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Pyroclastic flows (also called pyroclastic density currents) are hot ground-hugging mixtures of volcanic rock, volcanic ash (pulverized rock, glass, and crystals), and gas that can race away from vents at great speeds. They can form from column collapse (the partial collapse of an ash plume), dome collapse (called block-and-ash flows), the collapse of lava flow fronts, ‘boiling over’ from the vent, or phreatomagmatic eruptions. They range in concentrations of gas and solid material, which can alter their behavior and the areas impacted.
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A pyroclastic flow produced by collapse of a growing lava dome descends the northern flank of the Santiaguito lava dome in November 1967. Periodic larger collapses of Santiaguito have sent pyroclastic flows down the populated southern flanks. The most catastrophic of these traveled 10 km in 1929, when hundreds to thousands of people were killed. 
A large ash plume towers above Fuego in October 1974 as pyroclastic flows travel down the eastern flank (left). This was the largest Fuego eruption since 1932 at the time, and produced pyroclastic flows that traveled up to 7 km down the E, SE, SW, and W flanks. Prevailing winds distributed ashfall primarily to the SW, in the opposite direction from this view.
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.
Steam rises from a pyroclastic flow deposit on the NW flank of Ulawun in February 1970. Rainforest trees were blown down facing away from the volcano by a high-velocity pyroclastic surge at the margins of the 22 January pyroclastic flow, which traveled 5 km from the summit. Pyroclastic flow velocities on the upper part of the volcano were estimated to exceed 100 km/hour.
Pyroclastic surges at about 0805 on 1 August 1952 travel across the sea at the western coast of the island, 20 minutes after the start of the eruption. Several inches of ash and lapilli up to 12 mm in diameter fell on the deck of the fishing boat from which this photo was taken as it sailed away from the volcano at full speed.
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.
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.
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.
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.
A large 4 x 7 x 35 m block from a lava flow was transported about 900 m within a pyroclastic flow down the NE flank of Arenal on 5 September 2003. The flows were produced by lava flow fronts collapsing, descending to about 800 m elevation. Geologist Erick Fernandez from OVSICORI-UNA measures the dimensions of the block. 
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.
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.
Pyroclastic flows occurred frequently during the October 1974 eruption, and residents of nearby fincas (farms) are seen here carrying goods across a field in front of the pyroclastic flow descending the eastern flank behind them. Despite the frequency and magnitude of the pyroclastic flows, they affected relatively sparsely populated flanks of the volcano and produced no fatalities.
Pyroclastic flows descend the flanks of Arenal on 7 July 1987. This was part of the long eruption that began in 1968. The largest pyroclastic flow in this photo taken from the volcano observatory (2.5 km SSW of the summit) is traveling down the SSE flank.
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.
A large pyroclastic flow travels down the flank of the Santiaguito lava dome on 19 July 1989. The pyroclastic flow and ash plume are seen here from just west of El Palmar (10 km S of the dome) about 5 minutes after the start of the explosion. The Santa María summit is visible to the right of the plume, which rose to 4 km above the vent. The pyroclastic flow traveled 5 km down the Río Nimá and was one of the larger events since the major 1929-34 activity.
Collapse of the growing lava dome on 22 November 1994, produced a pyroclastic flow (block-and-ash flow) that swept 7.5 km down the south flank, overrunning several villages and killing 64 people. Six thousand people were evacuated as a result of this eruption.
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.
This is a pyroclastic flow deposit that was emplaced down the NE flank of Arenal on 5 September 2003, singeing vegetation on either side of the narrow valley. A series of pyroclastic flows were produced over two hours starting at 1055. The pyroclastic flows originated from the collapse of lava flows on the steep upper flank. Accompanying ashfall occurred to the W and NW.
A series of powerful eruptions at Redoubt began on 14 December 1989. This 18 December view from the Kenai Peninsula across Cook Inlet shows an umbrella cloud that appears to originate from a vent on the N flank, but is actually ash from a pyroclastic flow that is traveling down the Drift River valley to the north. Ash plumes during the initial days of the eruption reached heights of about 10 km. Several episodes of strong explosive activity and lava dome growth lasted until June.
A pyroclastic flow descending the east flank of Fuego on 8 January 1979, is viewed from the city of Antigua Guatemala to the NE. Intermittent explosive eruptions had been occurring since September 1977 and lasted until 8 August 1979.
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.
Pyroclastic flows descend the Barranca Honda and an adjacent drainage on the eastern flank of Fuego volcano in February 1973. Following strong steam emissions on 22 February 1973 small eruptions beginning 23 February were accompanied by pyroclastic flows down the Barranca Honda. Explosive eruptions continued until 3 March and resumed 13 and 22-23 March. This photo was taken from Finca Capetillo, near Alotenango. 
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.
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.
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.
Geologists investigate trees within in a pyroclastic flow deposit along the Río Tabacón on the NW flank of Arenal in November 1968. The trees are aligned parallel to the rapid pyroclastic flow direction. Devastating pyroclastic flows traveled down existing drainages during 29-31 July 1968. One of the largest, with an estimated volume of 0.0018 km3, entered the Río Tabacón. The deposits ranged to more than 30 m thick, but were typically about 10 m. Fumaroles on their surface were active for more than two years.
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.
An ash plume rises above a pyroclastic flow traveling down the SW flank of Colima on 16 April 1991, colored orange by the late-afternoon sun. The pyroclastic flow was produced by collapse of unstable parts of the summit lava dome. The black mass at the summit is a lava dome that began growing on 1 March. Later in the eruption, which ended in October, a lava flow traveled down the SW flank to 2,600 m elevation from the roughly 3,850-m-high summit.
