Pyroclastic surge

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The term surge (English surge = wave, wave), z. Partly correct ground surge or base surge , uses sedimentology and volcanology as an umbrella term for relatively low-particle, turbulent, and often hot flows of a gas (liquid) particle mixture. However, it is not a wave, but a transport and deposition process in the form of a pyroclastic flow .

Surges arise from volcanic explosions, water vapor and gas explosions or from meteorite impacts . The explosion of an atomic bomb can also trigger a surge if it was ignited in water or underground and the column of explosion breaks through to the surface of the earth. The term surge or base surge is often translated into German as a fundamental or pressure wave. But it has nothing to do with a pressure or detonation wave , since it is not a short-term fluctuation of the air pressure, but a very rapid flow or a current.

Definition and terminology

A surge in the sedimentological sense is a rapidly spreading density flow of a gas (liquid) particle mixture with a relatively low particle density (<1% to about 0.1%). It can be hot (up to approx. 800 ° C), but also relatively cool (around 80 ° C). The proportion of water droplets can be relatively high (so-called “wet” surges, e.g. in the case of phreatic explosions ) or very low (e.g. in “dry” pyroclastic surges). The speed varies from around 100 km / h to over 1000 km / h. The particles are transported in a turbulent manner and the speed fluctuates strongly.

The term for the phenomenon of low-particle density flows is not uniform in the literature. In the older literature outside of volcanology, surges are often referred to as base surges or ground surges . In the case of volcanic explosions, the term pyroclastic surge has largely become established in recent literature . In the older literature, however, the term base surge or base surge deposits can also be found to denote the deposits from phreatomagmatic explosions. In volcanology the definitions of pyroclastic density currents and surges overlap . Pyroclastic surges are surges whose particles consist of more than 75% pyroclastics . But genetic terms can also be found in the literature, e.g. For example, pyroclastic surges, which arise from volcanic eruptions similar to detonation, are also called "blast surges".

Origin of the term

In 1946, the US military detonated an atomic bomb at a depth of 30 meters in Bikini Atoll in the Pacific ( test series Crossroads Baker ). A relatively low explosion column was created from water droplets and aerosols . It was too dense to climb high, so it collapsed quickly. A turbulent gas-water mixture was created, moving radially away from the center of the explosion at a speed of about 100 km / h. Physicists called this phenomenon a base surge . In 1962 an atomic bomb was detonated 194 meters below ground in alluvial deposits on the NTS atomic bomb test site in Nevada . The explosion created a crater 370 meters in diameter and 98 meters deep. After the explosion column collapsed, there was a base surge of gas (from the atomic explosion), dust and sand. This base surge had a speed of 180 km / h originally.

In 1965 the Taal volcano erupted on the Philippine island of Luzon . The eruption began as a strombolian eruption with a glowing lava fountain. However, due to a break in the crater rim, water then flowed into the chimney. Then the type of eruption suddenly changed to phreatomagmatic . Several pyroclastic surges formed and destroyed the surrounding area. They left thin deposits with cross-stratification and dune stratification . It was recognized that these were the equivalents of the base surges of atomic bomb explosions.

In the late 1960s, the formation of the maars was associated with the surges, as very similar deposits had been found at the base of the maars' ejecta .

Such deposits were later found between the actual ignimbrites and it was concluded from this that hot, pyroclastic surges can arise in addition to the actual pyroclastic currents during magmatic eruptions. This led some researchers to regard the surges as a class of density currents of their own.

Emergence

The term surge is used for density currents that can have different origins (including non-volcanic). This can be:

  • Atomic bomb explosions at or just above the earth's surface (or atomic explosions at shallow depths, the explosions of which break through to the earth's surface)
  • Gas and (water) steam explosions
  • Meteorite impacts
  • Volcanic explosions
  • explosive interactions of pyroclastic currents with sea water when the currents enter the sea. This is where surges arose that flowed back onto the land.

Other processes can probably also produce surges. When the towers of the World Trade Center collapsed on September 11, 2001, a density cloud was created that was quite reminiscent of the base surges in atomic explosions.

Surge deposits

The term surge actually only refers to the transport and deposition phenomenon , not the deposits themselves. They are correctly referred to as surge deposits. However, it has become common practice in many specialist publications (as is the case with other transport and deposition phenomena) to also refer to deposits as surges.

The deposits of surges can be recognized relatively easily in the field . They form relatively thin layers , only centimeter to decimeter thick, with cross-layering , cross-layering with small angles, wavy structures and relatively large dune structures . They are often associated with planar layers , and sometimes only these planar structures are present.

However, the latter are often difficult to interpret in the case of pyroclastic surge deposits, since pyroclastic fall deposits can have very similar structures. Large ballistically transported pyroclasts can deform these layers due to the impact and form sag structures .

Surge deposits from meteorite impacts contain small, very characteristic spherules, which are interpreted as melting products of the impact, and shocked quartz .

See also: impact glass , pyroclastic flow deposition

literature

  • Greg Valentine, Richard V. Fisher: Pyroclastic surges and blasts. In: Haraldur Sigurdsson (Ed.): Encyclopedia of Volcanoes. Academic Press, San Diego et al. 2000, ISBN 0-12-643140-X , pp. 571-580.
  • KH Wohletz, MF Sheridan: A model of pyroclastic surge. In: Geological Society of America. Special Paper, 180, New York 1977, pp. 177-194.
  • Alain Burgisser, George W. Bargantz: Reconciling Pyroclastic Flow and Surge: the Multiphase Physics of Pyroclastic Density Currents. In: Earth and Planetary Letters. 202, Amsterdam 2002, pp. 405-418, doi : 10.1016 / S0012-821X (02) 00789-6 .

Individual evidence

  1. ↑ Flavio Dobran: Volcanic Processes - Mechanisms in Material Transports. Luwer Academic / Plenum Publ., New York 2001, ISBN 0-306-46625-2 .
  2. ^ GS Gohn, C. Koeberl, KG Miller. WU Reimold, JV Browning, CS Cockell, JW Horton Jr., T. Kenkmann, AA Kulpecz, DS Powars, WE Sanford, MA Voytek: Deep Drilling into the Chesapeake Bay Impact Structure. In: Science. 320, pp. 1740-1745, abstract
  3. ↑ M. Edmonds, RA Herd: Inland-directed base surge generated by the explosive interaction of pyroclastic flows and seawater at Soufrière Hills volcano, Montserrat. In: Geology. 33, Boulder, Col. 2005, pp. 245-248, doi : 10.1130 / G21166.1 .
  4. ^ Thoughts on what happened September 11, 2001
  5. ^ New York, Miami, Atlanta, New Orleans, Houston, Dallas-Fort Worth, St Louis, and Honolulu ( Memento of January 16, 2010 in the Internet Archive )

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