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 ?
P tephrostratigraphy related to the main explosive activity; 2) different pyroclastic density current deposits; 3) typical horseshoe-shaped amphitheatres and their associated debris avalanche deposits, and; 4) lava domes and lava flows.
2 m thick) related to a diluted pyroclastic density current.
At 9:09 pm, on November 13,1985, NRV, a small volume of mixed andesite and dacite magma was ejected as pumice fall and pyroclastic density currents during the Plinian eruption that pumped tephra into the atmosphere for more than 30 km (20 mi)--a commercial flight witnessed this eruption.
Turbulent pyroclastic density currents mechanically mixed with snow and produced meltwater at a rate of 0.
Processes included tephra fallout, pyroclastic density currents, multiple slope failures with debris avalanches and lahars, and interaction with the ice cap (Thouret et al, 1990).
The latter units correspond to diluted pyroclastic density currents (Figure 18).
In the stratigraphic section next to the right margin of the Quebrada La Hedionda (Figure 21), we can observe from base to top: 1) a lower debris avalanche deposits (at the road level, see the truck for scale); 2) in the middle a sequence of pyroclastic density currents (the layered part of the outcrop); and 3) an upper debris-avalanche deposit.
We took on the modeling of enormously complicated pyroclastic density currents, notably the classic, notorious May 1980 lateral blast that destroyed 500 square kilometers of forested terrain at Mount St.
Poor sorting, topographic control, nature and composition suggest deposition by pyroclastic density currents.
The strong topographic control, the crude sorting and composition suggest pyroclastic density currents originated by dome-collapse.