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A pyroclastic surge is a fluidised mass of turbulent gas and rock fragments that is ejected during some
volcanic eruption
Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are oft ...
s. It is similar to a pyroclastic flow
A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...
but it has a lower density or contains a much higher ratio of gas to rock, which makes it more turbulent and allows it to rise over ridges and hills rather than always travel downhill as pyroclastic flows do.
The speed of pyroclastic density currents has been measured directly via photography only in the case of Mount St. Helens
Mount St. Helens (known as Lawetlat'la to the indigenous Cowlitz people, and Loowit or Louwala-Clough to the Klickitat) is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United St ...
, where they reached 320-470 km/h, or . Estimates of other modern eruptions are around 360 km/h, or 100 m/s (225 mph). Pyroclastic flows may generate surges. For example, the city of Saint-Pierre in Martinique
Martinique ( , ; gcf, label=Martinican Creole, Matinik or ; Kalinago: or ) is an island and an overseas department/region and single territorial collectivity of France. An integral part of the French Republic, Martinique is located in ...
in 1902 was overcome by one. Pyroclastic surge include 3 types, which are base surge, ash-cloud surge, and ground surge.
Base surge
First recognized after the Taal Volcano eruption of 1965 in the Philippines, where a visitingvolcanologist
A volcanologist, or volcano scientist, is a geologist who focuses on understanding the formation and eruptive activity of volcanoes. Volcanologists frequently visit volcanoes, sometimes active ones, to observe and monitor volcanic eruptions, col ...
from USGS
The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, a ...
recognized the phenomenon as congruent to base surge in nuclear explosion
A nuclear explosion is an explosion that occurs as a result of the rapid release of energy from a high-speed nuclear reaction. The driving reaction may be nuclear fission or nuclear fusion or a multi-stage cascading combination of the two, tho ...
s. Very similar to the ground-hugging blasts associated with nuclear explosions, these surges are expanding rings of turbulent mixture of fragments and gas that surge outward at the base of explosion columns. Base surges are more likely generated by the interaction of magma and water or phreatomagmatic eruption
Phreatomagmatic eruptions are volcanic eruptions resulting from interaction between magma and water. They differ from exclusively magmatic eruptions and phreatic eruptions. Unlike phreatic eruptions, the products of phreatomagmatic eruptions cont ...
s.Becker, Robert John and Becker, Barbara (1998). "Volcanoes", p.133. J.H. Freeman and Company, US. . They develop from the interaction of magma (often basaltic) and water to form thin wedge-shaped deposits characteristic of maars.
Ash-cloud surge
These are the most devastating. They form thin deposits, but travel at great speed (10–100 m/s) carrying abundant debris such as trees, rocks, bricks, tiles etc. They are so powerful that they often blast and erode material (likesandblasting
Sandblasting, sometimes known as abrasive blasting, is the operation of forcibly propelling a stream of abrasive material against a surface under high pressure to smooth a rough surface, roughen a smooth surface, shape a surface or remove su ...
). They are possibly produced when conditions in an eruption column are close to the boundary conditions separating convection from collapse. That is, switching rapidly from one condition to the other.
Ground surge
These deposits are often found at the base of pyroclastic flows. They are thinly bedded, laminated and often cross-bedded.{{Cite journal, last1=Douillet, first1=Guilhem Amin, last2=Pacheco, first2=Daniel Alejandro, last3=Kueppers, first3=Ulrich, last4=Letort, first4=Jean, last5=Tsang-Hin-Sun, first5=Ève, last6=Bustillos, first6=Jorge, last7=Hall, first7=Minard, last8=Ramón, first8=Patricio, last9=Dingwell, first9=Donald B., date=2013-10-13, title=Dune bedforms produced by dilute pyroclastic density currents from the August 2006 eruption of Tungurahua volcano, Ecuador, url= , journal=Bulletin of Volcanology, language=en, volume=75, issue=11, pages=762, doi=10.1007/s00445-013-0762-x, issn=1432-0819, pmc=4456068, pmid=26069385 Typically they are about 1 m. thick and consist mostly of lithic and crystal fragments (fine ash elutriated away). They appear to form from the flow itself, but the mechanism is not clear. One possibility is that the head of the flow expands through entrainment of air (which is then heated). This then results in the flow front surging forward, which is then over-run by the rest of the flow.
See also
*Pyroclastic fall
A pyroclastic fall is a uniform deposit of material which has been ejected from a volcanic eruption or plume such as an ash fall or tuff. Pyroclastic air fall deposits are a result of:
# Ballistic transport of ejecta such as volcanic blocks, vol ...
*Pyroclastic rock
Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...
References