pyroclastic flow

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pyroclastic flow,

turbulent, fluidized mixture of rock, volcanic ash, and hot gas that moves like an avalanche away from a volcanic eruption. A pyroclastic flow may contain a mix of rock fragments ranging up to the size of boulders, with the heavier fragments moving along closer to the ground. The hot, expanding gases suspend ash and smaller rock particles, which can then support larger rock fragments; a lack of friction among the particles enables the mixture to travel great distances.

A flow may result from the collapse of the eruption column of a volcano or the collapse of a lava dome or lava flow. Flows that result from an eruption-column collapse are dominated by pumice and move more rapidly; flows can travel as fast as 100 mph (160 kph), reach temperatures that exceed 900°F; (500°C;), and in some instances climb slopes and cross stretches of open water. The speed at which they move, the heat and toxicity of their gases, and the debris that they carry make pyroclastic flows extremely destructive and deadly. A pyroclastic flow from the 1902 eruption of Mt. Pelée on Martinique killed nearly all of the 28,000 inhabitants of Saint-Pierre.

pyroclastic flow

[¦pī·rə¦klas·tik ′flō]
(geology)
Ash flow not involving high-temperature conditions.
References in periodicals archive ?
'Historically, Clark has pyroclastic flows and lahar.
It is classified as a complex stratovolcano because its eruptions consist of explosive eruptions as well as pyroclastic flows.
Quoting Japan's Meteorological Agency, NHK World reported that people have been advised to seek shelter and be on alert for volcanic rocks and pyroclastic flows.
It also issued warnings against large falling volcanic rocks within 3 kilometers of the crater and pyroclastic flows in areas within about 2 km of the crater.
The eruption of the volcano, which lies 40km south-west of the capital Guatemala City, sent deadly pyroclastic flows and ash across a ten-kilometre radius.
The eruptions have showered volcanic ash over a vast area and spewed deadly, fast-moving pyroclastic flows through nearby towns.
Rescue workers have been searching frantically for survivors and victims in the lava-ravaged landscape left by the eruptions, which showered volcanic ash over nearby towns and spewed pyroclastic flows throughout the area.
Volcan de Fuego, which means "Volcano of Fire," exploded on Sunday in its most devastating eruption in more than four decades, showering ash on a wide area and sending rapid pyroclastic flows through nearby towns.
Firefighters said they had seen some people who were trapped, but roads were cut by pyroclastic flows and they had been unable to reach them.
The alert level of the volcano had reached level 4 due to its intense unrest characterized by persistent tremors, continuous lava eruptions, and dangerous pyroclastic flows. The volcano is now down to level 2.
Officials cautioned people near Shinmoedake about the possibility of deadly pyroclastic flows. The flows are made up of lava blocks, pumice, ash and superheated volcanic gas that barrels down a volcano's slope to vaporize essentially anything in its path, resembling something like a tumbling avalanche that trades snow for piping-hot debris.
The agency also warned about the possibility of pyroclastic flows within a similar distance from Mt.