A plutonic rock may be classified mineralogically based on the actual proportion of the various minerals of which it is composed (called the mode). In any classification scheme, boundaries between classes are set arbitrarily. The International Union of Geological Sciences (IUGS) Subcommission on the Systematics of Igneous Rocks in 1973 suggested the use of the modal composition for all plutonic igneous rocks with a color index less than 90 (Image to the right). A second scheme (not shown) was proposed for those plutonic ultramafic rocks with a color index greater than 90.
The plotting of rock modes on these triangular diagrams is simpler than it may appear. The three components, Q (quartz) + A (alkali (Na-K) feldspar) + P (plagioclase), are recalculated from the mode to sum to 100 percent. Each component is represented by the corners of the equilateral triangle, the length of whose sides are divided into 100 equal parts. Any composition plotting at a corner, therefore, has a mode of 100 percent of the corresponding component. Any point on the sides of the triangle represents a mode composed of the two adjacent corner components. For example, a rock with 60 percent Q and 40 percent A will plot on the QA side at a location 60 percent of the distance from A to Q. A rock containing all three components will plot within the triangle. Since the sides of the triangle are divided into 100 parts, a rock having a mode of 20 percent Q and 80 percent A + P (in unknown proportions for the moment) will plot on the line that parallels the AP side and lies 20 percent of the distance toward Q from the side AP. If this same rock has 30 percent P and 50 percent A, the rock mode will plot at the intersection of the 20 percent Q line described above, with a line paralleling the QA side at a distance 30 percent toward P from the QA side. The third intersecting line for the point is necessarily the line paralleling the QP side at 50 percent of the distance from the side QP toward A. A rock with 25 percent Q, 35 percent P, and 40 percent A plots in the granite field, whereas one with 25 percent Q, 60 percent P, and 15 percent A plots in the granodiorite field. The latter is close to the average composition of the continental crust of the Earth.
Ideally it would be preferable to use the same modal scheme for volcanic rocks. However, owing to the aphanitic texture of volcanic rocks, their modes cannot be readily determined; consequently, a chemical classification is widely accepted and employed by most petrologists. One popular scheme is based on the use of both chemical components and normative mineralogy. Because most lay people have little access to analytic facilities that yield igneous rock compositions, only an outline will be presented here in order to provide an appreciation for the classification scheme.
The major division of volcanic rocks is based on the alkali (soda + potash) and silica contents, which yield two groups, the subalkaline and alkaline rocks. Furthermore as they are so common, the subalkaline rocks have two divisions based mainly on the iron content with the iron-rich group called the tholeiitic series and the iron-poor group called calc-alkaline. The former group is most commonly found along the oceanic ridges and on the ocean floor and is usually restricted to mafic igneous rocks like basalt and gabbro; the latter group is characteristic of the volcanic regions of the continental margins (convergent, or destructive, plate boundaries) and is comprised of a much more diverse suite of rocks.
Chemically the subalkaline rocks are saturated with respect to silica. This chemical property is reflected in the mode of the mafic members that have two pyroxenes, hypersthene and augite [Ca(Mg, Fe)Si2O6], and perhaps quartz. Plagioclase is common in phenocrysts, but it can also occur in the matrix along with the pyroxenes. In addition to the differences in iron content between the tholeiitic and calc-alkaline series, the latter has a higher alumina content (16 to 20 percent), and the range in silica content is larger (48 to 75 percent compared to 45 to 63 percent for the former). Hornblende and biotite phenocrysts are common in calc-alkaline andesites and dacites but are lacking in the tholeiites. Dacites and rhyolites commonly have phenocrysts of plagioclase, alkali feldspar (usually sanidine), and quartz in a glassy matrix. Hornblende and plagioclase phenocrysts are more widespread in dacites than in rhyolites, which have more biotite and alkali feldspar.
The alkaline rocks typically are chemically undersaturated with respect to silica; hence, they have only one pyroxene, the calcium-rich augite) and lack quartz but often have a feldspathoid mineral, nepheline. Microscopic examination of alkali olivine basalts (the most common alkaline rock) usually reveals phenocrysts of olivine, one pyroxene (augite), plagioclase and perhaps nepheline.
Now that we have completely confused you,let's look at a much simpler classification. We call this a field classification because it requires little detailed knowledge of rocks and can be easily applied to any igneous rock we might pick up while on a field trip. It utilizes texture, mineralogy and color. The latter is a particularly unreliable property, but the classification realizes that certain fine-grained (aphanitic) igneous rocks contain no visible mineral grains and in their absence color is the only other available property. Students the thus cautioned to use color only as a last resort.
To employ this classification we must first determine the rock's texture. Click HERE if you wish to go to a discussion of igneous rock textures. You might remember we have five basic textures; phaneritic (coarse), aphanitic (fine), vesicular, glassy and fragmental (our classification doesn't bother with the latter because we often term all fragmental igneous rocks tuffs). Examine your rock and determine which textural group it belows to. If it is glassy, vesicular or fragmental you cannot determine mineralogy and hence the name is simply obsidian for a glass, tuff for a fragmental or pumice/scoria for a vesicular rock (the latter are differentiated on the basis or color and size of the vesicles or holes).
For the phaneritic and some aphanitic rocks you must determine the mineralogy. Often it is only necessary to identify one or two key minerals, not all of the minerals in the rock. For instance quartz and potassium feldspar (k-feldspar) are restricted to granites and rhyolites. Amphibole is only abundant in diorite or andesite, although minor amounts can be present in granite. How am I getting these names? Let's take an example. I pick up my first specimen and notice that it is distinctly coarse grained (phaneritic). This means that it must be one of the rocks in the row labeled coarse (i.e., granite, diorite, gabbro or peridotite). I next place the rock under a binocular microscope and identify the minerals plagioclase and pyroxene. I go to the bottom row of the chart (Minerals Present) and look for a match with my mineralogy. I find it in the third column (Ca-play, pyroxene) and read the name (gabbro) from the coarse row on the chart. Pretty simple!! Relax, when you actually begin your igneous rock identification we will walk you through it step by step. But remember to refer to the above classification diagram often as an aid.
A plutonic rock may be classified mineralogically based on the actual proportion of the various minerals of which it is composed (called the mode). In any classification scheme, boundaries between classes are set arbitrarily. The International Union of Geological Sciences (IUGS) Subcommission on the Systematics of Igneous Rocks in 1973 suggested the use of the modal composition for all plutonic igneous rocks with a color index less than 90 (Image to the right). A second scheme (not shown) was proposed for those plutonic ultramafic rocks with a color index greater than 90.