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Types and Processes Gallery - Pyroclastic Flows

Pyroclastic Flows
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.

This outcrop shows light-colored deposits from the 3,500-year-old Minoan eruption of Santorini filling a valley that eroded into darker tephra layers of Pleistocene age. The lower beige-colored unit filling the valley is a pumice-fall deposit from early in the eruption. It is overlain by laminated pyroclastic surge deposits that were produced when water came into contact with the magma reservoir as the volcano collapsed into the sea. The upper lighter-colored layer truncating both these deposits is a pyroclastic flow deposit.

Photo by Lee Siebert, 1994 (Smithsonian Institution).

A pyroclastic flow travels down the SE flank of Mayon volcano in the Philippines on 24 September 1984. An ash plume rises above the moving pyroclastic flow, which was the largest of a series of pyroclastic flows that occurred during an eruption that began on 9 September. The pyroclastic flow traveled 7 km from the summit vent; velocities of 50 m/s were estimated from photographs.

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

An ash plume rises above a pyroclastic flow traveling down the Buang valley on the upper NW flank of Mayon volcano in the Philippines on 12 September 1984. The front 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/s.

Photo by Olimpio Pena, 1984 (Philippine Institute of Volcanology and Seismology).

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 23 September 1984. These hot, ground-hugging flows can travel at velocities to about 100 km/hour and reach areas well beyond the flanks of a volcano. Their high temperatures make them lethal to anything in their path. Hot ash plumes rise above the denser basal portion that can contain abundant solid blocks and ash.

Photo by Chris Newhall, 1984 (U.S. Geological Survey).

Voluminous pyroclastic flows on 15 June 1991 descended all sides of Mount Pinatubo in the Philippines. 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. The dark hill at the center was completely surrounded by pyroclastic flows that traveled 14 km down this valley.

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

A pyroclastic flow on 23 June 1993 at Unzen volcano in southern Japan travels 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 partial collapse of the lava dome growing at the summit of Fugendake. This pyroclastic flow traveled 1 km through inhabited areas that 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 Setsuya Nakada, 1993 (Kyushu University).

A sequence of pyroclastic surge deposits exposed in a sea cliff on Niijima, in the northern part of the Izu Islands of Japan. These cross-bedded layers were produced during repeated erosion and deposition by multiple pyroclastic surge events. The eruptions accompanied the formation of a lava dome at Mukaijima on the southern part of the island. The flat airfall deposits cap the exposure.

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

A pyroclastic flow deposit from the 1929 eruption of Komagatake volcano, on the northern Japanese island of Hokkaido, overlies a brown pre-eruption surface. The upper part of the deposit contains large pumice that lacks fine-grained material between the clasts and the underlying unit is enriched in fine-grained material. A geological hammer provides scale.

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

A pyroclastic flow travels down the north flank of Augustine volcano in Alaska on 30 March 1986, three days after the start of a five-month long eruption. An ash plume rises above the pyroclastic flow. 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.

Photo by Betsy Yount, 1986 (Alaska Volcano Observatory, U.S. Geological Survey).

A volcanologist next to a 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.

Photo by Harry Glicken, 1986 (U.S. Geological Survey).

Volcanologist Jurgen Kienle holds a pumice clast at the toe of a 1986 pyroclastic flow deposit at Alaska's Augustine volcano. Thermal measurements more than 100 days after the eruption showed a maximum temperature of 525°C at a depth of 6 m. The flows traveled about 5 km from the summit and reached the sea on the N and NE coasts.

Photo by Lee Siebert, 1986 (Smithsonian Institution).

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

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

St. Helens
The material in the lateral blast from Mount St. Helens on 18 May 1980 had a velocity of at least 100 m/s (224 miles per hour), perhaps much higher. It completely removed large trees standing near the volcano, and reached a distance of 30 km, blowing down mature trees like matchsticks. The blast devastated 600 km2 over a broad area north of the volcano.

Photo by John Dvorak, 1980 (U.S. Geological Survey).

St. Helens
The lateral blast of the 18 May 1980 Mount St. Helens eruption removed large standing trees near the volcano. The high-velocity rocky flow left jagged stumps with splinters facing away from the volcano and parallel to the direction of movement of the flow. This tree is about 2 m high, located 9 km N of the crater on Harry’s Ridge.

Photo by Lee Siebert, 1984 (Smithsonian Institution).

St. Helens
The fine-grained gray layer behind the ruler was produced by the 18 May 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 that was erupted later on 18 May and underlain by a pumice deposit from an eruption in 1482 CE.

Photo by Lee Siebert, 1982 (Smithsonian Institution).

St. Helens
Pumice clasts from the 18 May 1980 eruption form the pumice plain immediately north of Mount St. Helens, shown in this 23 May photo. Pumiceous pyroclastic flows on 18 May traveled 8 km from the crater, as far as Spirit Lake. A geologist can be seen holding a large, light-weight block of pumice. Pumiceous pyroclastic flows were also erupted on 25 May, 12 June, 22 July, 7 August, and 16-18 October 1980.

Photo by Dan Miller, 1980 (U.S. Geological Survey).

St. Helens
Pyroclastic surges are dilute pyroclastic flows with a high proportion of gas. They originated from secondary phreatic explosions at Mount St. Helens in 1980 and 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 hot pyroclastic flow deposits from the May 18 eruption with melt water produced by glacial ice carried down by the collapse of the summit.

Photo by Norm Banks, 1980 (U.S. Geological Survey).

Long Valley
These curved columnar joints in the Bishop Tuff are exposed in Owens River Gorge SW of Long Valley caldera in California. The 5- to 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 may have formed as a result of volatiles produced when the hot Bishop pyroclastic flows overran and vaporized the ancestral Owens River.

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

Pyroclastic surge deposits exposed in gullies on the flanks of Cráter Elegante in the Pinacate volcanic field of NW México. This photo shows cross bedding produced by particles transported by saltation or dilute suspension in a high-velocity pyroclastic surge. 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. This type of bedding is common in areas near the rim of the maar.

Photo by Bill Rose, 1997 (Michigan Technological University).

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

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

Durango Volcanic Field
Pyroclastic surge deposits from La Breña maar in México's Durango volcanic field show both laminar and dune bedding. The thin beds (pen in the center for scale) were created by successive explosive eruptions that produced high-velocity 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 Jim Luhr, 1988 (Smithsonian Institution).

A convecting ash column rises above a small pyroclastic flow on the SW flank of Colima volcano in México on 16 April 1991. The pyroclastic flow, colored by the late-afternoon sun, was produced by the collapse of portions of the summit lava dome.

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

Pyroclastic flows travel down the east flank of Fuego volcano in Guatemala during October 1974 as part of one of the largest historical eruptions of the volcano. Ash plumes rise from the pyroclastic flows, which traveled up to 7 km from the summit at estimated average velocities of 60 km/hour. The denser basal portion of the pyroclastic flow follows topographic lows on the flanks of the volcano. A smaller pyroclastic flow is descending the gully to the right.

Photo by William Buell, 1974.

Soufriere Hills
A devastating pyroclastic flow on 25 June 1997 sweeps across the lower NE flank of Soufrière Hills volcano on Montserrat. More than two dozen people within the officially evacuated zone were killed. This eruption sent an ash 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 almost reached the sea.

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

Soufriere Hills
An ash plume expands above a pyroclastic flow sweeping down the E flank of the Soufrière Hills summit lava dome on 16 January 1997. The pyroclastic flow descended the Tar River valley to the sea, covering the new delta with new material that included blocks up to 5 m in diameter.

Photo by Richard Heard, 1997 (Montserrat Volcano Observatory).