The Eruption

The volcanic eruption of Mount St. Helens happened in several stages.

• Earthquakes caused by the uprising of magma within the mountain began onMarch 20, 1980.
• A vent opened up at the top of the mountain allowing the escape of steam and ash. Plumes of ash reached heights of seven miles.
• The vents later collapsed, causing magma to pool inside the mountain. This pool, called a cryptodome, pushed the entire north side of the mountain up and out.
• On Sunday, May 18, 1980, a magnitude 5.1 earthquake triggered an enormous landslide.
• This massive avalanche raced down the north face, exposing molten rock and setting off a lateral pyroclastic explosion of volcanic rock, hot gasses, and super-heated steam. It was followed by a vertical blast of volcanic rock.
• Debris from the massive landslide and explosion cascaded down the mountaininto the Toutle River Valley.
• The intense heat melted snow and ice, creating a mudflow that flowed rapidly down the Toutle River, knocking down trees and destroying everything in its path, including 221 homes.
• The mudflow eventually reached the Columbia River, filling the channel and stopping ship traffic.