Igneous Rocks


Staff member
Jun 3, 2020
Igneous rocks (from Latin ignis, fire) also named magmatic, are all those that have been formed by solidification of a rocky, hot and mobile material called magma. This process, called crystallization, results from the cooling of the minerals and the entanglement of their particles. These types of rocks are also formed by the accumulation and consolidation of lava, a word used for magma that cools down on the surface when it is expelled by volcanoes.

When the solidification of magma occurs within the lithosphere, the resulting rock is called plutonic or intrusive. If the cooling occurs, at least in part, at the surface or at a shallow depth, the resulting rock is called volcanic or extrusive and these, in turn, are subdivided into families based on the different textures, mineral associations and mode of occurrence. The forms that igneous bodies take during their crystallization delimit different igneous structures.

Igneous rocks gabbro andesite pegmatite basalt pumice porphyry obsidian granite tuff
(Igneous rocks: gabbro andesite pegmatite basalt pumice porphyry obsidian granite tuff)

There are various criteria to classify an igneous rock , each with defined objectives, such as the occurrence of rocks, grain size, texture and structure, mineral content or chemical composition.

Intrusive or Plutonic Igneous Rocks
They are rocks formed inside the earth's crust. When magma cools below the surface, it slows down, allowing better development of crystals, which due to this reach sizes that can be observed with the naked eye, generally cover large areas of land and reach the earth's surface through orogenic processes (tectonic deformations) or through external erosion processes. Within this type of rocks, some authors recognize an intermediate class, the hypabyssal, which includes rocks that have crystallized to a moderate depth and appear in the form of reefs or dikes, filling cracks; they are much less abundant than plutonics and are almost always associated with them.

Extrusive or volcanic igneous rocks
Typical volcanic rocks are formed by the rapid cooling of lava and pyroclastic fragments. This process occurs when magma is expelled by volcanic processes and cools rapidly due to the ambient temperature, developing small crystals that form fine-grained rocks (not noticeable to the naked eye) and pyroclastic rocks. The pyroclastic (from the Greek pyro , fire, and klastos , broken), are products of explosive volcanic eruptions and contain fragments of rock from different backgrounds can come in many shapes and sizes.

The igneous rocks within the two large groups are subdivided into different families taking into account the texture and the essential minerals (basic presence for a certain type), being mutually equivalent. The following table presents the essential minerals that determine the different varieties of igneous rocks:

S. No.Essential MineralsPlutonic RocksVolcanic Rocks
1Quartz, Feld K> Plag Na, Biotite, HornblendeGraniteRhyolite
2Quartz, PlagNa> = Feld K, biotite, hornblendeGranodioriteQuartz latite
3Feld K> Plag Na, Biotite, Augite, HornblendeSyeniteTrachyte
4Plag Na> = Feld K, biotite, hornblende, augiteMonsoonLatita
5Feld K> = <Feldspathoids, Plag Na, maficFeldspathoid SyeniteClinkstone
6Quartz, Plag Na, Biotite, HornblendeTonalitaDacite
7Plag Na, Hornblende, BiotiteGreenstoneAndesite
8Plag Ca, augite, hypersthene, olivine, hornblendeGabbroBasalt
9Plag Ca, olivine, pyroxene, hornblende, magnetite, ilmenite, etc.Peridotite (Dunite)
10Plag Ca, olivine, pyroxene, hornblendeHypabyssal Rock -
Pegmatite (Diabase)

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