The 1980 eruption of Mount St. Helens (VEI = 5, 1.2 km3 of material erupted) is the most significant volcanic eruption to occur in the lower 48 U.S. states in recorded history, exceeding the destructive power and volume of material released by the 1915 eruption of California's Lassen Peak (the 1912 eruption of Novarupta in Alaska was the most powerful historic eruption in the U.S., although Alaska was not a U.S. state at the time). The eruption was preceded by a two-month-long series of earthquakes and steam-venting episodes, caused by an injection of magma at shallow depth below the mountain which created a huge bulge and a fracture system on Mount St. Helens' north slope. An earthquake at 8:32 AM on May 18, 1980, caused the entire weakened north face to slide away, suddenly exposing the partly molten, gas- and steam-rich rock in the volcano to lower pressure. The rock responded by exploding into a super-heated mix of pulverized lava and older rock that sped toward Spirit Lake so fast that it quickly passed the avalanching north face.

A volcanic ash column rose high into the atmosphere and deposited ash in eleven U.S. States. At the same time, snow, ice, and several entire glaciers on the mountain melted, forming a series of large lahars (volcanic mudslides) that reached as far as the Columbia River. Less severe outbursts continued into the next day only to be followed by other large but not as destructive eruptions later in 1980. By the time the ash settled, 57 people (including innkeeper Harry Truman and geologist David A. Johnston) and thousands of animals were dead, hundreds of square miles (mi²) reduced to wasteland, over a billion U.S. dollars in damage had occurred, and the once-graceful face of Mount St. Helens was scarred with a huge crater open to the north. The area was later preserved, as it was, in the Mount St. Helens National Volcanic Monument.