A new lava dome or plug may be forming in the crater of Mount St. Helens, scientists said yesterday, after the volcano cleared its throat Thursday with its fifth eruption since it exploded to life May 18.

Two quick "pulses" blew steam and ash two miles into the air yesterday morning.

"Geologists report what appears to be a dome forming on the inner crater," said Don Faulkner, a spokesman for the Federal Emergency Management Agency.

Formation of a dome could bottle up pressure inside the volcano and prepare it for another eruption.

"The dome might be like a seal," said Elliott Endo, a U.S. Geological Survey geophysicist at the University of Washington. "At some stage it could be removed by a large eruption to free up the conduit system. We saw it before."

"No one knows enough about the mechanics of an eruption to say for sure," Endo said. "But a dome building suggests volcanic activity continuing for a bit."

Thursday's eruptions sent steam and ash eight miles into the sky and released "pyroclastic flows" -- fast-moving clouds of dense, superheated gas and volcanic ash -- down the mountain's north slope and left a light ash dust over parts of Washington.

The repeated eruptions are becoming commonplace to some Washington residents.

"We don't get as upset as we used to," said Lee Thomas, a civil deputy with the Skamania sheriff's office in Stevenson, about 30 miles southeast of the volcano. "It doesn't create quite the feeling among people it used to. They don't become quite as concerned."