A volcanite (also volcanic rock, effusive rock, igneous rock, effusive rock or extrusive rock ) is a rock that is formed as a result of continental or oceanic volcanic activity through rapid cooling of a rock melt on the earth's surface or near the surface (cooling <5 km depth to the surface). Volcanics are as Lakkolithe , chimney rock , lava or pyroclastic or pyroclastic sediments before. Together with the plutonites (deep rocks), which arise from melt ( magma ) that slowly cools in deeper areas of the earth's crust , they form the group of igneous rocks (magmatites). Occasionally, rocks that have solidified in the transition area between volcanic and plutonite rocks are referred to as sub- volcanic rocks .
In contrast to deep rocks, igneous rocks are often very fine-grained or even glassy , which leads to a more even color. The reason for this is the rapid cooling, which leaves too little time for large crystals to grow. If, however , crystals formed in the magma chamber are carried along and trapped in the fine base mass as fragments when the melt solidifies , one speaks of a rock with a porphyry structure , which is characteristic of many volcanic rocks. Igneous rocks solidified as a lava flow can contain many bubble cavities formed by volcanic gases . During the outflow of the lava flow, regulation of tabular or stem-like crystals parallel to the direction of flow can lead to the formation of a so-called flowing texture or a trachytic structure .
The mineral composition of volcanic rocks is very diverse and reflects a number of processes of magma formation and the cooling history of magma ( fractional crystallization ). Frequently occurring minerals are z. B. quartz , feldspar , foids , pyroxene , olivine , amphibole , magnetite and other oxides .
Often volcanic rocks also contain inclusions of secondary rock ( xenolites ) that fell into the magma chamber or was carried away by the chimney walls during the magma ascent . A typical example here are the olivine bombs found in the basalt tuffs of the Eifel . Analogous to the plutonites , the magma differentiation can be used to give a rough sequence of volcanic rocks according to their differentiation (little to high):
Classification and examples
Different classification methods can be used depending on the type of volcanic rock. The classification using the route iron diagram based on their mineral inventory is widespread . Fine-grained or glassy volcanic rocks are classified in the TAS diagram according to their chemical composition. Further classification schemes exist, for example, for ultramafic or alkali-rich volcanic rocks and for pyroclastic sediments.
Some examples of volcanic rocks are:
Volcanic rocks were formed in all geological ages and are also formed today in areas of active volcanic activity. Volcanics occur on all continents, especially in areas of recent or former continental plate margins , rifts and mantle plumes . The most important group in terms of volume are the basalts of the ocean floors , which are formed on the mid-ocean ridges ( MORB ).
- Martin Okrusch, Siegfried Matthes: Mineralogy. Introduction to special mineralogy, petrology and geology. 7th, completely revised and updated edition. Springer, Berlin a. a. 2005, ISBN 3-540-23812-3 .
- Roger W. Le Maitre (Ed.): Igneous Rocks. A Classification and Glossary of Terms. 2nd edition, reprinted, 1st paperback edition. Cambridge University Press, Cambridge u. a. 2004, ISBN 0-521-61948-3 .
- John D. Winter: An introduction to igneous and metamorphic petrology. Prentice Hall, Upper Saddle River NJ 2001, ISBN 0-13-240342-0 .