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).

Following a magnitude five earthquake on the morning of May 18, 1980, a landslide (green) unroofed the hot magma and surrounding hydrothermal system, 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]

此圖像屬於公有領域，因為其包含的內容最初來自美國內政部下屬的美國地質調查局。進一步資訊請參見USGS的版權方針。 Bahasa Indonesia ∙ català ∙ čeština ∙ Deutsch ∙ eesti ∙ English ∙ español ∙ français ∙ galego ∙ italiano ∙ Nederlands ∙ português ∙ polski ∙ sicilianu ∙ suomi ∙ Tiếng Việt ∙ Türkçe ∙ български ∙ македонски ∙ русский ∙ മലയാളം ∙ 한국어 ∙ 日本語 ∙ 中文 ∙ 中文（简体） ∙ 中文（繁體） ∙ العربية ∙ فارسی ∙ +/−