Recent satellite pictures of Yellowstone National Park in Wyoming from 2004 through 2006 are showing that an ancient volcano is starting to rise once again.

Molten rock is currently pushing up the remains of the volcano's caldera, which sits over the top of Yellowstone lake, but scientists are stressing that there is no immediate threat of an eruption or explosion. The molten rock field is estimated to be the size of the city of Los Angeles, California.

"There is no evidence of an imminent volcanic eruption or hydrothermal explosion. That's the bottom line. A lot of calderas worldwide go up and down over decades without erupting," said Robert Smith a seismologist from the University of Utah in a statement to the press.

The images show that between 2004 and 2006, the volcano's caldera rose 2.8 inches per year for a total rise of seven inches. This is the quickest rise ever observed from a caldera in Yellowstone since observations began in 1923.

Scientists are not sure how long the event has been happening and they are also not sure if the rise will cause an eruption in the distant future.

"Our best evidence is that the crustal magma chamber is filling with molten rock. 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," added Smith.

Volcanic activity in Yellowstone heats up underground water which create geysers of superheated water, and is responsible for the creation and continuous eruption of Old Faithful.