A Classification of Igneous Rocks

Igneous rocks can be classified according to several criteria such as method of formation and occurrence, mineralogical content, and textual features.

Igneous rocks can be classified into four types according to its method of solidification and mode of occurrence.
1.  Pyroclastic Rocks
these rocks re formed by the material ejected to the atmosphere by a volcanic eruption. These rocks mainly consist of fine ash particles but can have larger particles as well (known as lapilli, blocks and bombs). These are somewhat similar to Sedimentary rocks.
2.  Volcanic Rocks
These rocks are formed when lava solidifies at the surface of the earth and the rapid cooling gives rise to fine grained rocks or rocks with a glassy texture such as Obsidian.
3.  Hypabyssal
These rocks parts of igneous bodies that are close to the earth surface and usually contain a fine grained texture.
4.  Plutonic Rocks
These are the rocks that solidify beneath the surface, and the slow cooling gives rise to coarse grains.

According to the mineralogical content rocks can be classified as follows,
1.  Ultra Basic / Ultra mafic
Has less than 45% of SiO2, and dark in colour. Mafic minerals(Olivine. Pyroxene) are dominant. eg. Peridotite, Komatite
2.  Basic / Mafic
45-50% SiO2 and lighter in colour. eg Gabbro, Basalt
3.  Intermediate
Feldspar is dominant. eg. Diorite, Andesite
4.  Acidic / Felsic
Quartz is dominant eg. Granite Ryolite

The textural classification is as below,
1.  Phaneritic
Coarse grained texture - produced by slow cooling
2.  Aphanitic
Fine grained texture- produced by rapid cooling
3.  Glassy
Non crystalline - very rapid cooling
4.  Porphyritic, Vitrophirc - Phenocrysts in fine grained and glassy matrix respectively.
5. Cavity textures - Vesicular, Amygdaloidal, miarotitic.
6.  Intergrowths - Graphic textures