7.3 Classification of Igneous RocksClassification By Mineral Abundance

Igneous rocks can be divided into four categories based on their chemical composition: felsic, intermediate, mafic, and ultramafic. The diagram of Bowen’s reaction series (Figure 7.6) shows that differences in chemical composition correspond to differences in the types of minerals within an igneous rock. Igneous rocks are given names based on the proportion of different minerals they contain. Figure 7.13 is a diagram with the minerals from Bowen’s reaction series, and is used to decide which name to give an igneous rock.

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Figure 7.13 Classification diagram for igneous rocks. Igneous rocks are classified according to the relative abundances of minerals they contain. A given rock is represented 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 rock gets depends not only on composition, but on whether it is intrusive or extrusive. Source: Karla Panchuk (2018) CC BY-NC-SA 4.0, modified after Steven Earle (2015) CC BY 4.0 view source and others, with photos by R. Weller/Cochise College. Click for links to photos and notes on image construction.

To see how Figure 7.13 works, first notice the scale in percent along the vertical axis. The interval between each tick mark represents 10% of the minerals within a rock by volume. An igneous rock can be represented as a vertical line drawn through the diagram, and the vertical scale used to break down the proportion of each mineral it contains. For example, the arrows in the mafic field of the diagram represent a rock containing 48% pyroxene and 52% plagioclase feldspar. An igneous rock at the boundary between the mafic and ultramafic fields (marked with a vertical dashed line) would have approximately 20% olivine, 50% pyroxene, and 30% Ca-rich plagioclase feldspar by volume.

Classification By Grain Size

The name an igneous rock gets also depends on whether it cools within Earth (an intrusive or plutonic igneous rock), or whether it cools on the Earth’s surface after erupting from a volcano (an extrusive or volcanic igneous rock). For example, a felsic intrusive rock is called granite, whereas a felsic extrusive rock is called rhyolite. Granite and rhyolite have the same mineral composition, but their grain size gives each a distinct appearance.

The key difference between intrusive and extrusive igneous rocks—the size of crystals making them up—is related to how rapidly melted rock cools. The longer melted rock has to cool, the larger the crystals within it can become. Magma cools much slower within Earth than on Earth’s surface because magma within Earth is insulated by surrounding rock. Notice that in Figure 7.13, the intrusive rocks have crystals large enough that you can see individual crystals—either by identifying their boundaries, or seeing light reflecting from a crystal face. A rock with individual crystals that are visible to the unaided eye has a phaneritic or coarse-grained texture. The extrusive rocks in the second row have much smaller crystals. The crystals are so small that individual crystals cannot be distinguished, and the rock looks like a dull mass. A rock with crystals that are too small to see with the unaided eye has an aphanitic or fine-grained texture. Table 7.1 summarizes the key differences between intrusive and extrusive igneous rocks.

Table 7.1 Comparison of Intrusive and Extrusive Igneous Rocks
Magma cools within EarthLava cools on Earth’s surface
TerminologyIntrusive/ plutonicExtrusive/ volcanic
Cooling rateSlow: surrounding rocks insulate the magma chamber.Rapid: heat is exchanged with the atmosphere.
TexturePhaneritic (coarse-grained): individual crystals are large enough to see without magnification.Aphanitic (fine-grained): crystals are too small to see without magnification.

What this means is that two igneous rocks comprised of exactly the same minerals, and in the exactly the same proportions, can have different names. A rock of intermediate composition is diorite if it is course-grained, and andesite if it is fine-grained. A mafic rock is gabbro if it is course-grained, and basalt if fine-grained. The course-grained version of an ultramafic rock is peridotite, and the fine-grained version is komatiite. It makes sense to use different names because rocks of different grain sizes form in different ways and in different geological settings.

Does This Mean an Igneous Rock Can Only Have One Grain Size?

No. Something interesting happens when there is a change in the rate at which melted rock is cooling. If magma is cooling in a magma chamber, some minerals will begin to crystallize before others do. If cooling is slow enough, those crystals can become quite large.

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Now imagine the magma is suddenly heaved out 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 larger crystals will be surrounded by much smaller ones. An igneous rock with crystals of distinctly different size (Figure 7.14) is said to have a porphyritic texture, or might be referred to as a porphyry. The larger crystals are called phenocrysts, and the smaller ones are referred to as the groundmass.

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Figure 7.14 Porphyritic rhyolite with quartz and potassium feldspar phenocrysts within a dark groundmass. Porphyritic texture (when different crystal sizes are present) is an indication that melted rock did not cool at a constant rate. Source: Karla Panchuk (2018) CC BY-NC-SA 4.0. Photo by R. Weller/Cochise College (2011) view source