rock index: igneous

Igneous rocks crystallize from magmatic melts intruded at depth into country strata or extruded as lava at the surface. Left - click to enlarge image - texture and mineral composition of common igneous rocks: Increasing to the left: SiO2 content; viscosity; felsic: mafic composition; (K, Na) : (Fe, Mg, Ca) ratio; Increasing to the right: darkness; mafic : felsic composition; (Fe, Mg, Ca) : (K, Na) ratio; temperature of melting: ◘ a/p - aphanitic or porphyritic texture, derived from extruded magma (lava), ◘ 1. rhyolite ◘ 3. dacite ◘ 5. andesite ◘ 7. basalt phaneritic texture, emplaced as magma (plutonic) ◘ 2. granite ◘ 4. granodiorite ◘ 6. diorite ◘ 8. gabbro to peridotite 0-100: percentage mineral content: ◊ a. quartz ◊ b. K-feldspar ◊ c. Na-feldspar ◊ pl. plagioclase to Ca-rich plagioclase ◊ d. muscovite ◊ e. biotite ◊ f. amphiboles ◊ g. pyroxenes ◊ h. olivine
Plutonic igneous rocks are found in large igneous intrusions of crystallized magma. Plutonic rocks exhibit a fine-grained, aphanitic texture if the magma cooled close to the surface in volcanic necks or feeder pipes. Grain size increases to coarse-grained, phaneritic textures for magmas that cooled very slowly at great depth in very large magma chambers. Plutonic structures include batholiths, laccoliths, stocks, bysmaliths, and lopoliths
Diorite	Diorites are hard, intermediate plutonic igneous rocks composed mainly of plagioclase feldspars (usually andesine), hornblende, and/or pyroxenes. Depending upon mineral compostion, diorites are colored salt-and-pepper, gray, bluish gray, to dark gray, and may have a greenish cast.
Dunites	Dunites are gray to olive green to 'dun', coarse-grained or phaneritic plutonic igneous rock of ultramafic composition. Comprising greater than 90% olivine, with minor pyroxenes and chromite, dunites are olivine-rich end-members of the peridotite group of mantle-derived rocks.
Gabbro	Gabbro is a coarse-grained, mafic, plutonic igneous rock composed of varied percentages of pyroxene, plagioclase feldspar, amphiboles, and olivine. Gabbros are form at spreading centers in rift zones and mid-ocean ridges (so underlies oceanic crust). Gabbros can form as massive uniform intrusions or as layered ultramafic intrusions formed by settling of pyroxene and plagioclase feldspar (pyroxene-plagioclase cumulate).
Granite	Granite is typically a medium to coarse-grained or porphyritic, felsic, intrusive igneous rock (plutonic) that is usually pink to dark gray, sometimes black, depending on its chemistry and mineralogy. Granites primarily comprises orthoclase and plagioclase feldspars, quartz, hornblende, muscovite and/or biotite micas, with minor accessory minerals such as magnetite, garnets, zircon and apatite. Granites are the commonest basement rocks of the continental crust, many dating from the Precambrian.
Granodiorite	Granodiorites are intrusive (plutonic) igneous rocks that have a composition similar to the average composition of the upper continental crust. Granodiorites are intermediate between granite and quartz diorite. They contain more plagioclase (Na/Ca) than potassium feldspar. They usually include abundant biotite and hornblende, giving them a darker appearance than true granites. Mica may be present in well-formed hexagonal crystals, and hornblende may be present as needle-like crystals. Other mineral components of granodiorite include quartz, apatite, and sphene.
Peridotite	Peridotite is an ultramafic, ultrabasic (less than 45% silica), dense, plutonic igneous rock comprising mostly olivine and pyroxene. Peridotite emplaced in the continental crust is typically found in obducted ophiolite complexes, as xenoliths in basalts and kimberlite pipes, and as orogenic peridotite massifs and alpine peridotites. Most of the Earth's upper mantle (asthenosphere) is composed of peridotite.
Porphyry	Porphyry is classically a reddish-brown to purple igneous rock containing large phenocrysts of minerals such as feldpar or quartz embedded in a fine-grained matrix or groundmass of feldspar. More generally, the term porphyry encompasses any rock formed by a two-stage cooling of rising magma that has a texture of phenocrysts embedded in a finer textured matrix (such as granite) or with visible crystals in an aphanitic matrix (such as basalts, aphanites or phanerites).
Tonalites	Tonalites are phaneritic, felsic plutonic igneous rocks comprising greater than 20% quartz ( IUGS classification). Tonalite feldspars are plagioclases (typically oligoclase or andesine), and less that 10% are alkali feldspars (K-spars). Common accessory minerals include amphiboles and pyroxenes. "Tonalite" is an old synonym for quartz diorite, which contains only 5-20% quartz.
Tronhjemite	Trondhjemite (named for Trondheim, Norway) is a light-colored (leucocratic) intrusive igneous rock common in Archaean terranes, which is a tonalite variety with high albite and low anorthite and low orthoclase content. Trondjemite occurs in conjunction with tonalite and granodiorite as the TTG (tonalite-trondhjemite-granodiorite) ortho-gneiss suite.
Hypabassal igneous rocks occur in small intrusions, so are finer grained than rocks emplaced in larger intrusions. Hypabyssal structures include diapirs, dikes and dikelets, ring dikes, sills, volcanic necks and plugs, and cone sheets.
Aplite	Aplite (or haplite) refers to any fine-grained, hypabassal, igneous rock of simple composition, such as granite composed only of alkali feldspars, quartz, and muscovite mica.
Pegmatites are plutonic rocks with particularly large crystal grains (larger than 20 mm, and usually larger than 50 mm). Postulated mechanisms of formation include metamorphic, magmatic, and metasomatic explanations.
Pegmatites	Most pegmatites are composed of quartz, feldspars, and micas (granitic composition). More rarely, intermediate and mafic pegmatites contain amphiboles, Ca-plagioclase feldspar, pyroxenes and other minerals in recrystallized zones and apophyses associated with large layered intrusions.
Volcanic igneous rocks result from the extrusion of molten lava and explosion of pyroclastics from volcanoes. The commonest volcanic lavas correspond to basaltic (~80%), andesitic (~10%), and rhyolitic (~10%) compositions. ◘ mid-oceanic ridges ◘ subduction ◙ subduction zone magmas ◘ submarine trenches
Andesite	Andesite is an aphanitic or, more often, porphyritic extrusive igneous rock with an intermediate composition similar to that of plutonic diorite, containing 52-63 % silica (SiO2) by weight.
Basalt	Basalt is a hard gray or black, mafic igneous volcanic rock that typically contains a preponderance of calcic plagioclase feldspar and pyroxene; olivine can also be a significant constituent. Accessory minerals include iron oxides and iron-titanium oxides, providing basalt with a paleomagnetic signature. Basalts are usually fine-grained due to the rapid cooling of lava, though they can contain larger crystals in a fine matrix (porphyritic), be vesicular, or be a frothy scoria.
Dacite	Dacite is an aphanitic to porphyritic, light to dark gray to black, extrusive igneous rock of composition intermediate between that of andesite and rhyolite (63-68 % SiO2). Like andesite, dacite comprises mostly plagioclase feldspar with biotite, hornblende, and pyroxenes (augite and/or enstatite). Quartz is present within the groundmass or as rounded phenocrysts.
Keratophyres	Keratophyres are Na-rich extrusive and hypabyssal rocks, containing pure sodic plagioclase with or without quartz (soda rhyolite). Keratophyres usually comprise secondary albite or albite-oligoclase and chlorite, epidote, and calcite. Keratophyres are commonly associated with spilitic rocks and interbedded with marine sediments. Keratophyre dikes and sills are associated with some ophiolite complexes, and keratophyre lava flows are found in some island arcs.
Komatiites	Komatiites are ultramafic, mantle-derived volcanic rocks that occur mainly in Archean greenstone belts and are interpreted as forming by high degree partial melting of mantle plumes. Komatiites erupted as voluminous, highly mobile flows. They have low SiO2, low K2O, low Al2O3, and high to extremely high MgO.
Rhyolite	Rhyolite is an aphanitic or porphyritic, felsic, extrusive igneous rock with a composition similar to that of granite. Rhyolites are typically composed of quartz, alkali feldspar, and plagioclase feldspar; with biotite and pyroxene as accessory minerals.
Migmatites or diatextites, are very high-grade metamorphic rocks that are transitional to igneous rocks because they result from near melting of protoliths. Migmatites are produced by high temperature/high pressure prograde Barrovian regional metamorphism, and commonly occur within extremely deformed rocks that formed at the base of eroded, orogenic belts, typically within Precambrian cratonic blocks.

Rock types indexes: igneous : metamorphic : sedimentary