Powerful earthquakes rocked Mount St. Helens 25 years ago today, occurring in such quick succession that seismologists had trouble telling one from another.

Twenty-two quakes of magnitude 4.0 or greater happened in an eight-hour period, shaking avalanches of ice and snow off the sides of the mountain.

Reporters and photographers from around the country were beginning to converge on the area. So many airplanes and helicopters buzzed around the peak that the Federal Aviation Administration today imposed special flight restrictions.

The U.S. Forest Service restricted skiers, climbers and snowmobilers to below the tree line and closed many logging roads. People living at the base of the mountain were warned that the steady barrage of earthquakes could send avalanches tumbling down on them.

David Johnston, a U.S. Geological Survey volcanologist on vacation in Seattle from his post in Menlo Park, Calif., reported for duty at the University of Washington seismology center.

Editor’s note: Twenty-five years ago this month, the volcano Mount St. Helens awoke. Early bursts of steam and ash climaxed May 18 with the most violent volcanic eruption in the United States in the 20th century. When it was over, 57 people were dead, 234 square miles of forest had been destroyed and an ash cloud blocked out the sun over half the state. Today we start a chronicle of the events leading to that day.