WASHINGTON (Reuters) - A big blob of molten rock appears to be pushing up remnants of an ancient volcano in Yellowstone National Park in Wyoming, scientists reported on Friday.

The Old Faithful geyser is seen in Yellowstone National Park in a 2002 handout photo from the National Park Service. A big blob of molten rock appears to pushing up remnants of an ancient volcano in Yellowstone National Park in Wyoming, scientists reported on Friday. REUTERS/NPS/Handout

They say no volcanic explosion is imminent -- that already happened 642,000 years ago, creating the volcanic crater known as a caldera where part of Yellowstone Lake sits.

But satellite readings show just how volcanically active the area remains, the researchers reported in the journal Science.

From the middle of 2004 through 2006, the floor of the caldera rose 7 inches at a rate of 2.8 inches a year -- the biggest rise ever measured, they reported.

“There is no evidence of an imminent volcanic eruption or hydrothermal explosion. That’s the bottom line,” University of Utah seismologist Robert Smith said in a statement.

“A lot of calderas worldwide go up and down over decades without erupting.”

Yellowstone is North America’s largest volcanic field, produced by what is known as a hotspot, a plume of hot and molten rock squirting up from 400 miles beneath the planet’s surface.

Monstrous eruptions took place there starting 2 million years ago but activity bubbles along much more calmly now -- akin to similar volcanic fields such as the Campi Flegrei just outside Naples in Italy.

Beneath the field lies what is known as a magma chamber, which is actually similar to a wet sponge in structure.

“Our best evidence is that the crustal magma chamber is filling with molten rock,” Smith said. “But we have no idea how long this process goes on before there either is an eruption or the inflow of molten rock stops and the caldera deflates again.”

Heat from the chamber warms the park’s hundreds of hot springs and geysers, including “Old Faithful,” perhaps the world’s best-known geyser.

Established in 1872 as the first U.S. national park, Yellowstone also stretches to parts of Montana and Idaho.