what was its cause ?
An earthquake
1980 eruption of Mount St. Helens
An earthquake at 8:32:11 a.m. PDT (UTC−7) on Sunday, May 18, 1980, caused the entire weakened north face to slide away, creating the largest landslide recorded. This allowed the partly molten, high-pressure gas- and steam-rich rock in the volcano to suddenly explode northwards toward Spirit Lake in a hot mix of lava and pulverized older rock, overtaking the avalanching face.
where did the top of mt st helens land UNANSWERABLE when did the eruption take place? On March 27, 1980
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1980 eruption of Mount St. Helens [SEP] Introduction
An earthquake at 8:32:11 a.m. PDT (UTC−7) on Sunday, May 18, 1980, caused the entire weakened north face to slide away, creating the largest landslide recorded. This allowed the partly molten, high-pressure gas- and steam-rich rock in the volcano to suddenly explode northwards toward Spirit Lake in a hot mix of lava and pulverized older rock, overtaking the avalanching face. An eruption column rose into the atmosphere and deposited ash in 11 U.S. states and two Canadian provinces. At the same time, snow, ice and several entire glaciers on the volcano melted, forming a series of large lahars (volcanic mudslides) that reached as far as the Columbia River, nearly to the southwest.