BEIJING, March 16 (Xinhuanet) -- U.S. scientists say one of the largest supervolcanoes in the world is waking up, media reported Friday.

    The Yellowstone system, which lies beneath America's western states, Wyoming, Montana and Idaho, is active and expected to eventually blow its top, but probably not in the near future. Supervolcanoes can sleep for centuries or millennia before producing incredibly massive eruptions that can drop ash across an entire continent.

    The findings, reported this month in the Journal of Geophysical Research -- Solid Earth, suggest that a slow and gradual movement caused by a giant hotspot of molten rock beneath a volcano can shape a landscape more than sudden ground movements caused by the volcano¡¯s frequent earthquakes.

    "We think it's a combination of magma being intruded under the caldera and hot water released from the magma being pressurized because it's trapped," said lead study author Robert Smith from the University of Utah. "I don't believe this is evidence for an impending volcanic eruption, but it would be prudent to keep monitoring the volcano."

    For the past 17 years, researchers used GPS satellites to monitor the horizontal and vertical motion of the Yellowstone caldera -- a huge volcanic crater formed by a super-eruption more than 600,000 years ago.

    The movement of the caldera indicates what's going on underground where magma, or molten rock, is stored for the next eruption. When magma builds up, some of it starts to rise toward the surface, where it presses against the floor of the caldera. The pressure makes the caldera bulge, while a decrease in pressure makes it sink.

    The 45-by-30-mile caldera bulged and deflated significantly during the study period.

    Data shows that the caldera floor sank 4.4 inches from 1987 until 1995. From 1995 until 2000, the northwest rim of the caldera rose about 3 inches, followed by another 1.4-inch rise until 2003. Then between 2000 and 2003, the caldera floor sank a little more than an inch.

    And then from 2004 to 2006 the central caldera floor rose faster than ever, springing up nearly 7 inches during the three-year span.

    "The rate is unprecedented, at least in terms of what scientists have been able to observe in Yellowstone," Smith said.

    (Agencies)