Igneous rocks are formed when molten rock (magma derived from the mantle, or, pre-existing rocks molten by extreme temperature) cools and solidifies, with or without crystallization. Over 700 types of igneous rocks have been described, most of them intrusive. Igneous rock are geologically important because:
- their minerals and global chemistry gives information about the composition of the mantle where they were extracted from, and the temperature and pressure conditions that allowed this extraction, or (below)
- their minerals and global chemistry gives information about the composition of the country pre-existing rock that melted
- their absolute ages can be obtained from various forms of radiometric dating and thus can be compared to adjacent strata, allowing a time sequence of events
- their features are usually characteristic of a specific tectonic environment, allowing tectonic reconstitutions (see Plate tectonics)
- in some special circumstances they host important mineral deposits, of, for example, tungsten, tin or uranium, commonly associated with granites
- they can be explored as ornamental stone
=Modes of occurrence=
In terms of modes of occurrence, igneous rocks can be either intrusive (plutonic) or extrusive (effusive). Intrusive rocks crystallize within the crust interior. Extrusive rocks are the result of volcanic eruptions and, therefore, solidify in atmospheric conditions.
The most important distinction in igneous rocks textures is related to grain size. Phaneritic rocks contain minerals with grains (crystals) visible to the unaided eye and are commonly intrusive (as the slower cooling rates allow the formation of large crystals). In the extreme, such rocks may contain extremely large crystals, in which case they are termed pegmatitic. In extrusive rocks, where cooling is much more rapid, the individual mineral crystals are usually not visible and these rocks are termed aphanitic. Porphyritic textures are an intermediate situation between the previous two: the groundmass of the rock has an aphanitic texture, but crystals (termed in this particular occurrence as phenocrystals) are visible to unaided eye. If a molten magma cools at extremely high rates, allowing no crystallization, the result is a vulcanic glass called obsidian. Crystal shapes are also an important feature in defining correct igneous textures. They can be:
- Euedral, if the crystallographic shape is preserved
- Subeuedral, if only part is preserved
- Anedral, if the crystal presents no recognizable crystallographic direction
Igneous rocks can be subdivided according to two main chemical parameters:
- contents of silica:
- contents of quartz, alkali feldspar and feldspatoids: the felsic minerals
- felsic rock, with predominance of felsic minerals; these rocks (ex: granite) are usually light coloured
- mafic rock, with predominance of mafic minerals (all other minerals, excluding the felsic); these rocks (ex: basalt) are usually dark coloured
- ultramaphic rock, with more that 90% of mafic minerals (ex: dunite))
The following table is a simple subdivision of igneous rocks according both to their composition and mode of occurrence.
|Mode of occurrence||Acid||Intermediate||Basic|
=Geometry of the igneous body=
Igneous rocks can also classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes. Typical intrusive formations are batholiths, laccoliths, sills and dikes. The extrusive types usually are called lavas.
=Example of classification=
Granite is an igneous, intrusive rock (crystallyzed at depth), with felsic acid composition (rich in silica and with more tha 10% of felsic minerals) and phaneritic, subeuedral texture (minerals are visible for the unaided eye and some of them retain original crystallographic shapes).