Pyroclastic Flow - Volcanic Hazards PDF

*A **pyroclastic flow** (also known as a **pyroclastic density current** or a **pyroclastic cloud**)[^\[1\]^](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-1) is a fast-moving current of hot [gas](https://en.wikipedia.org/wiki/Volcanic_gas) and volcanic matter (collectively known as [teph...

*A **pyroclastic flow** (also known as a **pyroclastic density current** or a **pyroclastic cloud**)[^\[1\]^](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-1) is a fast-moving current of hot [gas](https://en.wikipedia.org/wiki/Volcanic_gas) and volcanic matter (collectively known as [tephra](https://en.wikipedia.org/wiki/Tephra)) that flows along the ground away from a [volcano](https://en.wikipedia.org/wiki/Volcano) at average speeds of 100 km/h (30 m/s; 60 mph) but is capable of reaching speeds up to 700 km/h (190 m/s; 430 mph).[^\[2\]^](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-2) The gases and tephra can reach temperatures of about 1,000 °C (1,800 °F).* *Pyroclastic flows are the most deadly of all [volcanic hazards](https://en.wikipedia.org/wiki/Volcanic_hazard)[^\[3\]^](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-3) and are produced as a result of certain [explosive eruptions](https://en.wikipedia.org/wiki/Explosive_eruptions); they normally touch the ground and hurtle 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.* ***Origin of term*** *\[(https://en.wikipedia.org/w/index.php?title=Pyroclastic_flow&action=edit&section=1)\]* A pair of rocks on a white surface Description automatically generated*Pyroclastic rocks from the [Bishop tuff](https://en.wikipedia.org/wiki/Bishop_tuff); uncompressed with [pumice](https://en.wikipedia.org/wiki/Pumice) (on left), compressed with [fiamme](https://en.wikipedia.org/wiki/Fiamme) (on right)* *The word pyroclast is derived from the [Greek](https://en.wikipedia.org/wiki/Greek_language) [πῦρ](https://en.wiktionary.org/wiki/%CF%80%E1%BF%A6%CF%81) (pýr), meaning \"fire\", and [κλαστός](https://en.wiktionary.org/wiki/%CE%BA%CE%BB%CE%B1%CF%83%CF%84%CF%8C%CF%82) (klastós), meaning \"broken in pieces\".^[\[4\]](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-4)[\[5\]](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-PerseusDict-5)^ A name for pyroclastic flows that glow red in the dark is **nuée ardente** (French, \"burning cloud\"); this was notably used to describe the disastrous 1902 eruption of [Mount Pelée](https://en.wikipedia.org/wiki/Mount_Pel%C3%A9e) on [Martinique](https://en.wikipedia.org/wiki/Martinique), a French island in the Caribbean.^[\[6\]](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-6)[\[note\ 1\]](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-7)^* *Pyroclastic flows that contain a much higher proportion of gas to rock are known as \"fully dilute pyroclastic density currents\" or [pyroclastic surges](https://en.wikipedia.org/wiki/Pyroclastic_surge). The lower density sometimes allows them to flow over higher topographic features or water such as ridges, hills, rivers, and seas. They may also contain steam, water, and rock at less than 250 °C (480 °F); these are called \"cold\" compared with other flows, although the temperature is still lethally high. Cold pyroclastic surges can occur when the eruption is from a vent under a shallow lake or the sea. Fronts of some pyroclastic density currents are fully dilute; for example, during the eruption of Mount Pelée in 1902, a fully dilute current overwhelmed the city of [Saint-Pierre](https://en.wikipedia.org/wiki/Saint-Pierre,_Martinique) and killed nearly 30,000 people.[^\[7\]^](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-8)* *A pyroclastic flow is a type of [gravity current](https://en.wikipedia.org/wiki/Gravity_current); in scientific literature, it is sometimes abbreviated to PDC (pyroclastic density current).* ***Causes*** *\[(https://en.wikipedia.org/w/index.php?title=Pyroclastic_flow&action=edit&section=2)\]* ----------------------- -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------  *This section **does not [cite](https://en.wikipedia.org/wiki/Wikipedia:Citing_sources) any [sources](https://en.wikipedia.org/wiki/Wikipedia:Verifiability)**. Please help [improve this section](https://en.wikipedia.org/wiki/Special:EditPage/Pyroclastic_flow) by [adding citations to reliable sources](https://en.wikipedia.org/wiki/Help:Referencing_for_beginners). Unsourced material may be challenged and [removed](https://en.wikipedia.org/wiki/Wikipedia:Verifiability#Burden_of_evidence). (March 2018) ([Learn how and when to remove this message](https://en.wikipedia.org/wiki/Help:Maintenance_template_removal))* ----------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- *Several mechanisms can produce a pyroclastic flow:* - *Fountain collapse of an [eruption column](https://en.wikipedia.org/wiki/Eruption_column) from a [Plinian eruption](https://en.wikipedia.org/wiki/Plinian_eruption) (e.g. [Mount Vesuvius\' destruction of Herculaneum and Pompeii](https://en.wikipedia.org/wiki/Eruption_of_Mount_Vesuvius_in_79) in 79 AD). In such an eruption, the material forcefully ejected from the vent heats the surrounding air and the turbulent mixture rises, through [convection](https://en.wikipedia.org/wiki/Convection), for many kilometers. If the erupted jet is unable to heat the surrounding air sufficiently, convection currents will not be strong enough to carry the plume upwards and it falls, flowing down the flanks of the volcano.