Volcanic Pyroclastic Flow
Volcano Terms and Definition

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Pyroclastic flow sweeps down the side of Mayon Volcano, Philippines, during an explosive eruption on 15 September 1984. Note the ground-hugging cloud of ash (lower left) that is billowing from the pyroclastic flow and the eruption column rising from the top of the volcano.
Pyroclastic Flow

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 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.

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.

Pyroclastic Flow St Helen Volcano
Explosive eruption of magma and solid-rock fragments or the collapse of a vertical eruption column of ash and larger rock fragments may generate pyroclastic flows. Photographs of an eruption at Mount St. Helens on July 22, 1980 , shows the development of a pyroclastic flow.

St Helen Eruption July 1980
Mount St. Helens on July 22, 1980

Pyroclastic flow rushes down Unzen Volcano, Japan Collapse of lava dome generates pyroclastic flow on Unzen Volcano, Japan, on March 23, 1993 The fall of fresh lava and hot rock debris from a lava dome or thick lava flow can generate scores of pyroclastic flows. The repeated collapse of a growing lava dome atop Unzen Volcano caused thousands of small but dangerous pyroclastic flows between 1991 and 1995.

Avalanching of hot, thick pyroclastic flow deposits in valleys downslope from Mount Pinatubo, Philippines, triggered many small pyroclastic flows for more than two years after its climactic eruption on June 15, 1991. Some of these flows formed deposits as long as 10 km, 1 km wide, and 10 m thick. Such secondary pyroclastic flows present post-eruption hazards that were not recognized until they were observed at Pinatubo

A series of pyroclastic flows from Redoubt Volcano in Alaska between December 1989 and April 1990 rapidly melted snow and ice that generated lahars in Drift River (valley in photo). The lahars swept 40 km to Cook Inlet. Most of the pyroclastic flows were caused by the repeated collapse of a lava dome growing high on the volcano's north flank.

This view across the upper Drift River valley is to the SW; the north flank is on the right side of the volcano, but the dome is not visible. As each pyroclastic flow swept down the volcano's snow- and glacier-covered north flank, the hot lava-dome fragments eroded and mixed with the snow and ice to form a torrent of water that swept into Drift River. The sudden surges of water eroded loose sediment on the valley floor and transformed into lahars.

In this part of the Drift River valley, many of the lahars covered the entire valley floor, but they were generally no more than a few meters thick.

Scientists use a wide variety of names to describe specific types of hot, dry flows of rock fragments and gas produced by erupting volcanoes. The terms below are used to describe either (1) the way in which a pyroclastic flow originates and moves; or (2) a predominant characteristic of the resulting deposit.

Ash Flow or Ash Cloud
A pyroclastic flow consisting primarily of ash-sized particles, including glass shards and mineral fragments

Block and Ash Flow
A pyroclastic flow consisting of ash and large lava fragments with few gas bubbles, which typically forms as a consequence of a collapsing lava flow or dome.

Base Surge
A turbulent, low-density flow of rock debris and water and (or) steam that moves at high speeds. Base surges may occur when an explosive eruption occurs from within a crater lake or an ocean.

Directed Blast
A volcanic explosion of rocks and magma (or both) with a low-angle component. When the rock debris from a directed blast falls to the ground, it behaves like a pyroclastic flow or surge and moves rapidly away from the volcano.

Nuée Ardente (glowing cloud)
A French term introduced in 1904 to describe pyroclastic flows erupted on Mount Pelée on the Island of Marginique. When viewed at night or in low light, pyroclastic flows may appear to glow red. The term is widely used to describe pyroclastic flows, but not to imply the way a flow is generated.

Pumice Flow
A pyroclastic flow consisting predominantly of pumice fragments, which contain many gas bubbles.

Pyroclastic Surge
A turbulent, low-density cloud of hot rock debris and gases that moves at extremely high speeds. Because surges are low density, they tend to spread over large areas and move up and over ridge crests easily. By contrast, pyroclastic flows are high-density masses of hot rock debris and gases that tend to be confined in valleys.

Volcano Glossary



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