Graphic By Wu-lung Chang
Shape of the magma chamber can be seen under outline of the Yellowstone caldera. The red slice is magma moving upward recently.

In the 2005 film "Supervolcano," the Yellowstone caldera erupts, disastrously spreading deep volcanic dust throughout the West. The film's tag line was, "This is a true story. It just hasn't happened yet."

The Yellowstone region really is a gigantic caldera, left by previous supervolcano eruptions. Stretching back to 4 million years ago, Yellowstone supervolcano blasts covered what's now the western half of the continental United States with inches to feet of volcanic ash, according to the University of Utah.

But could it happen again? Almost certainly not in the lifetime of anyone alive today. But a new study shows that magma deep beneath the ground surface, which could be the source of a future eruption, is rising at a record rate.

The caldera floor, the ground of Yellowstone, has risen about 3 inches a year for the past three years. The rise is more than three times greater than the fastest observed since monitoring began in 1923, the study says.

"It's a surprising finding to find that something has changed by a factor of three over historic trends," said Robert B. Smith, professor of geophysics at the University of Utah and lead author of the study. "It's very key, scientifically."

According to a study that Smith and colleagues published in the Nov. 9 edition of the magazine Science, a wide layer of magma has been rising. The hot rock drifts upward because it is less dense than the medium around it and thus buoyant; Smith compared it to a bubble going up through a bottle of honey.

Besides Smith, researchers on the team were postdoctoral associate Wu-Lung Chang, doctoral students Jamie M. Farrell and Christine Puskas of the U., and geophysicist Charles Wicks of the U.S. Geological Survey in Menlo Park, Calif.

About six miles beneath the surface, hot melted rock is moving into the magma chamber, causing the ground to deform. Most of the gigantic Yellowstone caldera is affected. The caldera is around 37 miles long by 25 miles wide; the area of ground deformation is about 37 miles by 19 miles.

The molten rock is shaped like a mattress, about 38 miles long and 12 miles wide "but only tens or hundreds of yards thick," says a U. press release.

In a telephone interview, Smith said the magma is moving in around the middle part of Earth's crust. It is by no means near the surface, he added, "but it's down where we seismically imaged a magma chamber."

Surface deformation was measured through GPS readings and measurements by a European Space Agency satellite that aimed radar at the caldera floor.

The Yellowstone supervolcano, formed because of geological plate tectonics, erupted most recently about 640,000 years ago. Meanwhile, the floor of the caldera rises and drops like, Smith said, "a living, breathing system."

The system's heat is what causes the famous geysers and hot pots of Yellowstone National Park.

Asked what the chances are for a gigantic eruption in the lifetime of people alive today, Smith said, "Exceedingly unlikely."

Before that happened, earth scientists would expect many earthquakes. Yellowstone has been the location of mighty quakes in the past, such as the 7.3-magnitude 1959 Hebgen Lake disaster in Montana that killed 28 people. But lately, the ground hasn't been as shaky.

Seismicity "has in fact relatively decreased in Yellowstone over the past two years," after the magma rise started. If an eruption were due soon, he would expect much more seismic activity, and for it to be based at a much more shallow level, two to five miles below the surface.

"We'd have deformation and it might be more concentrated" and happening at a faster rate, he said.

The supervolcano might not be about to go off, but "this is exciting science," said Smith.

"This is the first observation of something this big."