"There seems to be some movement in the lava dome," said Jeff Wynn, chief scientist at the U.S. Geological Survey's Cascade Volcano Observatory in Vancouver, Wash., about 50 miles south of the mountain.
The pressure could come either from a buildup of gases within the 8,364-foot volcano, which erupted with devastating force in 1980, or from molten rock moving into the dome, Wynn said. The volcano began stirring again last week.
Seth Moran, a seismologist at the observatory, estimated the initial movement at 4 centimeters, about an inch and a half.
Wynn said the movement "sort of suggests that we're getting closer" to an eruption that could hurl rocks and ash a few thousand feet into the air.
He emphasized that the estimates were highly preliminary and inexact because there is only one measuring device on the dome, estimating scientists will need about 48 hours to interpret the data more clearly.
Scientists were keeping a close eye on the 925-foot-tall dome of hardened lava that has grown inside the crater since the May 18, 1980, eruption that blew the top off the mountain.
Swarms of tiny earthquakes — more than 1,000 since the mountain began stirring last Thursday — have gradually increased, cranking up to a level not seen since 1986, when the volcano's last dome-building eruption occurred.
But neither the earthquake activity nor the apparent growth in the dome indicated a major eruption was likely, Wynn said. He predicted a relatively small explosion of rocks, ash and steam within a few days.
On Tuesday, the quakes were occurring at a rate of two or three a minute. The volcano was releasing three to four times the energy it was releasing Monday, Wynn said.
Scientists are trying to determine if the quakes are caused by steam from water seeping into the dome or by magma moving beneath the crater.
Early tests of gas samples collected above the volcano by helicopter Monday did not show unusually high levels of carbon dioxide or sulfur, which could indicate the movement of magma.
Seismologist George Thomas at the University of Washington said that on a scale of zero to 10, with 10 being the explosion at the mountain in 1980, the current activity would rate a one. Thomas said any rocks, ash or steam coming out of the volcano would most likely be contained within the crater itself.
"The alerts we're sending out are just to protect hikers and scientists doing research within the crater," he said.
The USGS issued a notice of volcanic unrest on Sunday. U.S. Forest Service officials closed hiking trails above the tree line at 4,800 feet. The visitors center and most other trails at the Mount St. Helens National Monument have remained open.
The volcano's 1980 blast killed 57 people, spawned mud flows that choked the Columbia River, leveled hundreds of square miles of forests and showered distant communities with volcanic ash.