Teachers Guide to Stratovolcanoes of the World
Eruption Feature

Mount St. Helens, Washington USA:
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Lateral Blast

The giant landslide at Mount St. Helens in 1980 had an effect like pulling a cork off a bottle of severely shaken soda. Once the side of the mountain was removed, the volcanic gases exploded out of the side of the volcano producing a lateral blast. This blast, traveling at speeds of up to 1,072 km/hr (670 mi/hr), quickly overtook the landslide and extended to up to 30.4 km (19 mi) from the volcano. In the areas closest to the volcano and up to about 13 km (8 mi) away the blast destroyed everything-trees, houses, wild life, etc. and the area was left barren as the moon. In the area between approximately 13 and 21 km from the volcano the blast toppled trees and left them lying in neat rows like tooth picks. Still further from the volcano the trees remained standing but were singed brown by the hot gasses of the blast leaving forests of "skeleton" trees with needles striped. Most of the 57 people killed in the eruption including volcanologist David Johnston were killed by asphyxiation from the lateral blast. The hot gases scorched their lungs. The blast and the landslide (debris avalanche) removed the upper 396 m (1,306.8 ft) of the volcano. The blast devastated 596 square kilometers (229 square miles) and destroyed timber valued at several millions of dollars.

The photograph in the poster shows Mount St. Helens entering a new dome building phase approximately 3 years after the cataclysmic 1980 eruption. The large crater opening to the north produced in the 1980 eruption are visible in the photograph.

See an animation of Mount St. Helens, courtesy of the Exploring the Environment.