Igneous Rocks Are Classified By Mineral Abundance
In the last section you learned that igneous rocks are classified into four categories based on their chemical composition: felsic, intermediate, mafic, and ultramafic. From the diagram of Bowen’s reaction series in Figure 7.5, it is clear 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 which they contain. Figure 7.11 is a diagram with the minerals from Bowen’s reaction series, and is used to decide which name to give an igneous rock.
To see how Figure 7.11 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. 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 ultramafic field of the diagram represent a rock containing 40% olivine and 60% pyroxene. An igneous rock at the boundary between the mafic and ultramafic fields (marked with a vertical dashed line) would have approximately 15% olivine, 75% pyroxene, and 10% Ca-rich plagioclase feldspar.
Igneous Rocks Are Also Classified By Grain Size
The name an igneous rock gets also depends on whether it cools within the 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. A key difference between intrusive and extrusive igneous rocks is the size of crystals making them up. The longer magma has to cool, the larger the crystals within it can become. Magma cools much more slowly within the Earth than on Earth’s surface because magma within the Earth is insulated by surrounding rock. Notice that in Figure 7.11, the intrusive rocks in the first row have crystals large enough so that you can identify individual crystals. This is referred to as phaneritic texture. The extrusive rocks in the second row have much smaller crystals. The crystals are so small that the rock looks like a dull mass, and it isn’t possible to see individual crystals. This texture is referred to as aphanitic. 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 the Earth||Lava cools on Earth’s surface|
|Terminology||Intrusive or plutonic||Extrusive or volcanic|
|Cooling rate||Slow: surrounding rocks insulate the magma chamber.||Rapid: heat is exchanged with the atmosphere.|
|Texture||Phaneritic: crystals are large enough to see without magnification. (coarse grained)||Aphanitic: crystals are too small to see without magnification. (fine grained)|
What this means is that two igneous rocks with exactly the same minerals making them up, and in the same proportions, can have different names. If the rock is of intermediate composition it is diorite if it is course-grained, and andesite if it is fine-grained. If the rock is mafic it is gabbro if it is course-grained, and basalt if fine-grained. If the rock is ultramafic, the course-grained version 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 geologic settings.
Does That Mean an Igneous Rock Will Only Ever Have Crystals of 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. 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 very rapidly, and the larger crystals will be surrounded by much smaller ones. An igneous rock with crystals of distinctly different size 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 or matrix. Figure 7.12 shows a porphyritic rhyolite with quartz and potassium feldspar phenocrysts within a darker groundmass.
Exercise 7.3 Which Mineral Will the Phenocryst Be?
As a magma cools below 1300°C, minerals start to crystallize within it. If that magma is then involved in a volcanic eruption, the rest of the liquid will cool quickly to form a porphyritic texture. The rock will have some relatively large crystals (phenocrysts) of the minerals that crystallized early, and the rest will be very fine grained or even glassy. Using the diagram shown here, predict what phenocrysts might be present where the magma cooled as far as line a in one case, and line b in another.
Classifying Igneous Rocks According to the Proportion of Dark Minerals
If you aren’t exactly sure which minerals are present in an intrusive igneous rock that you’re studying, there is a quick way to approximate the composition of that rock. In general, igneous rocks have an increasing proportion of dark minerals as they become more mafic. The dark minerals are those higher in iron and magnesium (e.g., olivine, pyroxene), and for that reason they are sometimes referred to collectively as ferromagnesian minerals. Figure 7.13 is the result of simplifying Figure 7.11 by grouping minerals as either light or dark.
By estimating the proportion of light minerals to dark minerals in a sample, it is possible to place that sample in Figure 7.13. Guides like the ones in Figure 7.14 are used to help visualize the proportions of light and dark.
It is important to note that the method of estimating the proportion of dark minerals is approximate as a means for identifying igneous rocks. At issue is the fact that Na-rich plagioclase feldspar is light in colour, whereas Ca-rich plagioclase feldspar can appear darker (Figure 7.15), especially when surrounded by darker minerals. Nevertheless, plagioclase feldspar does not qualify as ferromagnesian, so it falls in the light-coloured minerals field in Figure 7.13.
Exercise 7.4 Classifying Igneous Rocks By Proportion of Ferromagnesian Minerals
The four igneous rocks shown below have differing proportions of ferromagnesian silicates (dark minerals). Estimate those proportions using the diagrams in Figure 7.14, and then use Figure 7.13 to determine the likely rock name for each one.
Classifying Igneous Rocks When No Crystals Are Visible
The method of estimating the percentage of minerals works well for phaneritic igneous rocks, where crystals are visible with the naked eye or with the use of a magnifying lens. If an igneous rock is porphyritic but otherwise aphanitic as in Figure 7.12, the minerals present as phenocrysts can be used to identify the rock (e.g., Exercise 7.3). However, there are two cases where mineral composition cannot be determined by looking at visible crystals. These include volcanic rocks without phenocrysts, and glassy igneous rocks.
Volcanic Rocks Without Phenocrysts
Without the aid of visible crystals or phenocrysts, volcanic rocks can be classified on the basis of colour and sometimes on other textural features. As you may have noticed in Figure 7.11, the colour of volcanic rocks moves from light to dark as the composition goes from felsic to mafic. Figure 7.16 shows this progression with rhyolite (often a tan or pinkish colour), andesite (often grey), and basalt (ranging from brown to dark green to black).
Basalt is often accompanied by two textural features, vesicles and amygdules. As magma travels to the surface and erupts as lava, the decrease in pressure allows gases dissolved in the lava to be released and form bubbles. When the lava freezes around the bubbles, vesicles are formed (circular inset in 7.16). If minerals later form within the vesicles, the filled vesicles are called amygdules (box inset in Figure 7.16).