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Schematic representation of the relationship between the temperature gradient and the melting temperature of the rock with regard to the formation of magmas in various plate tectonic scenarios.

Magma ( neuter ; from ancient Greek μάγμα mágma , German 'kneaded mass' ) are rock melts that occur in parts of the upper mantle and the deeper earth's crust . The flowability of the magma is the cause of volcanism (see also volcano ) and is of great importance for rock formation , since igneous rocks or magmatites are formed from the solidifying magma .

These are divided into plutonites , when the magma in the earth's crust cools down slowly and can therefore form larger crystals , and into volcanics , when the magma cools faster when it escapes to the earth's surface (for example as lava or in the form of pyroclasts ). Granite is one of the plutonites , and basalt is one of the volcanites .

Properties and classification

Greatly simplified scheme for the classification of igneous rocks.

Depending on the composition and pressure conditions, the temperature of magma is between 700 ° C and 1250 ° C. Magmas are usually silicate rock melts, that is, to a large extent (40–75 percent by weight ) of silicon dioxide (SiO 2 ). Magmas are roughly differentiated according to their magnesium and iron concentration and the silicate content :

  • Felsic or rhyolite melts (SiO 2 content> 65%) (previously acidic )
  • intermediate or andesitic melts (SiO 2 content between 52% and 65%)
  • mafic or basaltic magmas (SiO 2 content <52%) (previously basic )

The frequently used division into acidic and basic magmas comes from the term silica , which is often used in general terms for SiO 2 . However, since the acid - base term is precisely defined in chemistry and can be misunderstood in connection with magmas, it is replaced by felsic (rich in feldspar and quartz , highly differentiated, light) and mafic (rich in magnesium and iron, primitive, dark) .


The formation of magma is a process that is only partially understood. It is known that the earth's mantle is largely solid up to the edge of the earth's core at a depth of several thousand kilometers. The occurrence of liquid melts can only be explained under special conditions under which there is partial rock melt . The magma collects in magma chambers , as the lighter magma rises through the heavier surrounding rock, fills cavities (which are created by tectonic activity) and expands through melting. These processes take place over a period of several tens of thousands to several hundred thousand years.

The crystallization of magma is also a complex process in which, in addition to the respective chemical composition of the initial melt , the pressure conditions , temperature , water content and the surrounding rock play a role. This leads by igneous differentiation and fractional crystallization to formation of different rocks. Elements promoting the rise of magmas are an increase in temperature, pressure relief and / or the presence of fluid phases ( H 2 O , CO 2 ). The lower density of the magmas also contributes to their rise.

Rock is melted in geologically particularly active areas, so magma is created at mid-ocean ridges through pressure relief, at subduction zones through the fluids supplied by the subducted plate and the lower melting point of the upper mantle, and mantle diapirs form due to an increase in temperature.

Mid-ocean ridges and subduction zones are phenomena of plate tectonics and characterize lithospheric plates drifting apart or colliding . At the mid-ocean ridges, the plates are torn apart, creating trenches and crevices into which melts rise from the upper mantle, mostly in the form of basaltic lavas. In subduction zones, material from the lithosphere is transported into the interior of the earth by the descending plate. With the material of the submerged plate, fluids such as water and carbon dioxide are also transported. Fluids lower the solidus temperature of the rock and lead to partial melting of the surrounding material. Manteldiapirs or plumes are narrow columns of melted material, the roots of which lie in great depths of the earth's mantle . These diapirs or hot spots also occur far away from plate boundaries and then often lead to the formation of intraplate volcanoes.


  • Myron G. Best: Igneous and Metamorphic Petrology . WH Freemann & Company, San Francisco CA 1982, ISBN 0-7167-1335-7 .
  • Wolfhard Wimmenauer: Petrography of igneous and metamorphic rocks . Enke Verlag, Stuttgart 1985, ISBN 3-432-94671-6 .
  • Hans-Ulrich Schmincke: Vulcanism . Scientific Book Society, Darmstadt 2000, ISBN 3-534-14102-4 .

Web links

Individual evidence

  1. cf. H.-U. Schmincke: volcanism. 2nd, revised and expanded edition. Darmstadt 2000, p. 21ff.
  2. cf. H.-U. Schmincke: volcanism. 2nd, revised and expanded edition. Darmstadt 2000, p. 25.
  3. cf. H.-U. Schmincke: volcanism. 2nd, revised and expanded edition. Darmstadt 2000, p. 27.
  4. cf. H.-U. Schmincke: volcanism. 2nd, revised and expanded edition. Darmstadt 2000, p. 26f.