Pyroclastic flow(also known as a pyroclastic density current)

A 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 80 km/hour or more. The temperature within a pyroclastic flow may be greater than 500° C, sufficient to burn and carbonize wood. The gas can reach temperatures of up to 1,000 degrees Celsius (1,832 F). The flows normally falldown ground and travel downhill, or spread laterally under gravity. Their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope. They range from high density flows that move down valleys and can move beneath water, to dilute flows that extend over mountains and can move across water.

Pyroclastic flows and surges form in several ways:

Ø gravitational collapse of a vertical eruption column (Sparks et al., 1978),

Ø the "boiling-over" of a highly gas-charged magma from a crater (Taylor, 1958),

Ø inclined blasts from the base of an emerging spine or dome (Lacroix, 1904),

Ø lateral blasts following release of pressure caused by collapse of part of a volcano edifice (Bogoyavlenskaya et al., 1985; Siebert et al., 1987)

Ø collapse of a growing dome (Mellors et al., 1988),

Ø ash fountaining (Hoblitt, 1986), and

Ø collapse from the front of a lava flow (Rose et al., 1977).

The term "pyroclastic" - derived from the Greek words pyro (fire) and klastos (broken) - describes materials formed by the fragmentation of magma and rock by explosive volcanic activity. Most volcanic ash is basically fine-grained pyroclastic material composed of tiny particles of explosively disintegrated old volcanic rock or new magma. Larger sized pyroclastic fragments are called lapilli, blocks, or bombs

Volcanic Ash -- Aggluntinate ... Airfall ... Ash ... Ashfall ... Blocks ... Bombs ... Bread-crust bombs ... Cinder ... Cow-dung bombs ... Fusiform bombs ... Glass ... Lapilli ... Pele's hair ... Pele's tears ... Pumice ... Pyroclastic-fallout ... Reticulite ... Ribbon bombs ... Scoria ... Spatter ... Spindle bombs ... Tephra ... Tephra clasts ... Tephra deposits ... Tephra fall ... Thread-lace scoria ... Volcanic ash ... Volcanic bombs ... Welded spatter ...

A pyroclastic flow is a fluidized mixture of solid to semi-solid fragments and hot, expanding gases that flows down the flank of a volcanic edifice. These awesome features are heavier-than-air emulsions that move much like a snow avalanche, except that they are fiercely hot, contain toxic gases, and move at phenomenal, hurricane-force speeds, often over 100 km/hour. They are the most deadly of all volcanic phenomena.