It's a little like being in a room with the proverbial 800-pound gorilla--every little burp or twitch keeps you on your toes. That's the way geologists feel about the caldera that sits under Yellowstone National Park. Scientists have detected accelerated activity on Earth's surface that they think is originating from deep inside the geological structure known as a hot spot, and they are attempting to determine whether the giant volcano is growing restless or just turning over in its sleep.
Compared to the Yellowstone caldera, the other volcanoes of the western United States are pip-squeaks. The 1980 eruption of Mount St. Helens lofted about 0.3 cubic kilometer of debris into the air and buried much of the surrounding countryside. But 640,000 years ago, a Yellowstone eruption ejected approximately 250 cubic kilometers of material, blanketed most of the western half of the country in ash up to 20 meters thick, and left a crater that now contains Yellowstone Lake and extends 2400 square kilometers. The geologic hot spot in Earth's mantle that drives the periodic explosions remains active, so whenever scientists detect something new going on under Yellowstone, they take notice.
In the 9 November issue of Science, a team from the University of Utah in Salt Lake City and the U.S. Geological Survey (USGS) in Menlo Park, California, reports that the Yellowstone Valley is now rising more than 3 times faster than usual. Using satellite radar surveys and a growing array of Global Positioning System stations around the caldera, they have tracked the valley's rate of uplift going from about 2 centimeters per year--the average pace from 1923 until 2004--to 7 centimeters annually over the past 3 years. "We've had a marked change," says geophysicist and co-author Robert Smith of the University of Utah.
The team is quick to reassure that no eruption looms; there are no seismic harbingers. Instead, earthquake activity in the valley has dropped by half to fewer than 1000 tremors a year. Somehow, faster uplift and fewer earthquakes are related to what's going on some 80 kilometers below Yellowstone, where a plume of magma fuels the area's famous geothermal activity. By working to link the processes that drive Yellowstone, Smith says, scientists can learn more about the forces that govern many of Earth's seismic systems.
Such comparisons of calderas would be valuable, says David Hill, a seismologist with USGS who was not involved with the study. Hill agrees that Yellowstone's sudden rise probably doesn't mean an imminent eruption. The caldera in Long Valley, California--which he has been watching for many years--experienced a 10-centimeter uplift within 6 months in 1997, accompanied by a swarm of earthquakes, but then it settled down and has been quiet ever since.