A vast "plume" of hot rock, which scientists say is responsible for Yellowstone's famous geysers and other geothermal features, is coming into sharper focus in research coordinated by University of Utah scientists that sheds new light on our planet's inner workings.

Geophysicist Robert Smith and colleagues monitored seismic waves from 800 earthquakes to assemble the most complete image ever published of the 500-mile "plumbing" system under Yellowstone National Park, suggesting its bottom is at least 400 miles deep in the Earth's mantle, directly under the southwest Montana town of Wisdom.

The plume's angled rise offers proof that the mantle, the 1,800-mile thick rocky shell surrounding the Earth's core, is in motion, Smith said.

The mysterious plume and a related magma chamber just a few miles below Yellowstone are the subject of four papers published last month in the Journal of Volcanology and Geothermal Research.

Smith presents the findings Friday at the American Geophysical Union meeting in San Francisco.

One of the papers, co-authored with two of Smith's former graduate students, Anthony Lowry and Katrina DeNosaquo, used gravity measurements to determine the magma chamber is 20 percent larger than previously believed.

The chamber, which Smith describes as a banana-shaped reservoir with its ends pointed upward, runs the entire 40-mile width of the Yellowstone caldera southwest to northeast. (A caldera is the crater-like basin of a volcano.)

The newly imaged part extends for an additional 10 miles beyond the caldera's northeast edge, possibly following an ancient fault line, said DeNosaquo, who left the U. to work for the petroleum industry after earning her master's degree in 2007.

It is possible, but not a certainty, that Yellowstone's next cataclysmic eruption will be larger than previously thought, DeNosaquo said. This super-volcano has blown about every 650,000 years, the last eruption being 642,000 years ago. But scientists have no way of predicting the next event, which would bury much of the West in ash.

Scientists believe chunks of hot rock break off the plume to feed the magma chamber that extends from 4 to 10 miles below the surface. This reservoir is about 8 to 15 percent molten rock, while the plume is about 1 to 2 percent melt, Smith said.

The main plume study, the fruit of the Yellowstone Geodynamics Project and funded by the National Science Foundation, deployed up to 200 seismograph stations spaced 10 to 22 miles apart to record seismic waves passing by Yellowstone between 1999 and 2005.

Such waves move more slowly through hot rock, so scientists were able to chart the edges of the plume by recording differences in wave speeds through the crust and mantle below Yellowstone. These observations are made possible by technological advances in seismic instrumentation and data integration.

"We are in whole new realm of modern seismology," Smith said. "It allows us to see things we've never seen before."

The source of the Yellowstone hotspot has been the subject of scientific controversy, with some contending it is the result of shallow churning in the upper mantle. Smith believes the new research will put to rest doubts about the plume, seen in the new map as an irregularly shaped tube 45 miles in diameter.

It appears to start at a depth of 410 miles, the base of the "global transition zone" separating the Earth's upper and lower mantles, although it could originate from a much deeper depth, according to Smith, a U. professor emeritus who has devoted a career to understanding the volcanic activity under the world's first national park.

The plume angles up to place 50 miles below the Yellowstone caldera where it hits cooler surface rock under the Earth's crust and spreads out to the west like a 300-mile pancake.

"The plume is tilted [downward] to the west at a 60-degree angle. That surprised us," Smith said. "This plume material wants to come up vertically, it wants to buoyantly rise. But it gets caught in the 'wind' of the upper mantle flow, like smoke rising in a breeze."

Smith believes the plume's position indicates this mantle "breeze" moves horizontally to the east two inches a year.

Most of the world's volcanoes occur along the periphery of continental plates. But Yellowstone's hotspot has produced a super-volcano in the North American interior, giving scientists a unique "window" into the Earth. Other famous hotspots are under oceans, giving us the Hawaiian Islands and Iceland.