The quake at 2:34 a.m. likely triggered the overnight collapse of large section of rock at the north end of the growing lava dome, U.S. Geological Survey scientists reported Sunday from the Cascades Volcano Observatory.

Much of the smooth surface of the ridge, which is created as rock extrudes from the vent, has now been removed by rockfalls over the past few weeks.

After years of quiet, the mountain rumbled awake last September, and in October a flow of molten rock reached the surface, marking a renewal of domebuilding activity that had stopped in 1986.

A deadly eruption in 1980 killed 57 people and sent a river of hot mud and ash down the Toutle River Valley.

USGS and the University of Washington continue to monitor the mountain.

Scientists say a more explosive eruption, possibly dropping ash within a 10-mile radius of the crater, is possible at any time.