Pyroclastic surge

The term Surge ( engl. = surge wave, billow ), partially correct or base Ground Surge Surge, uses the sedimentology and volcanology as a general term for relatively low particle, turbulent, often hot currents of gas ( liquid) - particle mixture. However, it is not a wave but a transport and deposition process in the form of a pyroclastic flow.

Surges occur during volcanic explosions, steam and gas explosions or meteorite impacts. The explosion of a nuclear bomb can cause a surge, if it was detonated in the water or underground and the explosion column breaks through to the surface. The term surge or base surge is often translated as basic or pressure wave into German. It has nothing to do with a pressure or detonation wave, since it is not a short-term fluctuation in the air pressure, but a very rapid flow or a current.

Definition and terminology

A surge in sedimentological sense is a rapidly spreading density current of a gas ( liquid ) particles mixture with relatively low particle density (< 1% to about 0.1%). It can be hot ( up to 800 ° C), but also relatively cool ( around 80 ° C). The proportion of water droplets can be relatively high ( so-called "wet" Surges, eg phreatic explosions ) or very low (eg in "dry" pyroclastic surges ). The speed varies from about 100 km / h to 1000 km / h The transport of the particles and the turbulent velocity fluctuates greatly.

The term for the phenomenon of low particulate density currents is not uniform in the literature. In the older literature outside of volcanology Surges are commonly referred to as base surges or ground surges. In volcanic explosions, the term pyroclastic surge has largely prevailed in the recent literature. In the older literature, but will also use the term base surge or base surge deposits, identifying the deposits from phreatomagmatic explosions. In volcanology, the definitions of pyroclastic density currents and surges overlap. Pyroclastic surges are so Surges, whose particles consist of more than 75 % of pyroclastics. But genetic designations can be found in the literature, eg, be pyroclastic surges that result from detonation -like volcanic eruptions, also called " blast surges ".

Origin of the term

1946 lit the U.S. military in the Bikini Atoll in the Pacific in 30 feet of water a nuclear bomb (test series Crossroads Baker). There was a relatively low explosion column of water droplets and aerosols. She was too dense to rise high and therefore collapsed quickly. It was a turbulent gas -water mixture, the moving radially at a speed of about 100 km / h from the center of the explosion away. Physicists called this phenomenon base surge. 1962 an atomic bomb was detonated in 194 feet of water in alluvial deposits on the NTS nuclear test site in Nevada. The explosion created a crater with a diameter of 370 meters and a depth of 98 meters. After the collapse of the explosion a column base surge of gas was (from the nuclear explosion ), dust and sand. This base surge had a speed of originally 180 km / h

1965 broke the Taal Volcano on the Philippine island of Luzon. The eruption began as a strombolian eruption with a glowing lava fountain. But through a break in the crater rim then water flowed into the vent. Thereupon, the type of eruption changed abruptly to phreatomagmatisch. Several pyroclastic surges were formed and destroyed the surrounding area. They left small deposits with powerful cross stratification and dune stratification. It was recognized that there were the equivalents of base surges of atomic bomb explosions.

In the late 1960s, the emergence of the maars with Surges has been associated since they had found very similar deposits at the base of the ejecta of the crater lakes.

Later, it was also between the actual ignimbrites such deposits and concluded that even during magmatic eruptions hot pyroclastic surges may occur in addition to the actual pyroclastic flows. This meant that some researchers consider the Surges for a class of density currents.


The term is used for density surge currents that may have a different origin ( non- volcano ). This can be:

  • Atomic bomb explosions at or just above the soil surface ( or atomic explosions at shallow depths, break through their explosions on the earth's surface )
  • Gas and (water) vapor explosions
  • Meteorite impacts
  • Volcanic explosions
  • Explosive interaction of pyroclastic flows with sea water entering the streams into the sea. It was here that surges that flowed back to the country back.

Probably also other processes Surges can produce. Thus the collapse of the towers of the World Trade Center on September 11, 2001 it is a density cloud that reminded quite to the base surges in nuclear explosions.

Surge deposits

The term actually refers to Surge only the transport and deposition phenomenon, not the debris itself you are correctly referred to as surge deposits. However, it has become customary in many professional publications ( as with other transport and deposition phenomena as well) to describe the deposits as well as surges.

The deposits of Surges can be detected relatively easily in the field. They form relatively dünnbankige, only centimeter to decimeter - thick layers with cross stratification, cross- stratification with low angles, wave structures and relatively large dune structures. They are often associated with planar layers, and sometimes only these planar structures are present.

The latter are, however, in the case of pyroclastic surge deposits often difficult to interpret because pyroclastic fall deposits can have very similar structures. Large ballistically transported pyroclastic can deform these layers by the impact and form through bending structures ( sag structures).

Surge deposits of meteorite impacts include small, very characteristic beads ( " spherules " ), which are interpreted as products of impact melting, and shocked quartz.

See also: Impaktglas, pyroclastic flow deposit