Phreatic explosion

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Phreatic Explosions, Spirit Lake , Mount St. Helens
Scheme of a phreatic eruption

A phreatic explosion ( ancient Greek φρέαρ phréar , German 'fountain' , genitive φρέατος phréatos ), also phreatic eruption , phreatic eruption , hydrovolcanic or hydromagmatic steam explosion , is in the broader sense a volcanic explosion that results from a water vapor explosion in connection with magmatic explosion . During this process, superheated, external water, as a result of a sudden pressure release, turns into steam , which has approximately 1000 to 3000 times the volume of the water. This increase in volume becomes aExplosion crater blasted underground. The rock is smashed, ejected and deposited as a wall around the crater . Phreatic eruptions can therefore be assigned to physical explosions ; they only contain broken "old rock", no juvenile rock (strictly speaking no pyroclasts ).

In the original definition, the term was limited to groundwater (= phreatic water). Today we speak simply of external water (as opposed to juvenile water , which is carried up with the magma ). Phreatic explosions are often followed by phreatomagmatic outbreaks and are therefore grouped together with this type of explosion (and other phenomena) under the umbrella term hydrovolcanism or hydromagmatism .

Emergence

A phreatic explosion occurs when external water seeps or flows near the magma (but without direct contact) and is overheated by the magma or by gases rising from the depths. If the overlying rock cracks due to decomposing solfataras or suddenly becomes permeable due to an event (e.g. earthquake) and some steam can escape, the pressure relief leads to explosive evaporation of the superheated water and thus to a phreatic explosion.

Phreatic outbursts can e.g. B. suddenly occur on volcanoes with solfataras activity. Solfataras are often strongly decomposing rocks and thus contribute to the (sudden) permeability of the rock. However, phreatic outbreaks are usually short-lived, as the (flowing) water reservoir is quickly exhausted. They only convey steam, a mixture of steam and particles and crushed old rock, which can consist of sedimentary rocks , plutonic and metamorphic rocks , but also of older volcanic or pyroclastic rocks . The latter are still counted as "old rock" as they do not belong directly to the eruption. The explosion typically takes place at a depth of a few tens to around 100 meters.

A phreatic explosion can expose conveyor chimneys for the magma. Rising magma is often the direct cause of the explosion. A phreatic explosion can therefore be followed by phreatomagmatic outbreaks in the narrower sense if groundwater then comes into direct contact with magma. Further violent water vapor explosions can follow, which results in mixing with pyroclasts or in the ejection of exclusively pyroclastic material. The sequence from phreatic to phreatomagmatic explosions occurs relatively frequently, and the two processes can often only be separated by a detailed analysis of the ejection material. Many older authors have therefore not distinguished the two processes.

Closely related to the phreatic explosion is the eruption mechanism of a geyser . In contrast, a geyser has a constant, narrow tube from an underground water reservoir to the surface of the earth. The geothermal heat heats the water in the reservoir to over 100 ° C, with the pressure of the water in the tube initially preventing it from boiling. Only when the steam pressure exceeds the water pressure do individual steam bubbles rise up the narrow channel. As a result, the pressure in the underground water reservoir drops rapidly, and the superheated water suddenly turns into steam and hurls the water in the tube and in the underground reservoir. Often times, not only water and steam, but also minerals and rock particles dissolved in the water are emitted.

Formation of surges

Phreatic (and especially phreatomagmatic) eruptions can also generate high- energy "(pressure) waves" ( English surges or base surges ), which are made up of a mixture of gases, water vapor, (old) rock particles and possibly also in the case of phreatomagmatic eruptions of volcanic ash . The term “(pressure) wave” in this context is somewhat misleading, however, as it is not a question of short-term pressure differences, but a vapor-particle flow. That is why the technical term surge has become common for this phenomenon . Like pyroclastic density currents, surges can propagate close to the ground at high speed and with high destructive force. Since they do not contain pyroclasts , strictly speaking, they cannot be called pyroclastic surges. The deposits of these surges are similar to the deposits that are created by the base surges of atomic bomb explosions.

Maars and their formation

Phreatic explosions are responsible for the creation of numerous maars in which only old material was ejected. Other maars, however, are surrounded by ramparts made of pyroclastic deposits or rocks. For this reason, an emergence from purely magmatic eruptions was discussed earlier . However, most of the ring walls are underlain at the base of old rock vent breccias . They show that these magmatic eruptions were preceded by phreatic eruptions (and phreatomagmatic eruptions). Some authors therefore refer to this sequence from phreatic to magmatic bursts as phreatomagmatic bursts.

See also

Remarks

  1. In the literature there are diverging values ​​for the expansion of water vapor compared to water; up to 1000 times with Simper, but 3000 times with Parfitt and Wilson; the temperature of the steam also plays a role here.
  2. ^ Translation at leo.org

literature

  • Elisabeth A. Parfitt and Lionel Wilson: Fundamentals of Physical Volcanology. Blackwell Publishing, Malden MA 2008, ISBN 978-0-63205443-5 .
  • Hans Pichler and Thomas Pichler: volcanic areas of the earth. Spectrum Akademischer Verlag, Heidelberg 2007, ISBN 978-3-8274-1475-5 .
  • Hans-Ulrich Schmincke: Pyroclastic rocks: In: Hans Füchtbauer (Hrsg.): Sediment-Petrology, Part 2: Sediments and sedimentary rocks. 4th edition, E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart 1988, ISBN 3-510-65138-3 , pp. 731-778.
  • Gerd Simper: Understanding and experiencing volcanism. Feuerland Verlag, Stuttgart 2005, ISBN 3-00-015117-6 .

Web links

Commons : Phreatic explosions  - collection of images, videos and audio files