Pyroclastic Flow

Pyroclastic flow is a term that deserves a place in the popular lexicon. One of the most destructive volcanic processes, pyroclastic flows are mixtures of hot gas and rock that travel gravitationally down the flanks of a volcano at highway velocities. Their elevated temperatures and great mobility make them lethal to anything in their path. These flows include a basal component containing blocks of pumice or dense rocks and a dilute cloud of hot gas and ash that rises convectively above the moving flow. They can be formed by collapse of dense materials within a rising eruption column or directly from the volcanic vent. Pyroclastic flows include vesicle-rich pumice flows and dense block-and-ash flows formed by gravitational collapse of growing lava domes and active lava-flow fronts. Their travel paths typically follow topography. Pyroclastic surges, in contrast, are more dilute gas-rich flows that can have broader lateral extent less sensitive to topography. The dilute part of pyroclastic flows can become detached from the bedload portion and travel long distances over water. Extremely large-volume pyroclastic flows known as ignimbrites are produced during catastrophic eruptions and can travel long distances from a volcano, producing deposits tens to a few hundred meters thick.

Photo by Lee Siebert, 1994 (Smithsonian Institution).

Santorini

This spectacular outcrop shows light-colored deposits from the 3500-year-old Minoan eruption of Santorini volcano filling a valley cut in darker, bedded ashfall layers of Pleistocene age. The lower, beige-colored unit filling the ancient valley is a pumice-fall deposit from vertical explosions early in the eruption. It is overlain by laminated pyroclastic-surge deposits produced when water gained access to the magma reservoir as the volcano collapsed into the sea. The upper whitish layer truncating both these deposits is a pyroclastic-flow deposit.

Photo by Ernesto Corpuz, 1984 (Philippine Institute of Volcanology and Seismology).

Mayon
Ash clouds rise above a pyroclastic flow traveling down the Buang valley on the upper NW flank of Mayon volcano in the Philippines on September 12, 1984. The toe of the advancing pyroclastic flow is visible at the lower right. These pyroclastic flows traveled down to 100 m elevation at rates of about 20 m/sec.
Pyroclastic flows are hot avalanches of rock, ash, and gas that sweep down the flanks of volcanoes at high velocities. This photo shows a relatively small pyroclastic flow at Mayon volcano in the Philippines on September 23, 1984. These hot, ground-hugging flows can travel at velocities to about 100 km/hr and reach areas well beyond the flanks of a volcano. Their high temperatures make them lethal to anything in their path. Billowing ash clouds rise above the denser basal portion, which can consist of vesiculated pumice or dense lava clasts.

A pyroclastic flow sweeps down the SE flank of Mayon volcano in the Philippines on September 24, 1984. A thick column of ash rises above the surface of the moving pyroclastic flow, which was the largest of a series of pyroclastic flows that occurred during an eruption that began on September 9. Flow velocities of 50 m/sec were estimated from timed 35-mm photographs. The pyroclastic flow traveled 7 km from the summit vent, which is hidden behind the far left side of the ash column.

Photo by Rick Hoblitt, 1991 (U.S. Geological Survey).

Pinatubo

Voluminous pyroclastic flows on June 15, 1991, from Mount Pinatubo in the Philippines, swept all sides of the volcano. The flat, light-colored areas in the foreground are pyroclastic-flow deposits that filled the Marella River valley on Pinatubo's SW flank to a depth of 200 m, more than the height of the Washington Monument. The dark hill at the center was completely surrounded by pyroclastic flows, which traveled 14 km down this valley.

Photo by Setsuya Nakada, 1993 (Kyushu University).

Unzen

A pyroclastic flow on June 23, 1993, at Unzen volcano in southern Japan, sweeps down the flanks of the volcano into the Senbongi residential district of Shimabara city. Pyroclastic flows had been occurring at Unzen since May 1991 as a result of collapse of the unstable margins of a lava dome growing at the summit of Fugen-dake. This pyroclastic flow traveled 1 km through inhabited areas, which had been evacuated since August 1991. One resident who had returned to watch his house burn was killed by a second pyroclastic flow.

Photo by R.V. Fisher, 1979 (University of California Santa Barbara).