An ash plume rises above the surface of a pyroclastic flow descending the flanks of Pavlof on the Alaska Peninsula in 1975. Incandescent lava can be seen on the upper cone. Intermittent phreatomagmatic to magmatic eruptions began 13 September 1975 and continued until at least March 1977. Possible lava flows or lahars were reported in October 1975 and December 1976.
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.
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.
Following strong steam emission at Fuego on 22 February 1973 small eruptions beginning the next day were accompanied by pyroclastic flows down the Barranca Honda on the E flank. Explosive eruptions continued until 3 March and resumed 13 and 22-23 March. Less ash was erupted in 1973 than during the previous eruption in 1971, but pyroclastic flows were more voluminous and traveled farther downslope. This photo was taken from Finca Capetillo, near Alotenango.
Residents of Antigua Guatemala have observed eruptions at Volcán de Fuego for several centuries. A larger ash plume towers above the city in this October 1974 view of one of Fuego's largest historical eruptions. Pyroclastic flows are also descending the east flank (left). The local topography diverted pyroclastic flows and lahars down drainages to the east and south.
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.
A pyroclastic flow descends drainages on the east flank of Fuego in October 1975 as an ash plume (right) towers above the summit. Intermittent minor eruptions took place 28 May, 23 July to 4 August, and 19 September to 21 October 1975. On 11-12 and 16 October ash fell in, and west of, Antigua. This photo was taken from Finca Capetillo, near Alotenango. Pyroclastic flows frequently travel down these same drainages and had occurred during the two previous eruptions in 1973 and 1974. 
One of the largest recorded eruptions in the Kuril Islands took place 11-16 June from Sarychev Peak. This NASA Space Shuttle view on 12 June shows an eruption plume that rose to 16-21 km altitude; pyroclastic flows reached the sea and extended the shoreline in some areas. The main explosive phase ended on 16 June, but weak explosions producing ash plumes continued prior to arrival of a field team on 26-28 July, when no eruptive activity was observed.
A new cone, Volcán Bárcena, was rapidly constructed during an eruption that began on 1 August 1952 at the southern end of Isla San Benedicto. This photo, taken from off the west coast of the island only four minutes after the start of the eruption, also shows the onset of pyroclastic surges. Lava extrusion occurred in September, November, and December. On 8 December activity shifted to a vent on the SE flank, producing a lava delta that extended about 700 m to sea. Activity ended in late February 1953.
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.
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.
Pyroclastic flows traveling down the flanks of Ngāuruhoe volcano on 19 February 1975 as an ash plume rises above the summit crater. The eruption column rose 12 km above the vent and ash fell 160 km away in Hamilton City. Explosive activity had begun on the 12th and continued until the 23rd.
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. 
A block-and-ash flow travels down the flank of Fugendake in 1991. These were created by collapse of a lava dome that began on 24 May. The size of the flows gradually increased and on 3 June the largest flow of the eruption to that point swept 3 km down the Mizunashi River valley into Kita-Kamikoba village, killing 43 people and destroying many houses. On 8 June block-and-ash flows traveled 5.5 km to within 50 m of a major highway and destroyed an additional 73 houses.
A pyroclastic flow descends the SW flank of Merapi volcano on 14 June 1984. A long period of lava dome growth began on 2 October 1972, and continued intermittently until 1990, occasionally punctuated by partial dome collapse that formed pyroclastic flows (block-and-ash flows) that travel down the flanks.
A large ash plume towers above Fuego in October 1974 as pyroclastic flows sweep down the SE (left) and NE flanks (center). Incandescent ejecta can be seen at the base of the ash plume. Prevailing winds distributed ashfall primarily to the SW. More than 0.2 km3 of ash was erupted during four distinct explosive pulses of 4 to 17 hours duration between 14 and 23 October.
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.
This photo shows an ash plume during the eruption of Spurr on 18 August 1992. A lighter plume rises above pyroclastic flows descending the SE flank to the right, and the summit lava dome complex is to the left. The 18 August event was the second of three brief powerful explosive eruptions in 1992.
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.
A powerful eruption at Fuego produced this ash plume at sunrise in October 1974. This view from Antigua Guatemala to the NE shows major pyroclastic flows moving down the eastern flank. Pyroclastic flows traveled 7 km but caused no fatalities. The 1974 eruption, the largest at Fuego since 1932, began with mild eruptions and small pyroclastic flows on 10 October. The eruption intensified on 14 October, with major Vulcanian eruptions occurring on that day, as well as 17-18, 19-20, and 23 October.
Ash rises above a pyroclastic flow racing down the eastern flank of Fuego in October 1974 that fills the entire field of view. The direction of movement of the pyroclastic flow was from right to left. Pyroclastic flows of comparable magnitude also traveled down the W and SW flanks.
A pyroclastic flow descends the western flank of Colima at 0830 on 22 November 1998. The new growing lava dome had almost entirely filled the 1994 crater by the morning of the previous day, after which lava started flowing out of the summit crater area and producing block-and-ash flows at 3-5-minute intervals that mostly traveled south within the eastern branch of Barranca El Cordobán to distances of more than 4 km.
An explosive eruption from Galunggung on 7 August 1982 is accompanied by a pyroclastic flow advancing over the crater rim towards a reflector station that was being measured using the tripod to the lower left. The photo was taken from Butik Pasir Bentag, about 2 km from the crater. The 1-km-high ash-covered cliff to the left is a wall of the breached caldera.
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.
A pyroclastic flow travels down the SE flank of Merapi on 7 June 2006. Renewed lava dome growth had begun at Merapi in April 2006. Ashfall was reported 5 km from the summit on 27 April. Beginning in May, pyroclastic flows descended the flanks of the volcano, sometimes reaching populated areas.
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.