View An Igneous Rock Diagram

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QAP Diagram for Plutonic Rocks

For granitoids and other deep-seated rocks
Igneous Rock Classification Diagrams Click the image for a larger version. (c) 2008 Andrew Alden, licensed to About.com (fair use policy)

The official classification of igneous rocks fills an entire book. But the great majority of real-world rocks can be classified using a few simple graphical aids. The triangular (or ternary) QAP diagrams display mixtures of three components whereas the TAS graph is a conventional two-dimensional graph. They're also very handy for just keeping all the rock names straight. These graphs use the official classification criteria from the International Union of Geological Societies (IUGS). Click on any of them to see a larger version.

The QAP ternary diagram is used to classify igneous rocks with visible mineral grains (phaneritic texture) from their feldspar and quartz content. In plutonic rocks, all of the minerals are crystallized into visible grains.

Here is how it works:

  1. Determine the percentage, called the mode, of quartz (Q), alkali feldspar (A), plagioclase feldspar (P), and mafic minerals (M). The modes should add up to 100.
  2. Discard M and recalculate Q, A and P so that they add up to 100 -- that is, normalize them. For example, if Q/A/P/M are 25/20/25/30, Q/A/P normalizes to 36/28/36.
  3. Draw a line on the ternary diagram below to mark the value of Q, zero at the bottom and 100 at the top. Measure along one of the sides, then draw a horizontal line at that point.
  4. Do the same for P. That will be a line parallel to the left side.
  5. The point where the lines for Q and P meet is your rock. Read its name from the field in the diagram. (Naturally, the number for A will also be there.)
  6. Notice that the lines that fan downward from the Q vertex are based on values, expressed as percentage, of the expression P/(A + P), meaning that each point on the line, regardless of the quartz content, has the same proportions of A to P. That's the official definition of the fields, and you can calculate your rock's position that way too.

Notice that the rock names at the P vertex are ambiguous. Which name to use depends on the composition of the plagioclase. For plutonic rocks, gabbro and diorite have plagioclase with a calcium percentage (anorthite or An number) above and below 50, respectively.

The middle three plutonic rock types -- granite, granodiorite and tonalite -- are together called granitoids. (Read more about granitoids.) The corresponding volcanic rock types are called rhyolitoids, but not very often.

A large proportion of igneous rocks aren't suited for this classification method:

  • Aphanitic rocks: These are classified by chemical, not mineral content.
  • Rocks without enough silica to yield quartz: These instead contain feldspathoid minerals and have their own ternary diagram (F/A/P) if they are phaneritic.
  • Rocks with M above 90: Ultramafic rocks have their own ternary diagram with three modes (olivine/pyroxene/hornblende).
  • Gabbros, which can be further classified according to three modes (P/olivine/pyx+hbde).
  • Rocks with isolated larger grains (phenocrysts) may yield distorted results.
  • Rare rocks including carbonatite, lamproite, keratophyre, and others that are "off the chart."

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QAP Diagram for Volcanic Rocks

For volcanic rocks with visible grains
Igneous Rock Classification Diagrams Click the image for a larger version. (c) 2008 Andrew Alden, licensed to About.com (fair use policy)

Volcanic rocks usually have very small grains (aphanitic texture) or none (glassy texture), so the procedure usually takes a microscope and is rarely performed today. 

To classify volcanic rocks by this method requires a microscope and thin sections. Hundreds of mineral grains are identified and carefully counted before using this diagram. Today the diagram is useful mainly to keep the various rock names straight and to follow some of the older literature. The procedure is the same as with the QAP diagram for plutonic rocks.

Many volcanic rocks aren't suited for this classification method:

  • Aphanitic rocks must be classified by chemical, not mineral content.
  • Rocks with isolated larger grains (phenocrysts) may yield distorted results.
  • Rare rocks including carbonatite, lamproite, keratophyre, and others are "off the chart."

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TAS Diagram for Volcanic Rocks

The default method for most lavas
Igneous Rock Classification Diagrams Click the image for a larger version. (c) 2008 Andrew Alden, licensed to About.com (fair use policy)

Volcanic rocks are usually analyzed with bulk chemistry methods and classified by their total alkalis (sodium and potassium) graphed versus silica, hence the total alkali silica or TAS diagram. 

Total alkali (sodium plus potassium, expressed as oxides) is a fair proxy for the alkali or A-to-P modal dimension of the volcanic QAP diagram, and silica (total silicon as SiO2) is a fair proxy for the quartz or Q direction. Geologists usually use the TAS classification because it's more consistent. As igneous rocks evolve during their time beneath the Earth's crust, their compositions tend to move upward and rightward on this diagram.

Trachybasalts are subdivided by the alkalis into sodic and potassic types named hawaiite, if Na exceeds K by more than 2 percent, and potassic trachybasalt otherwise. Basaltic trachyandesites are likewise divided into mugearite and shoshonite, and trachyandesites are divided into benmoreite and latite.

Trachyte and trachydacite are distinguished by their quartz content versus total feldspar. Trachyte has less than 20 percent Q, trachydacite has more. That determination requires studying thin sections.

The division between foidite, tephrite and basanite is dashed because it takes more than just alkali versus silica to classify them. All three are without any quartz or feldspars (instead they have feldspathoid minerals), tephrite has less than 10 percent olivine, basanite has more, and foidite is predominantly feldspathoid.