Cryptovolcanism

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Shiprock, USA, a volcanic vent
Stanley's nut, a laccolith
Drachenfels in the Siebengebirge, a spring crest

Kryptovulkanismus (from Greek. Κρύπτω krypton, hide '), also Subvulkanismus (of lat. Sub, under'), is a form of volcanic activity , which extends between the Oberflächenvulkanismus and the Tiefenvulkanismus takes place. The rising magma still solidifies within the earth's crust, but close to the earth's surface, and forms volcanic rocks or, in the case of greater solidification depth, gangue rocks due to the longer cooling time .

to form

During the ascent, the magma penetrates through existing crevices and cavities in the earth's crust, and expands them by melting or displacing neighboring rock. When it reaches the surface of the earth, volcanic eruptions occur and it flows out as lava. In some cases, however, the penetrating magma gets stuck, for example due to its viscosity or due to a combination of cooling and a lack of supply. Depending on the geological situation, sub-volcanic intrusive bodies of the most varied of shapes arise .

The magma can penetrate in a club-like manner and the overlying layers bulge or dome-like, creating a swelling crest (see below). If it spreads near the earth's surface along a surface layer in the horizontal direction over greater distances, the result is a storage aisle (Sill), which has an approximately constant thickness over its spread. If the overlying layers bulge like a bulge in such a case, it is a laccolith . If the magma solidifies in columns, so forming a transition , however, it is frozen in the rise tube of a volcano, it is considered vent referred.

These structures become visible in a special way when the top layer has been removed and they remain as a hardened part . Some examples of sub-volcanic forms are:

Some sub-volcanoes show a regulation of tabular inserts, such as feldspar crystals, which, when the magma flows, arrange themselves in such a way that their long axis points in the direction of flow (flow structure). In some sub-volcanoes, the cooling creates a plate-like secretion structure parallel to the outer edge, or a columnar structure perpendicular to it. In the case of sub-volcanoes that are already largely eroded, all of these structural features can be used to reconstruct the former shape of the solidified body.

In the case of chimneys, the upper part of which has already been eroded, it is often not possible to say exactly how deep the chimney that is open today originally lay below the surface. In some cases, however, fragments of secondary rock that fell into the chimney and were later captured by the solidified magma give an indication of the minimum thickness of the former overburden.

Cryptodome

In the cryptodome , also known as the spring dome , the rising viscous magma formed club-shaped or spherical rock bodies within the soft covering layer (mostly made of tuff ), which it could bulge like a dome, but could not break through. Intermediate or felsic volcanic rocks such as trachyte , latite , phonolite or rhyolite are typical of spring crests . Examples of such swell knolls are:

At Usu , a volcano on the Japanese island of Hokkaidō , several new cryptodomes have been built in the last few centuries.

literature

  • Hans Murawski, Wilhelm Meyer: Geological dictionary . 11th edition. Elsevier / Spektrum, Heidelberg 2004, ISBN 3-8274-1445-8 .
  • Dieter Richter: General Geology . 3. Edition. de Gruyter Verlag, Berlin - New York 1985, ISBN 3-11-010416-4 , pp. 266 f .
  • Gerd Simper: Understanding and experiencing volcanism . Feuerland Verlag, Stuttgart 2005, ISBN 978-3-00-015117-0 .

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