Nii-jima

A spectacular sequence of pyroclastic-surge deposits are exposed in a sea cliff on Nii-jima, in the northern part of the Izu Islands of Japan. The dramatic cross-bedded layers were produced during episodic erosion and deposition by laterally moving ash clouds. The eruptions accompanied formation of a lava dome at Mukai-jima on the southern part of the island. Flat-bedded airfall deposits cap the exposure.

Photo by Shinji Takarada, 1992 (Geological Survey of Japan).

Komaga Take

A pyroclastic-flow deposit from the 1929 eruption of Komaga-take volcano, on the northern Japanese island of Hokkaido, overlies a brown, pre-eruption surface. The upper part of the deposit contains large blocks of pumice; layers both depleted and enriched in finer-grained material occur at the base. A geological hammer provides scale.

Photo by Lee Siebert, 1986 (Smithsonian Institution).

Augustine
A pyroclastic flow sweeps down the north flank of Augustine volcano in Alaska on March 30, 1986, 3 days after the start of a 5-month long eruption. A large convecting ash column rises above the moving pyroclastic flows. As with many Augustine eruptions, early pyroclastic flows were pumice rich; later in the eruption block-and-ash flows were produced by collapse of a growing lava dome.
A volcanologist observes a large, 6-m-high block that was carried about 4 km down the north flank of Augustine volcano in Alaska during the 1976 eruption. Blocks of this size and larger are fragments of the summit lava dome that were carried within block-and-ash flows produced by periodic collapse of the growing dome. This photo was taken during a quiet phase of the 1986 eruption and shows the steaming summit lava dome.

Volcanologist Jurgen Kienle hoists a light-weight block of pumice at the toe of a 1986 pyroclastic-flow deposit at Alaska's Augustine volcano. A pyroclastic-flow apron was formed by the accumulation of many individual lobes. Thermal measurements more than 100 days after the eruption showed a maximum temperature of 525 degrees Centigrade at a depth of 6 m. Pumiceous pyroclastic flows during the 1986 eruption traveled about 5 km from summit and reached the sea on the north and NE coasts of Augustine Island.

Photo by Christina Neal, 1990 (Alaska Volcano Observatory, U.S. Geological Survey).

Redoubt

Pyroclastic-flow deposits from the April 15 (lower 2/3 of section) and April 21 (upper 1/3 of section), 1990 eruptions of Redoubt Volcano in Alaska are exposed in a gully along the western margin of the piedmont lobe of Drift glacier. The shovel at the base of the section provides scale. The somewhat larger April 15 pyroclastic flow carried large blocky fragments of a lava dome that had been growing in the summit crater.

Photo by Lee Siebert, 1984 (Smithsonian Institution).

St. Hellens
Pumice fragments from the May 18, 1980 eruption form a broad plain below Mount St. Helens in this May 23 photo. Pumiceous pyroclastic flows on May 18 traveled 8 km from the crater of Mount St. Helens to as far as Spirit Lake. A geologist can be seen holding a large, light-weight block of pumice. In addition to May 18, pumiceous pyroclastic flows were erupted on May 25, June 12, July 22, August 7, and October 16-18, 1980.
The powerful lateral blast from Mount St. Helens on May 18, 1980 swept 30 km away from the volcano, blowing down giant trees like matchsticks. The blast, traveling at velocities up to 1100 km per hour, devastated 600 sq km over a broad area nearly 180 degrees wide north of the volcano.
Pryroclastic surges originating from secondary phreatic explosions at Mount St. Helens in 1980 produced these cross-bedded layers. They were deposited from successive, rapidly moving horizontal clouds of gas, ash, and rock fragments that resulted from the interaction of still-hot pyroclastic-flow deposits from the May 18 eruption with groundwater and fragments of Mount St. Helens glaciers carried down by the collapse of the summit.
The gray layer behind the ruler was produced by the May 18, 1980 lateral blast of Mount St. Helens. The deposit is about 50 cm thick at this location 13 km NE of the volcano. The blast deposit is overlain by airfall pumice produced later on May 18, and underlain by a pumice deposit from an eruption in 1482 AD.

