Phreatic eruption

A phreatic explosion ( ancient Greek φρέαρ phréar, fountains ', genitive φρέατος phréatos ), also phreatic eruption phreatic eruption, hydrovulkanische or hydro- magmatic steam explosion, in the broader sense is a volcanic explosion that results from a steam explosion associated with magmatic activity. It is super-heated, external water due to a sudden depressurization to steam, which is about the 1000 -. Has up to 3000 times the volume of water [note 1 ] This increase in volume is busted a blast crater in the ground. The rock is thereby destroyed, ejected and deposited around the crater as Wallpaper. Phreatic eruptions are thus belong to the physical explosions; they contain only zersprengtes " Altgestein ", no juvenile rock ( ie not strictly pyroclastics ).

In the original definition, the term was limited to groundwater ( = phreatisches water). Today is simply spoken of external water (as opposed to juvenile water that is promoted up with the magma ). Phreatic explosions are often followed by phreatomagmatic eruptions, and are therefore also along with this explosion type grouped under the umbrella terms Hydrovulkanismus or Hydromagmatismus ( and other phenomena ).

Formation

A phreatic explosion occurs when external water seeping or flowing (but without direct contact ) and from the magma or by ascending from the depths of gases is superheated in the vicinity of the magma. If now the auflagernde rock suddenly permeable approximately by decomposing solfataras activity cracked or by an event (eg, earthquake ) and can escape some steam, it comes as a result of pressure relief to the explosive vaporization of the superheated water and thus to the phreatic explosion.

Phreatic eruptions can, for example, volcanoes with solfataras activity occur suddenly. Solfataras are often heavily gesteinszersetzend and thereby contribute to the ( sudden ) permeability in the rock. However Phreatic eruptions are usually short- lived, as the ( subsequently flowing ) water reservoir rapidly exhausted. They promote only steam, a steam - particle mixture and smashed Altgestein, which may consist of sedimentary rocks, plutonic and metamorphic rocks, but also from older volcanic rocks or pyroclastics. The latter are still to " Altgestein " instead, because they do not directly belong to the outbreak. The explosion is typically a few tens to about 100 meters depth instead.

Through a phreatic explosion may occur to expose conveyor vents for the magma. Frequently rising magma is the direct cause of the explosion. A phreatic explosion can therefore s.str of phreatomagmatic eruptions. be followed when ground water then comes in direct contact with magma. It may be followed by further violent steam explosions, and it comes to mixing with pyroclastics or for ejection of pyroclastic material exclusively. The sequence of phreatic explosions to phreatomagmatic is relatively common, and the two processes are often separated only by a detailed analysis of the ejecta. Many older writers have in failing to distinguish the two processes.

Closely related to the phreatic explosion is the mechanism of a geyser eruption. In contrast, a geyser has a stable, narrow tube of an underground water reservoir to the surface. Geothermal heat heats the water of the reservoir to about 100 ° C, wherein the pressure of the water in the first tube to prevent boiling. Only when the vapor pressure exceeds the water pressure, the narrow channel single vapor bubbles rise upwards. This causes the pressure in the underground water reservoir rapidly declines, and the superheated water goes abruptly into steam and throws the water in the tube and in the underground reservoir from. It is often not only water and steam, but also expelled dissolved minerals and rock particles in the water.

Formation of Surges

Phreatic ( and especially phreatomagmatic ) eruptions can also energy-rich " (pressure) waves " (English surges [ Note 2 ] or base surges ) generate that in a mixture of gases, water vapor, (old) rock particles and possibly case of phreatomagmatic eruption and volcanic ash exist. The term " (pressure) wave " in this context is somewhat misleading because it is not about short-term pressure differences, but a vapor -particle current. Therefore, the technical term surge has become the norm for this phenomenon. Surges can be similar to the pyroclastic density currents with high speed and high destructive power spread close to the ground. Since they do not contain pyroclastics, they can be taken strictly not called pyroclastic surges. The deposits of these surges are similar to the deposits that are produced from the base surges of atomic bomb explosions.

Maars and their emergence

Phreatic explosions are responsible for the emergence of numerous maars in which only scrap material was ejected. Other Maare, however, are surrounded by ring of ramparts pyroclastic deposits or rocks. Therefore, before a formation was discussed by purely magmatic eruptions. However, most ringwalls are underlain at the base of the existing chimney to evacuate from Altgestein breccias. They prove that preceded these magmatic eruptions, phreatic eruptions ( and phreatomagmatic eruptions). Some authors therefore refer to this sequence of phreatic to magmatic eruptions as phreatomagmatic eruptions.