Yellowstone: Monitoring the Fire Below

Signs of Restlessness

he magma chamber responsible for Yellowstone’s volcanic activity is invisible and inaccessible, buried about 8 km beneath the surface. Therefore, its geologic moves must be analyzed indirectly. A hundred or so researchers from universities, the USGS, and other institutions check satellite data, seismometers, Global Positioning System (GPS) stations, temperature gauges, infrared cameras, gas sampling apparatus, and other equipment to monitor Yellowstone’s every geologic move. A spike in activity at any one of the monitored aspects may mean nothing. But if a number of features collectively and rapidly—over days or weeks—act strangely, the Yellowstone Volcano Observatory (YVO) would issue an alert of volcanic unrest. This year, the YVO adopted the new USGS standard alert-level system, which assesses the volcano at one of four degrees: from Normal (no eruptions), through Warning (highly hazardous eruption underway or imminent).

Yellowstone Volcano Observatory scientist-in-charge Jake Lowenstern captures gases rising from a thermal feature at Norris Geyser Basin.

David Rasmussen


The Monitoring—Walking the ground at Yellowstone is like treading on the back of a giant, lumbering beast. The caldera floor rises and falls over the years, indicating that fluids—maybe magma, maybe gas, maybe water and steam—are moving in and out of the rock beneath Yellowstone. “We watch the GPS stations to see how they’re moving north, south, east, west, up, or down over time,” says Jake Lowenstern, a USGS geologist who heads the Yellowstone Volcano Observatory. A newer system called Interferometric Synthetic Aperture Radar (InSAR) uses satellites to measure uplift and subsidence. Once or twice a year, the European Space Agency’s ERS-2 satellite passes over Yellowstone and beams out radio waves. By measuring the time of the return signal, precise changes in the ground level relative to the satellite can be detected and then checked against the GPS data.

Recent Unrest—Currently, the floor of the Yellowstone caldera is in an uplift phase. From October 2004 through May 2006, it rose a maximum of 10 cm at both the Yellowstone Lake and the White Lake GPS stations. The Yellowstone Volcano Observatory considers the shift business as usual for such an active volcano, warranting a “Normal” alert level.


The Monitoring—Each year 1,000 to 3,000 earthquakes shake Yellowstone. These vibrations are caused by rock breaking in the volcanic system and by movement along faults. The molten magma’s pressure on the brittle rock above the chamber can trigger strain and breakage, as can high-pressure hydrothermal fluids winding through fractures.

“When you see a series of earthquakes get shallower and shallower, that means fluids are rising,” says Bob Smith, a geophysics professor at the University of Utah who runs Yellowstone’s seismographic network. Major shifts, like when the floor of the Yellowstone caldera switched from rising to falling in 1985, can be accompanied by intense swarms of earthquakes. Indeed, unusual earthquake activity is the first place scientists look for clues of upcoming eruption.

Recent Unrest—The last destructive earthquake in the area, a magnitude 7.5, was at Montana’s Hebgen Lake in 1959. More than three swarms have occurred since 1985; the last was in 2004 when 400 earthquakes rumbled the northwest section of the park over a three-day period. Many small earthquakes (magnitudes 1, 2, 3, or 4) occur at Yellowstone every day, some strong enough for people near the epicenters to feel. Geologists consider Yellowstone’s recent seismic activity to be at “background levels”: i.e., typical for such a geologically active area. To learn about Yellowstone’s latest earthquakes, visit the University of Utah’s Recent Earthquake Activity site.

A typical roadside sign in Yellowstone National Park.

David Rasmussen


The Monitoring—The daily air temperature at Yellowstone averages a near-freezing 2.2 degrees C. But the magma chamber can heat the soil to upwards of 100 degrees C, and the groundwater flowing into springs, geysers, and mud pools can be beyond boiling. Last summer, on separate occasions, three Yellowstone visitors scalded themselves by walking off-trail and unwittingly into three of Yellowstone’s 10,000 thermal features.

Rising surface temperatures could mean that magma is inching closer to the surface, or that underground heat and fluid flow has shifted in some manner. It also can be a tip-off to groundwater pressure changes, which can alter the boiling point of underground fluids. Over large areas, surface temperature is detectable by satellite. But in 2002 a research team flew an infrared camera aboard a twin-engine aircraft over Yellowstone Park to get a initial sense of its surface heat patterns in higher resolution. “It’s like sensing the temperature of skin—in this case, the “skin” of the hot ground or hot water coming off of thermal features,” says park geologist Cheryl Jawoworski.

The black cylinder in the belly of this twin-engine plane is an infrared camera that senses emitted subsurface heat. Utah State University graduate student Deepak Lal oversaw the most recent thermal data flights over Yellowstone’s Upper, Midway, and Lower Geyser Basins.

David Rasmussen

In fall 2005, Jawoworski and National Park Service colleagues commissioned flights over the park’s geyser basins, which include Old Faithful, to establish a solid set of baseline measurements for Yellowstone’s heat flux. The team will be able to compare future heat maps to this baseline to detect temperature changes and alert visitors to danger zones before they learn the hard way. Other scientists are testing another novel technique to image Yellowstone’s heat aerially: infrared cameras strapped to 2 m wide helium balloons.

Recent Unrest—In summer 2003, a portion of Norris Geyser Basin turned up the heat. Ground thermometer measurements revealed that cool soil along the Back Basin Trail shot up to 94 degrees C. A new mud pot also formed, spurting hot, acidic ooze onto the trail. These events indicated changes not in magmatic activity, but in underground water pressure. By looking back at the heat patterns on the 2002 aircraft map, park staff decided to reroute the Back Basin Trail and add wooden boardwalks to prevent accidents on hot ground. Jawoworski and her colleagues are still analyzing the results from the fall 2005 baseline flight.


The Monitoring—“We know that magmas are down there, they're putting off a lot of heat,” says Lowenstern. “They also put out a lot of gas. And one way that geologists and volcanologists can understand what magmas are doing is to look at that gas.” Lowenstern and his colleagues manually collect these gases in glass bottles, going face to face with acrid emissions of carbon dioxide, sulfur, helium, and hydrogen sulfide. They find the gases dissolved in the hot waters around Yellowstone and percolating from soils and hissing steam vents called fumaroles.

Bison at Yellowstone.

Analyses can reveal the chemical compositions and sources of the gas, whether from magma or shallow groundwater. By sampling gases 30 times a year in different locations, Lowenstern and his colleagues can understand what areas of Yellowstone are more affected by the magma chamber. Knowing if any unhealthy gases are building in particular areas also helps inform visitor safety.

Recent Unrest—A few months before the temperatures increased at Norris, a line of new fumaroles opened in a forested portion of the basin. Beyond this, no major changes in gas activity have occurred recently at Yellowstone. But even normal gas emissions can prove deadly in concert with certain weather and geographical conditions. In March 2004, biologists discovered five dead bison near the Gibbon River in the Norris Geyser Basin. The bison were lying on their sides with their legs straight out. Gas testing and blood and lung samples revealed that gas vents uphill from the bison emitted denser-than-air carbon dioxide and hydrogen sulfide, which stagnated in the low river valley on a cold, windless night. The group of bison roaming into the valley was asphyxiated en masse.

Related Links

   USGS Yellowstone Volcano Observatory: Volcano Monitoring at Yellowstone National Park

   USGS Fact Sheet: Tracking Changes in Yellowstone's Restless Volcanic System

   USGS Volcano Hazards: Monitoring Volcanic Gases

   USGS new common alert-level system for volcanoes