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Mount St. Helens Erupts a Pyroclastic Flow Target Name:   Earth's Volcanos Produced by:   USGS, J.W. Vallance Copyright:  Public Domain Date Released:  July 22, 1980 Related Documents Terrestrial Volcanoes The Nature of Volcanoes Principal Types of Volcanoes Download Options Name Type Width x Height Size helens1.jpg JPEG 146K This image shows Mount St. Helens, 5 minutes into eruption onset on July 22, 1980. It was taken by J.W. Vallance from a helicopter between 6:25 and 6:31 p.m. The vertical eruption column of ash and pumice rises directly from the crater, whereas the ash cloud rising through the white clouds in the background rises from a pyroclastic flow that is rushing down the north flank of the volcano. The pyroclastic flow was generated by the collapse of a "fountain" of erupting magma. The fountain was a sustained event that increased in intensity over a period of many seconds, like a huge garden hose pointed upward and whose water pressure is gradually increased. Pumice fragments from the "fountain" then fell to the crater floor and formed a fast-moving pyroclastic flow. Viewed from the southeast, the pyroclastic flow is clearly separate from the darker vertical eruption column. Apparently, the dark color reflects a greater proportion of ash, suggesting that more of the erupting magma was being fed preferentially into the vertical column rather than being fed into the pyroclastic flow.

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