page_title: Pyroclastic flow
section_title: Size and effects
hierarchical_section_title: Pyroclastic flow / Size and effects
caption_attribution_description: English: Building remnant in Francisco Leon destroyed by pyroclastic surges and flows during eruption of El Chichon volcano in Mexico 1982. Reinforcement rods in concrete bend in direction of flow.
context_page_description: A pyroclastic flow is a fast-moving current of hot gas and volcanic matter that flows along the ground away from a volcano at velocities of 100 km/h on average but is capable of reaching speeds up to 700 km/h. The gases and tephra can reach temperatures of about 1,000 °C.
  Pyroclastic flows are the most deadly of all volcanic hazards and are produced as a result of certain 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.
context_section_description: Flow volumes range from a few hundred cubic meters (yards) to more than 1,000 cubic kilometres (~240 cubic miles). Larger flows can travel for hundreds of kilometres (miles), although none on that scale has occurred for several hundred thousand years. Most pyroclastic flows are around 1 to 10 km³ (about ¼ to 2½ cubic miles) 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 lofts above it because of the turbulence between the flow and the overlying air, admixing and heating cold atmospheric air causing expansion and convection.
  The 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 and Herculaneum, Italy, for example, were engulfed by pyroclastic surges on August 24, 79 AD with many lives lost.
  The 1902 eruption of Mount Pelée destroyed the Martinique town of St. Pierre. 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 two of its 30,000 residents.
  A pyroclastic surge killed volcanologists Harry Glicken and Katia and Maurice Krafft and 40 other people on 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 (0.2 in) of ash.
  On 25 June 1997, a pyroclastic flow travelled down Mosquito Ghaut on the Caribbean island of 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.
caption: Building remnant in Francisco Leon destroyed by pyroclastic surges and flows during eruption of El Chichon volcano in Mexico 1982. Reinforcement rods in concrete bent in the direction of the flow.
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