1980 eruption of Mount St. Helens

On May 18, 1980, at 8:32 a.m., Mount St. Helens in Skamania County, Washington, United States, experienced a catastrophic explosive eruption which had a volcanic explosivity index of 5. It was the first to occur in the contiguous United States since the much smaller 1915 eruption of Lassen Peak in California. The main eruption was preceded by a series of volcanic explosions, pyroclastic flows, and phreatic blasts beginning in March 1980. It has often been considered the most disastrous volcanic event in U.S. history.

The eruption was preceded by a series of earthquakes and steam-venting episodes caused by an injection of magma at shallow depth below the volcano that created a large bulge and a fracture system on the mountain's north slope. An earthquake at 8:32:11 am PDT (UTC−7) on May 18, 1980, caused the entire weakened north face to slide away, a sector collapse which was the largest subaerial landslide in recorded history. This allowed the partly molten rock, rich in high-pressure gas and steam, to suddenly explode northward toward Spirit Lake in a hot mix of lava and pulverized rock, overtaking the landslide. An eruption column rose 80,000 feet (24 km; 15 mi) into the atmosphere and deposited ash in 11 U.S. states and various Canadian provinces. At the same time, snow, ice, and several entire glaciers on the volcano melted, forming a series of large lahars (volcanic mudslides) that reached as far as the Columbia River, nearly 50 miles (80 km) to the southwest.