What is an igneous stone

The formation of igneous rocks

Igneous rocks are the result of the solidification of magma and lava. In relation to the place of origin, igneous rocks are subdivided into intrusive and extrusive rocks.

Volcanites and plutonites

While intrusive rocks (plutonites) crystallize under the earth's surface, extrusive rocks (volcanites) are formed on the earth's surface.

Vulcanites (examples): Rhyolite, volcanic glasses, dacite, pitch stone, pumice, ignimbrite, basalt, trachyte and andesite.

Plutonite (examples): Granite, quartz porphyry, pegmatite, granodiorite, anorthosite, norite, dunite, gabbro, peridotite, syenite, diorite

Rocks from magma and lava

The formation of the minerals that make up a rock (so-called main and secondary parts) from the liquid rock melt (magma) takes place in several phases in which the main magma is split up into partial magmas due to changes in chemical composition.

First crystallize in the orthomagmatic phase the minerals with the highest melting point (700 to 1200 ° C), followed by the minerals of the pneumatolytic and hydrothermal phase.
The geologists Karl Harry Friedrich Rosenbusch and Norman Levi Bowen dealt with the crystallization sequences as early as the end of the 19th century. The Rosenbusch-Bowen series describes that the first rock-forming minerals from the rock melt are dark, mafic (i.e. magnesium and iron-rich) minerals. Then light, calcium-rich minerals solidify, followed by sodium-rich plagioclases, further feldspars and quartz.

The composition of the rocks can vary considerably locally. The reason for this is the composition of the magma.
A distinction is made between light, acidic rocks with a silicon dioxide content of more than 65%, intermediate rocks with 55 to 65% silicon dioxide, basic rocks with a high content of iron and magnesium at 45 to 55% silicon dioxide and ultrabasic rocks with a silicon content below 45% . Igneous rocks of the so-called Pacific clan are primarily responsible for acidic, calcium-rich rocks such as granite and rhyolite. The rocks of the Atlantic clan are basic and rich in sodium, e.g. basalt and gabbro.

Based on the crystal shape and size, conclusions can be drawn about the formation conditions.

Intrusive rocks (also called plutonites, deep rocks or abyssal rocks) coarse-grained with hypidiomorphic, directionlessly arranged minerals, because the minerals were able to crystallize well at depths of 1 to over 10 km below the earth's surface due to the slow cooling of the magma in the earth's interior. Overlying layers of rock prevented cavities and pores in the rock through the effect of pressure. For this reason, the rocks are extremely compact.

Extrusive rocks (alternatively effusion rocks, volcanic rocks or effusiva) are fine-grained, partly also glassy. The crystals are xenomorphic. Sometimes volcanic rocks show cavities as a sign of the degassing of the lava. Some of these are oval or elongated, thus indicating the direction of flow of the molten rock.

⇒ The difference between lava and magma

See also:
⇒ Black granite
⇒ The oldest rock in the world
⇒ cycle of rocks

⇒ Murawski, H. (1992): Geological Dictionary. Ferdinand Enke Verlag Stuttgart
⇒ Maresch, W., Medenbach, O .; Trochim, H.-D. (1987): The colored natural guide rocks. Mosaik Verlag GmbH Munich *
⇒ Yassamanov, N.A. (1991): Small Spectrum of Sciences - Geology: Excursion to Earth
⇒ Georgi, K.-H. (1983): The Rock Cycle - An Introduction to Geology
⇒ Zepp, H. (2011): Grundriß Allgemeine Geographie: Geomorphologie
⇒ Ahnert, F. (2009): Introduction to Geomorphology

Last updated: May 20, 2019

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