PYROCLASTIC FLOW

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pyroclastic flows

Hot (700 °C)

fast moving (100’s mph)

mixture of pyroclastics, volcanic gas, and entrained air

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Pyroclastic Flow or Nuee Ardente (French: Fiery Cloud)

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Pyroclastic flowA pyroclastic flow is a ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side of a volcano as fast as 100 km/hour or more. The temperature within a pyroclastic flow may be greater than 500° C, sufficient to burn and carbonize wood. Once deposited, the ash, pumice, and rock fragments may deform (flatten) and weld together because of the intense heat and the weight of the overlying material.

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Pyroclastic flows from the June 15 eruption swept down each of the volcano's river valleys as far as 12 to 16 km (light-colored areas in lower photograph). With rainfall averaging between 2 to 4 m per year in this part of the Philippines, most of it falling in the monsoon season from June to October, thousands of small

but destructive lahars originated from these pyroclastic-flow deposits.

Pyroclastic Flows Fill Valleys

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Augustine Volcano (1,282 m) erupts a relatively short eruption column directly from a summit vent as a pyroclastic flow sweeps down the volcano's north flank. The

prevailing wind is blowing ash and gas southeast from the volcano to form an eruption cloud; surface wind is blowing ash across the ground from earlier pyroclatic flows.

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Pyroclastic Flows Melt Snow and Ice

Headwaters of Azufrado River. Hot rock fragments of the pyroclastic flows and surges quickly eroded and mixed with Ruiz's snow and ice, melting about ten percent of the volcano's ice cover. In places, channels 100 m wide and 2-4 m deep were eroded into the

icecap. Flowing mixtures of water, ice, pumice and other rock debris then poured from the summit and sides of the volcano into rivers draining the volcano. In one river,

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Eruption cloudA cloud of tephra and gases that forms downwind of an erupting volcano is called an eruption cloud. The vertical pillar of tephra and gases rising directly above a vent is an eruption column.

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Pyroclastic flows descend the south-eastern flank of Mayon Volcano, Philippines. Maximum height of the eruption column was 15 km above sea level, and volcanic ash fell within about 50 km toward the west. There were no casualties from the 1984 eruption because more than 73,000 people evacuated the danger zones as recommended by scientists of the Philippine Institute of Volcanology and Seismology.

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Pyroclastic flows descend the south-eastern flank of Mayon Volcano, Philippines. Maximum height of the eruption column was 15 km above sea level, and volcanic ash fell within about 50 km toward the west. There were no casualties from the 1984 eruption because more than 73,000 people evacuated the danger zones as recommended by scientists of the Philippine Institute of Volcanology and Seismology.

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Effects of pyroclastic flows

A pyroclastic surge associated with a major dome collapse and pyroclastic flows on June 25, 1997, swept through the village

of Dryers, located about 3 km NW of English's Crater.

Destroyed home Aerial view of highway bridge abutments (structures on both sides of the river channel) that once supported a bridge across the Muddy River

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Letusan Galunggung 1982, Tasikmalaya, Jawa Barat

http://volcano.und.nodak.edu/vwdocs/

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