Pilots reported the plume rose between 16,000 and 20,000 feet in the air, scientists at the Cascades Volcano Observatory said.

The rockfall coincided with a magnitude 3.1 earthquake shortly after 9 a.m. Monday at the mountain, scientists said. Such events are expected during growth of the lava dome, they said.

"There is no evidence of an explosion associated with this event," the observatory said in a statement.

Clouds obscured the crater at the time.

"We don't know how much steam and how much ash," Cynthia Gardner, scientist in charge at the observatory, told The Columbian. "These are very short-lived events."

Lava has continued to push into the crater - most recently forming a sheer rock fin - since the 8,364-foot mountain reawakened with a drumfire of low-level seismic activity in September 2004.

The crater was formed by the volcano's deadly May 18, 1980, eruption that killed 57 people and blasted about 1,300 feet off the then-9,677-foot peak.