANIC PUBLIC LANDS Tracking Changes in Yellowstone’s Restless Volcanic System The world-famous Yellowstone geysers and hot springs are In the 1970s, a resurvey of benchmarks discovered the fueled by heat released from an unprecedented uplift of the enormous reservoir of magma Yellowstone Caldera of more (partially molten rock beneath than 28 inches (72 cm) over fi ve decades. More recently, the ground). Since the 1970’s, new and revolutionary sat- scientists have tracked rapid ellite-based methods for tracking the Earth’s shifting uplift and subsidence of the ground motions have en- ground and signifi cant changes abled University of Utah, U.S. Geological Survey, and other in hydrothermal (hot water) scientists to assemble a more features and earthquake activity. precise and detailed picture of Yellowstone’s ground In 2001, the Yellowstone Volcano movements. Global Position- Observatory was created by the ing System (GPS) stations like U.S. Geological Survey (USGS), this one in the Norris Geyser Basin can detect changes in the University of Utah, and elevation and horizontal shifts Yellowstone National Park to of 1 inch or less per year, helping scientists understand strengthen scientists’ ability to the processes that drive track activity that could result in Yellowstone’s active volcanic and earthquake systems. hazardous seismic, hydrothermal, (Photo courtesy of Christine or volcanic events in the region. Puskas, University of Utah.) No actual volcanic eruption has occurred in this way, the water level of Yellowstone Lake lowstone Caldera, a shallow, oval depression, the Yellowstone National Park region of Wyo- would appear to rise at the south end. 53 miles long and 28 miles across (85 by 45 ming, Montana, and Idaho since a lava fl ow To test this idea, in 1975–77 scientists from km), in the middle of the park. This caldera poured out about 70,000 years ago. However, the University of Utah and the U.S. Geo- was formed 640,000 years ago during the the area’s many geysers and hot springs indi- logical Survey (USGS) resurveyed a series of most recent of Yellowstone’s great volcanic cate that the underlying volcanic system re- benchmarks along roads throughout Yellow- eruptions. In that eruption, 240 cubic miles mains active. In the early 1970’s, Earth scien- stone National Park. These benchmarks were (1,000 km3) of molten rock (magma) was tists found evidence that this system was more originally installed and their elevations and blasted into the atmosphere and scattered on active than even these spectacular hydrother- locations precisely determined in 1923. The the Earth’s surface—more than 1,000 times mal (hot water) features had suggested. results of the resurvey showed dramatically the volume erupted at Mount St. Helens in One important clue was found along the that the central part of the park had indeed 1980! The ground then collapsed into the south shore of Yellowstone Lake, where trees risen. The greatest uplift appeared to be in the partly emptied magma reservoir, forming an were dying because their trunks were partly area of Le Hardy Rapids on the Yellowstone enormous craterlike depression. below water and a boat dock had become River, a few miles downstream from the lake’s Later eruptions of many large lava fl ows, completely submerged. At the same time, the outlet. There the ground had risen 28.5 inches some as thick as 400 feet (120 m), buried the water level at the lake’s outlet on its north side (72 cm) since 1923, raising the nearby north original caldera fl oor and most of the caldera appeared little changed. end of Yellowstone Lake much more than the walls. Mount Washburn, a prominent land- Scientists reasoned that these observations south end. Scientists then focused their atten- mark in the park, is a section of the caldera could be explained by southward tilt of the tion on what could be causing the ground to rim that escaped burial. The most recent area around Yellowstone Lake, including the rise and whether the uplift was still going on. series of eruptions at Yellowstone, 160,000 to lake basin itself. If the entire area was being 70,000 years ago, covered much of the cal- pushed up, perhaps the ground at the north Yellowstone Caldera and its resurgent domes dera fl oor with more than 20 thick lava fl ows, end of the lake was rising more than the south The 1975–77 survey showed that the area including the Elephant Back fl ow, which can end, like a giant bathtub lifted at one end. In of recent uplift was located within the Yel- be seen west of Fishing Bridge. U.S. Department of the Interior USGS Fact Sheet 100-03 U.S. Geological Survey 2004 111°W 110.5° 110° The uplift detected in the 1970’s was cen- tered near Le Hardy