[^\[8\]^](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-9)* - *Fountain collapse of an eruption column associated with a [Vulcanian eruption](https://en.wikipedia.org/wiki/Vulcanian_eruption) (e.g., [Montserrat](https://en.wikipedia.org/wiki/Montserrat)\'s [Soufrière Hills](https://en.wikipedia.org/wiki/Soufri%C3%A8re_Hills) volcano has generated many of these deadly pyroclastic flows and surges). The gas and projectiles create a cloud that is denser than the surrounding air and becomes a pyroclastic flow.* - *Frothing at the mouth of the vent during degassing of the erupted lava. This can lead to the production of a rock called [ignimbrite](https://en.wikipedia.org/wiki/Ignimbrite). This occurred during the eruption of [Novarupta](https://en.wikipedia.org/wiki/Novarupta) in 1912.* - *Gravitational collapse of a [lava dome](https://en.wikipedia.org/wiki/Lava_dome) or spine, with subsequent avalanches and flows down a steep slope (e.g., Montserrat\'s Soufrière Hills volcano, which caused nineteen deaths in 1997).* - *The [directional blast](https://en.wikipedia.org/wiki/Lateral_eruption) (or jet) when part of a volcano collapses or explodes (e.g., the [eruption of Mount St. Helens on May 18, 1980](https://en.wikipedia.org/wiki/1980_eruption_of_Mount_St._Helens)). As distance from the volcano increases, this rapidly transforms into a gravity-driven current.* ***Size and effects*** *\[(https://en.wikipedia.org/w/index.php?title=Pyroclastic_flow&action=edit&section=3)\]* A rock structure with wires Description automatically generated with medium confidence*Building remnant in Francisco Leon destroyed by pyroclastic surges and flows during eruption of [El Chichon](https://en.wikipedia.org/wiki/El_Chichon) volcano in Mexico in 1982. Reinforcement rods in the concrete were bent in the direction of the flow.**A scientist examines pumice blocks at the edge of a pyroclastic flow deposit from [Mount St. Helens](https://en.wikipedia.org/wiki/Mount_St._Helens)*A group of people lying on the ground Description automatically generated*The casts of some victims in the so-called \"Garden of the Fugitives\", [Pompeii](https://en.wikipedia.org/wiki/Pompeii)* *Flow volumes range from a few hundred cubic meters to more than 1,000 cubic kilometres (240 cu mi). Larger flows can travel for hundreds of kilometres, although none on that scale has occurred for several hundred thousand years. Most pyroclastic flows are around one to ten cubic kilometres (^1^⁄~4~--2+^1^⁄~2~ cu mi) and travel for several kilometres. Flows usually consist of two parts: the basal flow hugs the ground and contains larger, coarse boulders and rock fragments, while an extremely hot [ash plume](https://en.wikipedia.org/wiki/Ash_plume) lofts above it because of the turbulence between the flow and the overlying air, admixing and heating cold atmospheric air causing expansion and convection.[^\[9\]^](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-10) Flows can deposit less than 1 meter to 200 meters in depth of loose rock fragment.[^\[10\]^](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-11)* *The [kinetic energy](https://en.wikipedia.org/wiki/Kinetic_energy) of the moving cloud will flatten trees and buildings in its path. The hot gases and high speed make them particularly lethal, as they will incinerate living organisms instantaneously or turn them into carbonized fossils:* - *The cities of [Pompeii](https://en.wikipedia.org/wiki/Pompeii) and [Herculaneum](https://en.wikipedia.org/wiki/Herculaneum), Italy, for example, were engulfed by pyroclastic surges in 79 AD with many lives lost.[^\[11\]^](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-12)* - *The 1902 eruption of [Mount Pelée](https://en.wikipedia.org/wiki/Mount_Pel%C3%A9e) destroyed the [Martinique](https://en.wikipedia.org/wiki/Martinique) town of [St. Pierre](https://en.wikipedia.org/wiki/Saint-Pierre,_Martinique). Despite signs of impending eruption, the government deemed St. Pierre safe due to hills and valleys between it and the volcano, but the pyroclastic flow charred almost the entirety of the city, killing all but three of its 30,000 residents.