Much of Yellowstone National Park is a giant collapsed volcano, or a caldera. In an enormous eruption roughly 640,000 years ago, this volcano spit out around 240 cubic miles of rock, dirt, magma and other material. Around 70,000 years ago its last eruption filled in that gaping hole with flows of lava. The area has enjoyed an uneasy peace since then, the land alternately rising and falling with the passing decades. New satellite data indicate that this uplift and subsidence is caused by the movement of magma beneath the surface and may explain why the northern edge of the park continues to rise while the southern part of the caldera is falling.

Charles Wicks, Daniel Dzurisin and their colleagues at the U.S. Geological Survey studied radar images of the caldera captured by the European Space Agency's ERS-2 satellite during two passes over the park. Using a technique called interferometry--whereby radar measurements from two different vantage points are combined to give a measure of height--the scientists confirmed measurements on the ground that showed the land rising. But the images also revealed that a roughly 12-mile-wide circle of land centered at the northern rim of the caldera is still rising while land to its south is sinking. The source of that uplift, according to data revealed in today's Nature, lies more than seven miles underground.

Therefore, magma movement must be the cause of the rise and fall, Dzurisin explains. "It's just too deep to be caused by pressurization of the hydrothermal system," he says. "A small amount of magma has either moved up or been intruded to a depth of [seven miles] or perhaps it was already there and it's been pressurized."

Although previous studies had hinted at new magma moving beneath Yellowstone, this represents the first compelling evidence, according to Dzurisin. Such magma movement would also explain recent surface phenomena including new cracks and hot springs as well as the more frequent eruption of Steamboat Geyser. "If you do pressurize or increase the volume of a source [seven miles] deep, you put the ground in tension and that would be conducive to new fractures giving access to the surface for hot waters that previously hadn't had that access," he adds.

This new magma does not mean that Yellowstone will erupt again in the near future; much more significant signs such as more earthquakes, more focused ground deformations and the escape of volcanic gases would point to that. But it does point to continued activity at one of the world's largest volcanic systems. "We don't know if the next event will be a continuation of the series of lava flows that filled in the caldera or the beginning of a new cycle that will create a new caldera," Dzurisin says. "Eruptions are far enough apart that there is a very low probability of the next eruption happening in our lifetimes or anytime soon. The flipside is: the system has been active for millions of years and it's going to erupt again sometime."