March 1980 — The volcano began to show signs of unrest. Earthquakes and steam eruptions continued for several weeks.

8:32 a.m., May 18, 1980 — The volcano exploded in a massive eruption and landslide, flattening 230 square miles of forest and killing 57 people. A plume of ash extended 15 miles into the sky and coated towns 250 miles away.

Summer 1980-October 1986 — Repeated minor eruptions built a 925-foot-tall dome of hardened lava inside the crater left by the eruption.

1982 — Congress and President Reagan created the 110,000-acre Mount St. Helens National Volcanic Monument for research, recreation and education. Inside the monument, the environment was left to respond naturally to the eruptions.

1998 — The first major seismic activity since 1986 occurred, with earthquakes located as deep as 6 miles forcing magma to within about a mile of the dome, scientists said.

2001 — Another flurry of small earthquakes struck, but once again, no magma surfaced.

Sept. 23, 2004 — The first of thousands of tiny, shallow earthquakes were recorded.

Sept. 26 — The U.S. Geological Survey declared a notice of volcanic unrest, closing the crater and upper flanks of the volcano to hikers and climbers.

Oct. 1 — The mountain briefly belched smoke and ash. The quakes subsided but resumed a short time later. Four more steam and ash explosions occurred through Oct. 5.

Oct. 11 — Molten rock reached the surface, marking a new period of dome-building that had stopped in 1986.

March 8, 2005 — An explosion shot gritty volcanic ash to about 36,000 feet.

May 6 — The Johnston Ridge Observatory, five miles from the mountain, reopened. It had been closed since Oct. 2.

Today — The 25th anniversary of the 1980 eruption.

Sources: Mount St. Helens National Volcanic Monument, U.S. Geological Survey

The Associated Press