40,000 years ago: Mount St. Helens erupted onto the scene, the youngest of the five steep-sloped volcanoes in the Cascade range.
4,000 years ago: Most of the volcano visible today was built from eruptions since then, making it the most active volcano in the Cascades.
3,600 years ago: An eruption occurred that was roughly four times as large as the May 18, 1980, event.
1792: Capt. George Vancouver names the volcano after the British ambassador to Spain, Alleyne Fitzherbert, also known as Baron St. Helens.
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1847-57: Explorers report eruptions of Mount St. Helens
March 20, 1980: A shallow 4.1 magnitude earthquake rumbles beneath Mount St. Helens, signaling its latest reawakening. Earthquake activity increases over the next few days.
March 27, 1980: A loud boom and a puff of ash and smoke mark the first eruption in 123 years.
April 30, 1980: A growing bulge on the north flank of the volcano concerns scientists.
May 18, 1980: A 5.1-magnitude earthquake a mile beneath the summit triggers the largest landslide in recorded history, followed by a lateral blast and mudflow that leveled 230 square miles of forest, killed 57 people and caused billions of dollars in property damage. An ash plume reached 80,000 feet in less than 10 minutes and the ash fallout covered 22,000 square miles.
1980-86: Lava from ongoing eruptions adds 97 million cubic yards of material to the crater floor.
2004-2008: An additional 125 million cubic yards of material spills onto the crater floor, enough to pave seven highway lanes three feet thick from Portland to New York City. Since 1980, about seven percent of the crater created by the 1980 eruption has refilled with lava.
February 2008-present: Mount St. Helens has been quiet.
Source: United States Geological Survey