On March 16, 1980, a series of small earthquakes were the first signs of activity on Mount St. Helens. On March 27, after hundreds of additional earthquakes, Mount St. Helens erupted for the first time in over 100 years. The blast created a 200 to 250 foot wide crater through the volcano's ice cap and covered the snow in the southeast sector with dark ash. Within a week, the crater had grown to about 1,300 feet. On average, eruptions occurred about one per hour during March and then decreased to about one per day by April 22, when the first period of activity stopped. Small eruptions resumed from May 7 to May 17. By that time, more than 10,000 earthquakes had shaken the volcano. The north flank had now grown outward by about 450 feet to form a prominent bulge. This bulge grew about 6.5 feet a day. This type of growth provided strong evidence that molten rock (magma) had risen high into the volcano. A cryptodome was also developing beneath the bulge that had yet to erupt onto the surface.

On May 18, 1980, at 8:32 a.m., a 5.1 magnitude earthquake occurred. At the same time, the volcano's bulge slid away in a huge landslide -- actually the largest debris avalanche recorded in Earth's history. Most of the debris went westward as far as 14 miles down the valley of the Toutle River's north fork. The total avalanche volume equaled about 3.3 billion cubic yards. The landslide removed the northern flank, including part of the cryptodome. The removal of this very hot and highly pressurized body of magma resulted in the immediate depressurization of the volcano's magmatic system triggering powerful eruptions that blasted laterally through the debris and removed the upper 1,000 feet of the cone. The blast accelerated the avalanche to at least 300 miles per hour. Within a few minutes, an eruption cloud rose from the former summit's crater, and fifteen minutes later, it had risen to over 15 miles high. Within an hour, the loss of pressure on top of the volcano initiated a Plinian eruption that sent a massive tephra plume high into the atmosphere. Pyroclastic flows poured out of the crater at 50 miles an hour, spreading as far as five miles. The Plinian phase continued for nine hours producing a high eruption column, numerous pyroclastic flows, and ash falling downwind of the eruption. Lahars (surges of water caused by melting snow and ice eroded and mixed with loose rock debris) flowed down the volcano into river valleys, ripping trees from their roots and destroying roads and bridges. The largest and most destructive lahar occurred in the North Fork Toutle River and destroyed bridges and homes before flowing into the Cowlitz River about 50 miles downstream from the volcano. The blast devastated an area of nearly 19 miles from west to east and more than 12.5 miles northward. Within 6 miles of the summit, virtually no trees remained of what was once a dense forest. Two hundred and thirty miles of the devastated area was blanketed by deposits of hot debris carried by the blast. Throughout the day, 520 million tons of ash blew eastward across the U.S. and caused complete darkness in Spokane, Washington. Major ash falls occurred as far away as central Montana, and smaller amounts of ash were observed as far away as the Great Plains.