Image:Computer animation showing May 18, 1980 St Helens landslide.png
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These illustrations show the landslide (green) and directed blast (red) that occurred during the first few minutes of the eruption of Mount St. Helens in 1980.
Before the eruption, an estimated 0.11 km³ of dacite magma had intruded into the volcano (equivalent to sphere about 600 m in diameter!). The rising magma forced the volcano's north flank (right side of illustration) outward about 150 m and heated the volcano's ground water system, causing many steam-driven explosions (phreatic eruptions).
The hot magma and surrounding hydrothermal system were unroofed by the landslide (green), and the resulting rapid depressurization caused a series of steam- and volcanic-gas-driven explosions. The explosions burst through part of the landslide, blasting rock debris northward. The resulting pyroclastic surge quickly overran the landslide and spread over ridges and valleys across an area of 550 km².
USGS image (by T.R. Alpha) and text from [1] (http://volcanoes.usgs.gov/Imgs/Gif/Drawings/30210600-084_caption.html) Template:PD-USGov-Interior-USGS
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