The land is rising because magma and hydrothermal fluids are migrating into the volcano's underground chambers, Smith explained.
"It's really kind of a sponge, where you have interlaced open spaces with magma and solid rock between. Only 10 percent [of the chambers are] actually made up of molten rock."
The current uplift was detected by global positioning system stations and radar instruments on an orbiting satellite.
Kenneth Pierce of the U.S. Geological Survey in Bozeman, Montana, has studied ancient water levels in Lake Yellowstone to track elevation changes in the underlying portions of the caldera (see a map of Yellowstone National Park).
He said the newly detected upswell is part of a natural cycle that is common to calderas.
"Since about 14,000 years ago, the Yellowstone caldera has inflated and deflated about six to eight times without a volcanic eruption," he said.
Smith agreed. "Calderas go up and down," he said. "We use the term 'restless' to describe these systems."
But, he added, "occasionally they burp."
When the volcano might "burp" next isn't clear, because highly accurate, modern instruments have only been around for a few years, Smith explained.
"It's hard to predict what's going to happen in the future when you have such a short record," Smith said.
One thing that is certain is that the Yellowstone volcano isn't dead.
Many scientists believe the volcano is produced by a "hot spot" in the Earth's mantle, a plume of hot rock rising from hundreds of miles below.
Since the hot spot first rose 16 million years ago, it has produced at least 140 eruptions throughout the northwestern United States, Smith said.
To gauge any future volcanic activity, the park is continuously monitored by instruments associated with the Yellowstone Volcano Observatory, which is jointly administered by the University of Utah, the U.S. Geological Survey, and Yellowstone National Park, he added.
"These provide information of public safety, daily [and] in real time."
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES