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Igneous rock

Igneous rock forms when molten rock (magma) cools and solidifies, with or without crystallization. Igneous rocks are classified according to texture, fabric, chemical composition, and field relationship or mode of occurrence. In terms of modes of occurrence, the two categories are intrusive (plutonic[?]) or extrusive (effusive[?]). In terms of their composition, igneous rocks range from having high concentrations of magnesium (so-called mafic rocks) to high concentrations of iron (felsic rocks). A final categorisation splits igneous rocks into acid (high silica content), basic (low silica content) and intermediate (intermediate silica content).

Modes of occurrence The intrusive rocks are 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 are called lavas. Over 700 types of igneous rocks have been described, most of them intrusive.

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[?]. Obsidian is a type of igneous rock that has cooled extremely rapidly, rendering it similar to glass.

Igneous rock are geologically important in that their ages can be obtained from various forms of radiometric dating and thus can be used to adjacent strata.

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

Simple subdivision of igneous rocks
Composition
Mode of occurrenceAcidIntermediateBasic
ExtrusiveGraniteDiorite[?]Gabbro
IntrusiveRhyoliteAndesite[?]Basalt

See also: List of minerals



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