^\[[citation\ needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)\]^* - *A pyroclastic surge killed [volcanologists](https://en.wikipedia.org/wiki/Volcanologist) [Harry Glicken](https://en.wikipedia.org/wiki/Harry_Glicken) and [Katia and Maurice Krafft](https://en.wikipedia.org/wiki/Katia_and_Maurice_Krafft) and 40 other people on [Mount Unzen](https://en.wikipedia.org/wiki/Mount_Unzen), in Japan, on June 3, 1991. The surge started as a pyroclastic flow and the more energised surge climbed a spur on which the Kraffts and the others were standing; it engulfed them, and the corpses were covered with about 5 mm (^1^⁄~4~ in) of ash.[^\[12\]^](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-13)* - *On June 25, 1997, a pyroclastic flow travelled down Mosquito Ghaut on the [Caribbean](https://en.wikipedia.org/wiki/Caribbean) island of [Montserrat](https://en.wikipedia.org/wiki/Montserrat). A large, highly energized pyroclastic surge developed. This flow could not be restrained by the Ghaut and spilled out of it, killing 19 people who were in the Streatham village area (which was officially evacuated). Several others in the area suffered severe burns.^\[[citation\ needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)\]^* ***Interaction with water*** *\[(https://en.wikipedia.org/w/index.php?title=Pyroclastic_flow&action=edit&section=4)\]* *Testimonial evidence from the [1883 eruption of Krakatoa](https://en.wikipedia.org/wiki/1883_eruption_of_Krakatoa), supported by experimental evidence,[^\[13\]^](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-Freundt2003-14) shows that pyroclastic flows can cross significant bodies of water. However, that might be a [pyroclastic surge](https://en.wikipedia.org/wiki/Pyroclastic_surge), not flow, because the density of a gravity current means it cannot move across the surface of water.[^\[13\]^](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-Freundt2003-14) One flow reached the [Sumatran](https://en.wikipedia.org/wiki/Sumatra) coast as far as 48 kilometres (26 nautical miles) away.[^\[14\]^](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-15)* *A 2006 BBC documentary film, Ten Things You Didn\'t Know About Volcanoes,[^\[15\]^](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-16) demonstrated tests by a research team at [Kiel University](https://en.wikipedia.org/wiki/Kiel_University), Germany, of pyroclastic flows moving over the water.[^\[16\]^](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-17) When the reconstructed pyroclastic flow (stream of mostly hot ash with varying densities) hit the water, two things happened: the heavier material fell into the water, precipitating out from the pyroclastic flow and into the liquid; the temperature of the ash caused the water to evaporate, propelling the pyroclastic flow (now only consisting of the lighter material) along on a bed of steam at an even faster pace than before.* *During some phases of the Soufriere Hills volcano on Montserrat, pyroclastic flows were filmed about 1 km (^1^⁄~2~ nmi) offshore. These show the water boiling as the flow passes over it. The flows eventually built a delta, which covered about 1 km^2^ (250 acres). Another example was observed in 2019 at [Stromboli](https://en.wikipedia.org/wiki/Stromboli) when a pyroclastic flow traveled for several hundreds of meters above the sea.[^\[17\]^](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-18)* *A pyroclastic flow can interact with a body of water to form a large amount of mud, which can then continue to flow downhill as a [lahar](https://en.wikipedia.org/wiki/Lahar). This is one of several mechanisms that can create a lahar.^\[[citation\ needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)\]^* ***On other celestial bodies*** *\[(https://en.wikipedia.org/w/index.php?title=Pyroclastic_flow&action=edit&section=5)\]* *In 1963, NASA astronomer [Winifred Cameron](https://en.wikipedia.org/wiki/Winifred_Cameron) proposed that the lunar equivalent of terrestrial pyroclastic flows may have formed sinuous [rilles](https://en.wikipedia.org/wiki/Rille) on the [Moon](https://en.wikipedia.org/wiki/Moon). In a lunar volcanic eruption, a pyroclastic cloud would follow local relief, resulting in an often sinuous track. The Moon\'s [Schröter\'s Valley](https://en.wikipedia.org/wiki/Schroter%27s_Valley) offers one example.^[\[18\]](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-19)\[[non-primary\ source\ needed](https://en.wikipedia.org/wiki/Wikipedia:No_original_research#Primary,_secondary_and_tertiary_sources)\]^ Some volcanoes on [Mars](https://en.wikipedia.org/wiki/Mars), such as [Tyrrhenus Mons](https://en.wikipedia.org/wiki/Tyrrhenus_Mons) and [Hadriacus Mons](https://en.wikipedia.org/wiki/Hadriacus_Mons), have produced layered deposits that appear to be more easily eroded than lava flows, suggesting that they were emplaced by pyroclastic flows.[^\[19\]^](https://en.wikipedia.org/wiki/Pyroclastic_flow#cite_note-20)*