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.