The largest eruption of Mount St. Helens in recent history occurred on May 18, 1980, when a cataclysmic eruption removed most of the northern face of the mountain. At 8:32 a.m. PDT, the summit collapsed into a rock debris avalanche that released a lateral gas explosion.
The explosion knocked down forests for 150 miles, and the avalanche spread out over a 14-mile area. The volcano, which has been growing for 40,000 years, has had dozens of large and hundreds of small eruptions in its history. These have generally been grouped into six or more named activity periods prior to the modern period.