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Volcano Hazards: | Types and Effects
| Location | | Gas | Lahars | Landslides | Lava Flows | Pyroclastic Flows | Tephra | |
 July 22, 1980 |
Mount St. Helens erupts a pyroclastic flow during the second of three pulses on July 22, 1980. The vertical eruption column of ash and pumice rises directly from the crater, whereas the ash cloud rising through the white clouds in the foreground rises from a pyroclastic flow that is rushing down the north flank of the volcano. The series of photographs below (all by J.W. Vallance), taken from a helicopter between 6:25 and 6:31 p.m., show that 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. |
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The deep crater of Mount St. Helens is open to the north, which serves to direct pyroclastic flows northward from the vent, shown by the arrow. The crater walls are about 600 m high and the crater floor is about 1,000 m wide. |
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Eruption onset + 15 seconds. Initial explosive ejection of magma forms a growing vertical "fountain" of gases and pumice and ash fragments. A cauliflower-shaped ash cloud rises only slightly above the fountain. |
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Eruption onset + 30 seconds. Leading edge of a pyroclastic flow races out of the crater. The pyroclastic flow is growing in size quickly as slightly-cooled magma fragments (pumice and ash) and gases fall from the fountain and collide with the crater floor. |
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Eruption onset + 40 seconds. With an average velocity of about 40m/second (145 km/hour), the leading edge of the pyroclastic flow is now hidden by the white atmospheric clouds. The fountain of erupting magma is shooting up about 800 m above the vent before crashing back to the crater floor. |
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Eruption onset + 60 seconds. A vertical eruption column is now about 3,000 m above the crater rim and is beginning to grow very quickly. In the next 15 seconds, the rise velocity increased from about 60 m/second to about 130 m/second (470 km/hour). |
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Eruption onset + 5 minutes. 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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Helicopter stands by as a scientist collects a sample from a tongue-like pyroclastic flow deposit and mesures its temperature. The July 22 pyroclastic flow traveled about 6.5 km from the crater and the maximum temperature measured within about 1 hour of its formation was 660 degrees C. This light-colored deposit erupted on August 7, 1980, is nearly 2 m thick. |
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Whitish blocks of pumice cover the surface of this day old pyroclastic flow, erupted on October 22, 1980. Just a few centimeters beneath the surface, however, ash dominates the deposit by filling spaces between smaller pieces of pumice. The darker surface beneath the October pyroclastic flow is the deposit of pyroclastic flows formed on May 18, 1980, during the climactic eruption of Mount St. Helens. |
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