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Igneous rocks room classified based on texture and also composition. Texture defines the physical attributes of the minerals, such together grain size. This relates to the cooling background of the molten magma from which that came. Composition refers to the rock’s details mineralogy and chemical composition. Cooling background is additionally related to changes that can happen to the composition of igneous rocks.

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Figure \(\PageIndex1\): Granite is a standard coarse-grained (phaneritic) intrusive igneous rock. The various colors are distinctive minerals. The black color colors are likely two or three different minerals.

If magma cools slowly, deep in ~ the crust, the resulting absent is dubbed intrusive or plutonic. The slow-moving cooling procedure allows crystals to prosper large, providing the intrusive igneous absent a coarse-grained or phaneritic texture. The separation, personal, instance crystals in phaneritic texture are readily visible come the unaided eye.

Figure \(\PageIndex1\): Basalt is a classic fine-grained (aphanitic) extrusive igneous rock. This sample is greatly fine groundmass with a couple of small environment-friendly phenocrysts that space the mineral olivine.

When lava is extruded onto the surface, or intruded right into shallow fissures close to the surface and also cools, the result igneous absent is referred to as extrusive or volcanic. Extrusive igneous rocks have a fine-grained or aphanitic texture, in i beg your pardon the grains are too small to see through the unaided eye. The fine-grained texture suggests the conveniently cooling lava go not have actually time come grow huge crystals. These tiny crystals can be regarded under a petrographic microscope <1>. In part cases, extrusive lava cools so rapidly it go not construct crystals in ~ all. This non-crystalline material is no classified together minerals however as volcano glass. This is a usual component of volcanic ash and rocks favor obsidian.

Figure \(\PageIndex1\): Porphyritic texture

Some igneous rocks have a mix the coarse-grained minerals surrounded by a matrix of fine-grained material in a texture called porphyritic. The huge crystals are called phenocrysts and the fine-grained matrix is dubbed the groundmass or matrix. Porphyritic texture shows the magma human body underwent a multi-stage cooling history, cooling gradually while deep under the surface and also later increasing to a shallower depth or the surface ar where that cooled an ext quickly.

Figure \(\PageIndex1\): Pegmatitic texture

Residual molten material expelled indigenous igneous intrusions may form veins or masses comprise very big crystals that minerals favor feldspar, quartz, beryl, tourmaline, and also mica. This texture, which indicates a really slow crystallization, is called pegmatitic. A rock that chiefly is composed of pegmatitic structure is known as a pegmatite. Come give an instance of how big these crystals can get, transparent cleavage sheets the pegmatitic muscovite mica were provided as windows throughout the center Ages.

Figure \(\PageIndex1\): Scoria, a vesicular extrusive igneous rock

All magmas save gases liquified in a solution dubbed volatiles. Together the magma rises to the surface, the autumn in pressure causes the liquified volatiles come come bubbling out of solution, like the fizz in an opened up bottle of soda. The gas bubbles become trapped in the solidifying lava to produce a vesicular texture, v the holes specifically called vesicles. The type of volcanic absent with common vesicles is called scoria.

Figure \(\PageIndex1\): Pumice

An extreme version that scoria occurs when volatile-rich lava is really quickly quenched and also becomes a meringue-like froth the glass referred to as pumice. Some pumice is so full of vesicles that the thickness of the absent drops low enough that it will certainly float.

Figure \(\PageIndex1\): Obsidian (volcanic glass). Note conchoidal fracture.

Lava the cools extremely quickly may not type crystals at all, also microscopic ones. The resulting rock is referred to as volcanic glass. Obsidian is a rock consisting of volcanic glass. Obsidian together a glassy rock shows wonderful example of conchoidal fracture similar to the mineral quartz (see thing 3).

api/deki/files/8215/Igneous-rock-composition-as-volume-percentages-of-minerals-1024x724.jpg?revision=1" />Figure \(\PageIndex1\): Mineral composition of typical igneous rocks. The percentage of mineral is displayed on the upright axis. The portion of silica is presented on the horizontal axis. Rock names in ~ the top incorporate a constant spectrum of compositions grading indigenous one into another. Felsic refers to a predominance of the light-colored (felsic) minerals feldspar and silica in the kind of quartz. This light-colored mineral have an ext silica together a ratio of their overall chemical formula. Minor quantities of dark-colored (mafic) minerals choose amphibole and also biotite mica might be existing as well. Felsic igneous rocks space rich in silica (in the 65-75% range, meaning the rock would certainly be 65-75% weight percent SiO2) and also poor in iron and magnesium. Intermediate is a composition in between felsic and mafic. That usually has roughly-equal amounts of light and dark minerals, consisting of light seed of plagioclase feldspar and also dark minerals like amphibole. That is intermediate in silica in the 55-60% range. Mafic advert to wealth of ferromagnesian mineral (with magnesium and also iron, chemical symbols Mg and also Fe) add to plagioclase feldspar. That is mostly made that dark minerals choose pyroxene and also olivine, which space rich in iron and magnesium and reasonably poor in silica. Mafic rocks space low in silica, in the 45-50% range. Ultramafic refers to the extremely mafic rocks composed of mostly olivine and also some pyroxene which have even much more magnesium and also iron and also even less silica. This rocks are rare top top the surface, but make up peridotite, the rock of the top mantle. That is negative in silica, in the 40% or less range.

On the figure above, the height row has actually both plutonic and volcanic igneous rocks i ordered it in a continuous spectrum from felsic top top the left to intermediate, mafic, and also ultramafic toward the right. Rhyolite describes the volcanic and also felsic igneous rocks and granite refer to intrusive and also felsic igneous rocks. Andesite and also diorite similarly refer come extrusive and also intrusive intermediary rocks (with dacite and granodiorite applying to those rocks through composition between felsic and also intermediate).

Basalt and gabbro space the extrusive and also intrusive names for mafic igneous rocks, and peridotite is ultramafic, v komatiite together the fine-grained extrusive equivalent. Komatiite is a rarely rock since volcanic material that comes directly from the mantle is not common, return some examples can be uncovered in ancient Archean rocks <2>. Nature rarely has sharp boundaries and also the classification and naming the rocks regularly impose what appears to be sharp boundary names onto a continuous spectrum.

Figure \(\PageIndex1\): Igneous rock category table with composition as vertical columns and texture together horizontal rows.

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Aphanitic/Phaneritic Rock varieties with Images

Felsic Composition

Granite native Cape Cod, Massachusetts.
Fine Crystalline rock with larger glassy pieces floating in the groundmass.a vesicular igneous texture indicates that -->