The Igneous rock reference article from the English Wikipedia on 24-Jul-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Igneous rock

Igneous rocks are formed when molten rock (magma derived from the Earth's 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 formed beneath the surface of the Earth's crust (known as intrusive).

Igneous rock are geologically important because:

• their minerals and global chemistry gives information about the composition of the mantle, from where some igneous rocks are extracted, and the temperature and pressure conditions that allowed this extraction, and/or of other pre-existing rock that melted;
• their absolute ages can be obtained from various forms of radiometric dating and thus can be compared to adjacent geological 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: for example, tungsten, tin and uranium, are commonly associated with granites.

Table of contents

1 Classification 1.1 Mode of occurrence 1.2 Texture 1.2.1 Grain size 1.2.2 Crystal shapes 1.3 Chemical composition 1.4 Geometry of the igneous body 1.5 Example of classification 2 See also 3 External links

Classification

Igneous rocks are classified according to mode of occurrence, texture, chemical composition, and the geometry of the igneous body.

Mode 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.

Texture

The most important distinction in igneous rocks is texture, which is related to the size and shape of the constituent crystallite gains.

Grain size

According to the size of the grains, igneous rocks may be classified as pegmatic (very large grains), phaneritic (only large grains), porphyritic (some large grains and some small grains), apahnitic (only small grains) or glassy (no grains).

• 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

Crystal shape is also an important factor in the texture of an igneous rock. Crystals may be euhedral, subeuhedral or anhedral:

• Euhedral, if the crystallographic shape is preserved.
• Subeuhedral, if only part is preserved.
• Anhedral, if the crystal presents no recognizable crystallographic direction.

Chemical composition

Igneous rocks can be subdivided according to two main chemical parameters:

• contents of silica:
  • acid igneous rocks present a high silica content (e.g., granite)
  • basic igneous rocks have low silica content (e.g., basalt)
  • intermediate igneous rocks
• contents of quartz, alkali feldspar and feldspathoids: the felsic minerals
  • felsic rock, with predominance of felsic minerals; these rocks (e.g., granite) are usually light coloured, and less dense.
  • mafic rock, with predominance of mafic minerals (all other minerals, excluding the felsic); these rocks (e.g., basalt) are usually dark coloured, and more dense than felsic rocks.
  • ultramafic rock, with more that 90% of mafic minerals (e.g., dunite)

Note that some light coloured rocks, such as limestone or sandstone, cannot be classified as felsic because their origin is sedimentary not igneous.

The following table is a simple subdivision of igneous rocks according both to their composition and mode of occurrence.

	Composition
Mode of occurrence	Acid	Intermediate	Basic	Ultrabasic
Intrusive	Granite	Diorite	Gabbro	Peridotite
Extrusive	Rhyolite	Andesite	Basalt

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 than 10% of felsic minerals) and phaneritic, subeuedral texture (minerals are visible for the unaided eye and some of them retain original crystallographic shapes).

See also

• List of minerals
• List of rocks

External links

• Igneous Rocks

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