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Igneous rocks room classified based on texture and composition. Texture explains the physical features of the minerals, such together grain size. This relates come the cooling history of the molten magma native which the came. Composition refers to the rock’s particular mineralogy and chemical composition. Cooling history is also related to alters that can take place to the composition of igneous rocks.

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

If magma cools slowly, deep in ~ the crust, the resulting rock is called intrusive or plutonic. The slow-moving cooling process allows crystals to grow large, giving the intrusive igneous absent a coarse-grained or phaneritic texture. The separation, personal, instance crystals in phaneritic texture are conveniently visible to the unaided eye.

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

When lava is extruded top top the surface, or intruded into shallow fissures close to the surface and cools, the resulting igneous rock is called extrusive or volcanic. Extrusive igneous rocks have actually a fine-grained or aphanitic texture, in i m sorry the grains space too tiny to see through the unaided eye. The fine-grained texture suggests the conveniently cooling lava go not have time to grow huge crystals. These tiny crystals deserve to be regarded under a petrographic microscope <1>. In some cases, extrusive lava cools so rapidly it walk not construct crystals in ~ all. This non-crystalline product is no classified as minerals yet as volcano glass. This is a common component of volcano ash and also rocks choose obsidian.

Figure (PageIndex1): Porphyritic texture

Some igneous rocks have actually a mix that coarse-grained minerals surrounding by a matrix of fine-grained material in a texture dubbed porphyritic. The huge crystals are referred to as phenocrysts and the fine-grained matrix is dubbed the groundmass or matrix. Porphyritic texture suggests the magma human body underwent a multi-stage cooling history, cooling slowly while deep under the surface and also later rising to a shallower depth or the surface ar where the cooled more quickly.

Figure (PageIndex1): Pegmatitic texture

Residual molten product expelled from igneous intrusions may kind veins or masses include very big crystals that minerals prefer feldspar, quartz, beryl, tourmaline, and mica. This texture, which suggests a very slow crystallization, is dubbed pegmatitic. A rock the chiefly is composed of pegmatitic structure is known as a pegmatite. To give an instance of how huge these crystals deserve to get, transparent cleavage sheets the pegmatitic muscovite mica were offered as windows throughout the middle Ages.

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

All magmas save on computer gases dissolved in a solution dubbed volatiles. Together the magma rises come the surface, the fall in pressure reasons the dissolved volatiles to come bubbling the end of solution, like the fizz in an opened bottle the soda. The gas bubbles come to be trapped in the solidifying lava to produce a vesicular texture, through the feet specifically dubbed vesicles. The kind of volcanic rock with typical vesicles is dubbed scoria.

Figure (PageIndex1): Pumice

An excessive version of scoria occurs when volatile-rich lava is an extremely quickly quenched and becomes a meringue-like froth the glass called pumice. Part pumice is so full of vesicles that the thickness of the rock drops low enough that it will certainly float.

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

Lava that cools extremely easily may not kind crystals in ~ all, even microscopic ones. The resulting absent is referred to as volcanic glass. Obsidian is a absent consisting of volcanic glass. Obsidian as a glassy absent shows great 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 ingredient of common igneous rocks. The portion of minerals is shown on the upright axis. The percent of silica is shown on the horizontal axis. Absent names in ~ the top encompass a constant spectrum the compositions grading indigenous one right into another. Felsic refers to a predominance of the light-colored (felsic) mineral feldspar and also silica in the type of quartz. This light-colored mineral have much more silica together a proportion of their as whole chemical formula. Minor amounts of dark-colored (mafic) minerals prefer amphibole and also biotite mica may be existing as well. Felsic igneous rocks are rich in silica (in the 65-75% range, definition the rock would certainly be 65-75% load percent SiO2) and poor in iron and magnesium. Intermediate is a composition in between felsic and also mafic. It usually has roughly-equal quantities of light and also dark minerals, consisting of light grains of plagioclase feldspar and also dark minerals choose amphibole. It is intermediate in silica in the 55-60% range. Mafic ad to an abundance of ferromagnesian minerals (with magnesium and also iron, chemical icons Mg and also Fe) to add plagioclase feldspar. That is largely made the dark minerals favor pyroxene and olivine, which are rich in iron and magnesium and fairly poor in silica. Mafic rocks space low in silica, in the 45-50% range. Ultramafic refers to the exceptionally mafic rocks written of mainly olivine and also some pyroxene which have even much more magnesium and also iron and even less silica. This rocks are rare on the surface, yet make up peridotite, the rock of the upper mantle. That is poor in silica, in the 40% or less range.

On the number above, the height row has actually both plutonic and also volcanic igneous rocks i ordered it in a constant spectrum indigenous felsic on the left come intermediate, mafic, and also ultramafic towards the right. Rhyolite refers to the volcanic and felsic igneous rocks and also granite refer to intrusive and felsic igneous rocks. Andesite and also diorite similarly refer come extrusive and intrusive intermediate rocks (with dacite and also granodiorite applying to those rocks v composition between felsic and also intermediate).

Basalt and gabbro are the extrusive and intrusive names for mafic igneous rocks, and peridotite is ultramafic, with komatiite as the fine-grained extrusive equivalent. Komatiite is a rarely rock since volcanic product that comes directly from the mantle is no common, although some examples can be discovered in old Archean rocks <2>. Nature rarely has sharp boundaries and also the classification and naming that rocks often impose what appears to be spicy boundary names top top a consistent spectrum.

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

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Granite from Cape Cod, Massachusetts.
Fine Crystalline absent with larger glassy pieces floating in the groundmass.which igneous rock has a vesicular texture and a felsic composition -->