BILLINGS, Mont. - A newly discovered surface bulge in Yellowstone National Park may be responsible for some unexpected geothermal activity in recent years, according to a study by U.S. Geological Survey scientists. The bulge, about 25 miles across, rose 5 inches from 1997 to 2003 and may have triggered some thermal unrest at Norris Geyser Basin, including a sudden rise in temperatures, new steam vents and the awakening of Steamboat geyser.
The findings are part of a paper set to be published Thursday in the journal Nature.
Charles Wicks, one of the USGS scientists who worked on the study, said much of what happens beneath the park's surface remains a mystery, but more is being learned about the Yellowstone caldera, the huge bowl-shaped collapsed volcano in the middle of the park that last erupted 640,000 years ago.
Geologists discovered the dome on the northern rim of the caldera several years ago, and Wicks and others used satellite images and other tools to track its swelling.
Wicks and his colleagues theorize that molten rock moved out of the caldera and beneath the area of the inflating dome, which has been named the North Rim Uplift Anomaly. The floor of the caldera sank as the molten rock left.
Around the same time, some unusual activity began occurring in and around Norris Geyser Basin, according to the USGS findings.
Steamboat geyser erupted in May 2000 after nine years of dormancy, and then erupted five more times between 2002 and 2003. The nearby Porkchop geyser also sprang to life after 14 years of dormancy.
Ground temperatures at Norris, the hottest and most unstable geyser area in the park, rose so high in 2003 that Yellowstone officials closed some boardwalks out of fear that visitors might be burned.
And just north of Norris near Nymph Lake, a series of steam vents churned and emitted white clouds of gas.
Scientists studying the shore of Yellowstone Lake found that the caldera has been rising and falling for at least 15,000 years, sometimes swinging more than 10 feet.
Henry Heasler, Yellowstone's lead geologist, said research about the heaving caldera could play a role in predicting volcanic activity and help ensure the public's safety.
"We've known that the caldera breathes," Heasler said. "Now we're starting to get a much better idea of those respirations."