The powerful lateral blast of May 18, 1980 at Mount St. Helens totally removed large standing trees near the volcano, leaving jagged stumps with splinters facing away from the volcano parallel to the direction of movement of the blast cloud. "Warbonnet" stumps, such as this one (about 2 m high) on Harry's Ridge 9 km north of the crater, are a common feature near the volcano; farther away trees were felled and left in place.

Photo by R. V. Fisher, 1984 (University of California Santa Barbara).

Long Valley

Spectacular curved columnar joints in the Bishop Tuff are exposed in Owens River Gorge SW of Long Valley caldera in California. The 5-6-sided columns are about 1-3 m wide and curve downward to a common point, forming a feature known as a joint rosette. The rosettes are the site of large fossil fumaroles and often are overlain by fumarole mounds. These mounds are close to the Owens River Gorge, suggesting that they were formed as a result of volatiles produced when the hot Bishop ash flows overran and vaporized the ancestral Owens River.

Photo by Richard Waitt, 1988 (U.S. Geological Survey).

Pinacate

Pyroclastic-surge deposits surround the Cerro Colorado maar of the Pinacate volcanic field in NW México. These thin beds (note the coin for scale next to the the block in the center of the photo) were formed by successive explosions that produced laterally moving pyroclastic surges. The light-colored rock in the center of the photo is a ballistically ejected block that impacted onto the surface of earlier surge deposits, compressing them and forming a small pit.

Spectacular pyroclastic-surge deposits are exposed in gullies on the flanks of Cráter Elegante in the Pinacate volcanic field of NW México. This photo shows cross-bedded sandwave bed forms produced by particles transported by saltation or dilute suspension in a rapidly moving surge cloud. The direction of movement of the surge cloud, seen by the truncation of dune beds on the near-vent side, was from right to left. Sandwave beds predominate in areas near the rim of the maar.

Photo by Jim Luhr, 1988 (Smithsonian Institution).

Durango Volc Field

Pyroclastic-surge deposits from La Breña maar in México's Durango volcanic field show both laminar and dune bedding forms. The thin beds (note the pen in the center for scale) were created by successive explosive eruptions that produced high-velocity, laterally moving pyroclastic surges that swept radially away from the volcano. The direction of movement of the surge clouds was from right to left, as seen from the truncated dune beds on the near-vent side.

Photo by Alfredo Ramirez (pilot Ernesto Gómez Hofman), 1991 (courtesy Melchor Urzua, Protección Civil de Colima).

Colima

A convecting ash column roils above a small, but impressive-looking pyroclastic flow from Colima volcano in México on April 16, 1991. The pyroclastic flow, colored by the late-afternoon sun, was produced by collapse of unstable portions of the summit lava dome. It descended the SW flank but caused no damage to inhabited areas.

Photo by William Buell, 1974.

Fuego

Pyroclastic flows sweep down the east flank of Fuego volcano, Guatemala, in October 1974, during one of the largest historical eruptions of the volcano. Ash clouds rise off the base of the pyroclastic flows, which traveled up to 7 km from the summit at estimated average velocities of 60 km/hr. The travel direction of pyroclastic flows is influenced by topography. The denser basal portion of the pyroclastic flows follows topographic lows on the flanks of the volcano--note a smaller pyroclastic flow descending a gully at the right.

Photo by Paul Cole, 1997 (Montserrat Volcano Observatory).

Soufriere Hills
Cauliflower-like clouds of ash roil above the surface of a pyroclastic flow sweeping down the eastern flank of the summit lava dome on January 16, 1997. This was the largest single pyroclastic flow of the eruption to date. The pyroclastic flow descended the Tar River valley to the sea, covering the new delta with new material that included blocks 1-2 m in diameter (up to 5-m diameter at the head of the fan).

A devastating pyroclastic flow on June 25, 1997 sweeps across the lower NE flank of Soufrière Hills volcano on Montserrat. More than two dozen persons within the officially evacuated zone were killed by this pyroclastic flow. The June 25 eruption sent a plume to ~10-km altitude and produced pyroclastic flows and surges that overran both vacated and partly inhabited NE-flank settlements, destroying 100-150 houses in eight villages within the restricted zone. The pyroclastic flow traveled 4.5 km and reached almost to the sea.