Yellowstone: Monitoring the Fire Below
Signs of Restlessness
The 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.
UPS AND DOWNS
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.
EARTHQUAKES
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.
HEAT
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.
