A pyroclastic surge is a fluidised mass of turbulent gas and rock fragments that is ejected during some

volcanic eruption A volcanic eruption occurs when material is expelled from a volcanic vent or fissure. Several types of volcanic eruptions have been distinguished by volcanologists. These are often named after famous volcanoes where that type of behavior h ...

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, 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 ( ; or ; Kalinago language, Kalinago: or ) is an island in the Lesser Antilles of the West Indies, in the eastern Caribbean Sea. It was previously known as Iguanacaera which translates to iguana island in Carib language, Kariʼn ...

in 1902 was overcome by one. Pyroclastic surge include 3 types, which are base surge, ash-cloud surge, and ground surge.

Base surge

Base surges were first recognized after the

Taal Volcano Taal Volcano (; ) is a large caldera filled by Taal Lake in the Philippines. Located in the province of Batangas about south of Manila, the volcano is the second most List of active volcanoes in the Philippines, active volcano in the country ...

eruption of 1965 in the Philippines, where a visiting

volcanologist 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), founded as the Geological Survey, is an government agency, agency of the United States Department of the Interior, U.S. Department of the Interior whose work spans the disciplines of biology, geograp ...

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, th ...

s. Very similar to the ground-hugging blasts associated with nuclear explosions, these surges are expanding rings of a 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

maar A maar is a broad, low-relief volcanic crater caused by a phreatomagmatic eruption (an explosion which occurs when groundwater comes into contact with hot lava or magma). A maar characteristically fills with water to form a relatively shallow ...

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 (like

sandblasting 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. 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.

References

{{Volcanoes Volcanology he:זרם פירוקלסטי#נחשול פירוקלסטי

Base surge

Ash-cloud surge

Ground surge

See also

References