7.3 category of Igneous RocksClassification by Mineral Abundance

Igneous rocks deserve to be separated into 4 categories based upon their chemistry composition: felsic, intermediate, mafic, and also ultramafic. The diagram of Bowen’s reaction series (Figure 7.6) reflects that distinctions in chemical composition exchange mail to differences in the types of minerals in ~ an igneous rock. Igneous rocks are provided names based upon the ratio of different minerals lock contain. Number 7.13 is a diagram through the minerals from Bowen’s reaction series, and is provided to decide which name to provide an igneous rock.

You are watching: All igneous rocks have the same mineral compositions.

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Figure 7.13 classification diagram for igneous rocks. Igneous rocks space classified follow to the family member abundances of minerals they contain. A offered rock is stood for by a vertical line in the diagram. In the mafic field, the arrows represent a rock containing 48% pyroxene and 52% plagioclase feldspar. The name an igneous absent gets counts not just on composition, however on whether it is intrusive or extrusive. Source: Karla Panchuk (2018) CC BY-NC-SA 4.0, modified after Steven Earle (2015) CC through 4.0 view resource and others, v photos by R. Weller/Cochise College. Click for web links to photos and notes on photo construction.

To view how figure 7.13 works, first notice the range in percent follow me the upright axis. The interval in between each tick note represents 10% of the minerals within a rock by volume. An igneous rock have the right to be stood for as a vertical line attracted through the diagram, and the vertical scale used to failure the ratio of every mineral the contains. For example, the arrows in the mafic ar of the diagram stand for a rock containing 48% pyroxene and also 52% plagioclase feldspar. One igneous absent at the boundary between the mafic and ultramafic areas (marked v a vertical dashed line) would have around 20% olivine, 50% pyroxene, and also 30% Ca-rich plagioclase feldspar by volume.

Classification By serial Size

The surname an igneous absent gets also depends on even if it is it cools within planet (an intrusive or plutonic igneous rock), or whether it cools on the Earth’s surface ar after erupting native a volcano (an extrusive or volcanic igneous rock). For example, a felsic intrusive rock is called granite, vice versa, a felsic extrusive rock is referred to as rhyolite. Granite and also rhyolite have the exact same mineral composition, but their serial size gives each a distinctive appearance.

The vital difference between intrusive and also extrusive igneous rocks—the size of crystals making lock up—is connected to exactly how rapidly melted absent cools. The much longer melted rock needs to cool, the larger the crystals within it can become. Magma cools much slower within planet than on Earth’s surface since magma within planet is insulated by neighboring rock. Notice that in figure 7.13, the intrusive rocks have crystals large enough the you can see individual crystals—either by identifying your boundaries, or see light reflecting from a crystal face. A rock through individual crystals that are visible come the unaided eye has actually a phaneritic or coarse-grained texture. The extrusive rocks in the 2nd row have much smaller sized crystals. The crystals room so little that separation, personal, instance crystals can not be distinguished, and also the absent looks choose a dull mass. A rock with crystals that room too small to see with the unaided eye has actually an aphanitic or fine-grained texture. Table 7.1 summarizes the an essential differences in between intrusive and also extrusive igneous rocks.

Table 7.1 compare of Intrusive and also Extrusive Igneous Rocks
Magma cools in ~ EarthLava cools top top Earth’s surface
TerminologyIntrusive/ plutonicExtrusive/ volcanic
Cooling rateSlow: neighboring rocks insulate the magma chamber.Rapid: heat is exchanged v the atmosphere.
TexturePhaneritic (coarse-grained): separation, personal, instance crystals are big enough to watch without magnification.Aphanitic (fine-grained): crystals are too tiny to watch without magnification.

What this method is that 2 igneous rocks comprised of specifically the same minerals, and also in the specifically the very same proportions, can have various names. A absent of intermediate ingredient is diorite if the is course-grained, and also andesite if it is fine-grained. A mafic absent is gabbro if that is course-grained, and basalt if fine-grained. The course-grained variation of an ultramafic absent is peridotite, and also the fine-grained version is komatiite. It makes sense come use different names because rocks of different grain sizes type in various ways and also in various geological settings.

Does This average an Igneous Rock can Only have actually One serial Size?

No. Something exciting happens as soon as there is a change in the rate at i beg your pardon melted rock is cooling. If magma is cooling in a magma chamber, part minerals will start to crystallize prior to others do. If cooling is slow-moving enough, those crystals can come to be quite large.

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Now imagine the magma is suddenly heaved the end of the magma chamber and erupted from a volcano. The larger crystals will flow out with the lava. The lava will then cool rapidly, and the bigger crystals will certainly be surrounded by lot smaller ones. One igneous rock v crystals of distinctly different size (Figure 7.14) is said to have a porphyritic texture, or might be described as a porphyry. The larger crystals are referred to as phenocrysts, and also the smaller sized ones are referred to as the groundmass.

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Figure 7.14 Porphyritic rhyolite with quartz and potassium feldspar phenocrysts in ~ a dark groundmass. Porphyritic structure (when different crystal sizes space present) is an indication the melted rock did no cool in ~ a constant rate. Source: Karla Panchuk (2018) CC BY-NC-SA 4.0. Picture by R. Weller/Cochise college (2011) check out source