Mount St. Helens Eruptive Activity, 1980-1984

The Eruption

A magnitude 5.1 earthquake on May 18 (8:32 a.m. PDT) shook loose the steepened bulge on the volcano's north flank, resulting in the largest known landslide in historic time, 2.3 cubic km (0.56 cubic miles). The entire north flank was described by an aerial observal as "rippling" and "churning" moments before "the north side of the summit began sliding north along a deep-seated slide plane."

As the avalanche reached the north base of the cone, the topography it encountered caused it to be divided into three sections:
  1. Part of the avalanche slid into Spirit Lake, raising the lake bed roughly 180 feet, and damming its natural outlet. Water displaced by the avalanche surged up the surrounding hillslopes, washing the blown-down timber from the lateral blast into the lake.

  2. Part of the avalanche "ramped" up and over a 1,200 foot high ridge five miles north of the volcano (Johnston Ridge) depositing debris on top of the ridge and in the South Colwater Creek drainage.

  3. The bulk of the avalanche was deflected westward down the North Fork of the Toutle River valley. The front of the avalanche traveled a distance of 15 miles in about 10 minutes. The resulting deposit covers the valley floor to an average depth of 150 feet, but it is more than 500 feet deep in a few places (such as 1.5 miles west of Harry Truman's Lodge).

The hummocky avalanche deposit covers a total area of about 24 square miles. It consists of intermixed volcanic debris of various sizes, including blocks, pebbles, sand and silt, and blocks of glacial ice.

Continue to Lateral Blast

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Eruptive Activity

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Mount St. Helens