The 1980 activity of Mount St. Helens is summarized in U.S. Geological Survey (USGS) Professional Papers 1249 (Foxworthy and Hill, 1982) and 1250 (Lipman and Mullineaux, 1981). The activity began on March 15 with an increasing number of earthquakes beneath the volcano. The first phreatic eruption occurred on March 27, coincident with a high level of seismic activity. A summit crater formed and continued to enlarge for 2 months as phreatic activity continued. Tephra erupted during this time was composed of pulverized old rock, no new magma; however, viscous magma was intruding high into the cone, forming a cryptodome whose surface manifestation was the famous "bulge" on the north flank. This bulge grew outward at a maximum rate of 8.2 feet per day (2.5 meters per day) with no acceleration or other significant change until the climactic eruption.
The eruption at 8:32 a.m. P.D.T. May 18 was apparently triggered by a magnitude 5.1 earthquake that caused the unstable north flank to fail as three great retrogressive landslide blocks. The landslides developed into a complex debris avalanche that sped down the valley of the North Fork Toutle river, reaching its termination 16 miles west of the volcano in about 10 minutes. Unloading of the volcano by these landslides relieved pressure on the cryptodome and its associated hydrothermal system; the depressurized gases violently expanded and generated a northward-directed lateral explosion or blast. A pyroclastic surge (Moore and Sisson, 1981) or flow (Walker and McBroome, 1983) developed from the blast and fanned outward from the volcano, felling trees and killing most wildlife in a 212-square-mile (550-square-kilometer) area. Two columns convectively rose from the devastated zone and joined to reach a maximum height of 16 miles (25 kilometers) by 9:00 a.m.
The landslides and blast removed the upper 1,312 feet (400 meters) of the cone and left a crater 2,050 feet (625 meters) deep, 1.7 miles (2.7 kilometers) long, and 1.3 miles (2.0 kilometers) wide. About 30 minutes after the blast, debris falling from the unstable crater wall and lesser vesiculating dacitic magma from the roots of the cryptodome were explosively ejected in an eruption column that ranged from 9 to 10 miles (14-16 kilometers) in height throughout the morning. Dark-gray ash, consisting largely of lithic debris from this column, fell more than 930 miles (1,500 kilometers) away. The eruption column lightened in color and became more energetic at about noon, possibly as a fresh supply of gas-rich magma reached the surface; pumiceous pyroclastic flows spilled northward from the crater and covered part of the debris avalanche, forming the Pumice Plain. Light-gray magmatic ash from this 9 to 12 mile (14-19 kilometer) column fell on earlier, dark-gray lithic ash in eastern Washington and northern Idaho. The eruptive activity declined and ended that night.
Many mudflows formed on May 18, mostly by melting of snow and glacial ice. The
largest mudflow, down the North Fork Toutle River, formed as the debris
avalanche dewatered (Janda and others, 1981; Voight and others, 1981, 1983).
This flow destroyed or heavily damaged 200 homes and deposited more than 95
million cubic yards (72 million cubic meters) of sediment in the Cowlitz and
Columbia Rivers, where clogged shipping channels required costly dredging
(Schuster, 1981).
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