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PORTLAND, Oregon (Reuters) -- A large, slow-growing volcanic bulge in western Oregon is attracting the attention of seismologists who say that the rising ground could be the beginnings of a volcano or simply magma shifting underground.
Scientists said that the 100 square-mile bulge, first discovered by satellite, poses no immediate threat to nearby residents.
"It is perfectly safe for anyone over there," said Michael Lisowski, geophysicist at the United States Geological Survey's Cascades Volcano Observatory in Vancouver, Washington.
The bulge is rising at a rate of about 1.4 inches per year, according to a report issued by the U.S. Geological Survey.
The bulge is in a sparsely populated area 3 miles southwest of South Sister, a mountain 25 miles west of Bend, Oregon.
Lisowski said the unnamed bulge was created because of a big cavity, estimated to be about 4.5 miles below the surface, that is filling with fluid.
The fluid is likely magma, but could also be water. It was described in the report as a lake 1 mile across and 65 feet deep.
The bulge is a bare patch of land with no residents, and anyone in the area would not be able to see, feel or smell anything, seismologists said.
South Sister is one of three volcanic peaks called The Three Sisters, which are part of the Cascade mountain range. The range includes four of the 18 most active volcanoes in the United States, according to the U.S. Geological Survey.
The South Sister probably erupted last time about 2,000 years ago, seismologists said.
Further north, the 1980 eruption of Mount St. Helens killed 57 people, destroyed at least 230 square miles of forest and spewed ash for hundreds of miles.
Mount St. Helens has rumbled back to life recently, spitting lava, rocks and ash, but has not had another big eruption.
A lava dome is growing in the huge crater created in Mount St. Helens, but that event appears to be unrelated to the South Sister bulge, seismologists said.
"Growth of the new lava dome inside the crater of Mount St. Helens continues, accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash," the U.S. Geological Survey said in a daily report.
Scientists said they would continue to monitor the bulge, most likely over a number of years.
"We haven't seen anything like this in the Cascade range," Lisowski says, "although we have only been looking in the last 20 years."
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