Rapids, between two resurgent domes—sections of the caldera fl oor that had earlier been pushed upward and faulted. Such resurgent domes form when magma rises to shallow levels beneath 45°N a caldera and slowly reinfl ates a previously depleted magma reservoir, pushing the over- lying caldera fl oor upward to form a dome. The pressure of the infl ating magma reser- Mount Washburn voir may even force some of the molten rock M to emerge at the surface as lava. The fact that the uplift documented in t Y Sour Creek the 1970’s was centered within the caldera Dome LeHardyRapids near the resurgent domes seemed to indi- Fishing B cate that the magma reservoir was again 44.5° Mallard exerting pressure upward. What could be Lake Dome Yellowstone causing this? Was new magma entering Yel- Lake lowstone’s enormous reservoir from below? Perhaps volcanic gas was escaping from the magma reservoir, percolating upward and era causing the surface to bow upward? Perhaps some of Yellowstone’s hot water had become trapped beneath the surface, exerting pres- sure on the caldera fl oor from below? New volcano-monitoring data soon forced scien- 44° tists to think again about what was occurring beneath Yellowstone’s active caldera. The region of Yellowstone National Park in Wyoming, Montana, and Idaho is famous for its spec- tacular geysers and extensive hot springs, indications that an underlying volcanic system remains The ups and downs of the caldera’s movements active. The central part of the park is occupied by the great, partly fi lled depression of the Yellow- To learn more about the changing ground stone Caldera, formed in a giant volcanic eruption 640,000 years ago that blasted 240 cubic miles levels in the Yellowstone area, scientists con- (1,000 km3) of molten rock (magma) into the atmosphere—more than 1,000 times the volume erupted at Mount St. Helens in 1980. Later eruptions largely fi lled the caldera and pushed up two resurgent ducted additional surveys across the eastern domes within it—the Sour Creek and Mallard Lake Domes. No actual volcanic eruption has oc- part of the caldera nearly every year from curred in the Yellowstone region for about 70,000 years. However, since the 1970’s, scientists have 1983 to 1998. In the 1990’s, new and revolu- detected signifi cant changes in this remarkable volcanic and hydrothermal (hot water) system, including rapid ground uplift and subsidence and bursts of earthquake activity. tionary satellite-based methods for tracking the Earth’s changing ground surface—the Global Positioning System (GPS) and In- -111.5° -111° -110.5° -110° terferometric Synthetic Aperture Radar (In- SAR)—were applied by University of Utah, A new satellite-based tech- USGS, and other scientists to assemble a more 45° nique known as Interferomet- ric Synthetic Aperture Radar detailed picture of how and when the ground (InSAR) allows direct and moves above Yellowstone’s magma reservoir. precise measurement of the These new data reveal that Yellowstone is vertical changes in ground level. This InSAR image of the in nearly continuous but frequently changing area around the Yellowstone movement—the fl oor of the caldera contin- Caldera (dotted line) shows ued to rise until 1984, stopped rising during vertical changes during the 4-year period 1996–2000. 1984–85, and then subsided for the next 10 ° 44.5 The ringed pattern centered years. Parts of the central caldera began rising northwest of Yellowstone again in 1995, but a more complex pattern Lake is a prominent area of dome-shaped uplift. Each of uplift and subsidence has prevailed since complete cycle of colors in 2000. InSAR data show that between 1995 the color bands represents a and 1997 a large area along the northwest little more than one inch (28.3 mm) of vertical change. Yellow rim of the Yellowstone Caldera, centered near triangles are continuous GPS Norris Geyser Basin, started to rise. The pic- 0 28.3 mm 44° 01020MILES stations; white dots are loca- ture that emerges from all these data is of a Range Change tions of earthquakes in the 01020KILOMETERS period 1996–2000. dynamic system in which the caldera fl oor is in almost constant motion—episodes of uplift 1000 14,000 THE YELLOWSTONE 900 12,000 Diagrammatic caldera subsidence VOLCANO OBSERVATORY 800 (averag Increased scientifi c surveillance of 700 e 19 mm/yr) 10,000 Diagrammatic caldera uplift Yellowstone in the past 30 years has erage 22 mm/yr) R (av eturn to 600 Autumn 1985 swarm p t artial uplif 8,000 detected unmistakable changes in its 500 akes vast underground volcanic system, hqu art e e 6,000 similar to historical changes observed 400 ulativ Cum No at many other large calderas (vol- 300 seismic 4,000 data canic depressions) in the world.