science

TAS classification

TAS classification is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand TAS classification rather than just read about it. In short: TAS stands for Total Alkali Silica. The TAS classification can be used to assign names to many common types of volcanic rocks based upon the relationships between the combined alkali and silica contents.

Key takeaways

TAS classification belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect TAS classification to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of TAS classification from memory before moving on to harder problems.

Reference excerpt

TAS stands for Total Alkali Silica. The TAS classification can be used to assign names to many common types of volcanic rocks based upon the relationships between the combined alkali and silica contents. These chemical parameters are useful because the relative proportions of alkalis and silica are important in determining both normative mineralogy and actual mineralogy. The classification can be simple to use for rocks that have been chemically analyzed. Except for the following quotation from Johannsen (1937), this discussion is based upon Le Maitre et al (2002).

Use of the TAS classification Before using the TAS or any other classification, some particular guidance by Johannsen (1937) should be kept in mind.

Many and peculiar are the classifications that have been proposed for igneous rocks. Their variability depends in part upon the purpose for which each was intended, and in part upon the difficulties arising from the characters of the rocks themselves. The trouble is not with the classifications but with nature which did not make things right. … Rocks must be classified in order to compare them with others, previously described, of similar composition and appearance. If this cannot be done on a genetic basis, then an artificial system must answer in order to serve as a card index to rock descriptions. Although this may be an evil thing, it is, at least, the least of several evils. The TAS classification cannot be applied to all volcanic rocks—as discussed in detail by Le Maitre et al (2002). Certain rocks cannot be classified/named using the diagram. For others, additional chemical, mineralogic, or textural criteria must be used, as e.g. lamprophyres. The TAS classification should be applied only to rocks for which the mineral mode analysis cannot be determined. Otherwise, a scheme based on mineralogy, such as the QAPF diagram, or one of the other diagrams available for igneous rocks may be suitable. Before classifying rocks using the TAS diagram, the chemical analyses must be recalculated to 100% excluding water and carbon dioxide.

The TAS diagram

The names provided by Le Maitre et al. (2002) for fields in the TAS diagram are listed below. B (Basalt)—Use normative mineralogy to subdivide. O1 (Basaltic andesite) O2 (Andesite) O3 (Dacite) R (Rhyolite) T (Trachyte or trachydacite)—Use normative mineralogy to decide. Ph (Phonolite) S1 (Trachybasalt)—*Sodic and potassic variants are hawaiite and potassic trachybasalt.

S2 (Basaltic trachyandesite)—*Sodic and potassic variants are mugearite and shoshonite. S3 (Trachyandesite—*Sodic and potassic variants are benmoreite and latite. Pc (Picrobasalt) U1 (Basanite or tephrite)—Use normative mineralogy to decide. U2 (Phonotephrite) U3 (Tephriphonolite) F (Foidite)—When possible, classify/name according to the dominant feldspathoid. Melilitites also plot in this area and can be distinguished by additional chemical criteria.

(*)Sodic as used above means that Na2O - 2 is greater than K2O, and potassic that Na2O - 2 is less than K2O. Yet other names have been applied to rocks particularly rich in either sodium or potassium—as are ultrapotassic igneous rocks.

References Albert Johannsen, A Descriptive Petrography of the Igneous Rocks. Volume 1, Introduction, Textures, Classifications, and Glossary. The University of Chicago Press, Chicago, Illinois, 1937. R. W. Le Maitre (editor), A. Streckeisen, B. Zanettin, M. J. Le Bas, B. Bonin, P. Bateman, G. Bellieni, A. Dudek, S. Efremova, J. Keller, J. Lamere, P. A. Sabine, R. Schmid, H. Sorensen, and A. R. Woolley, Igneous Rocks: A Classification and Glossary of Terms, Recommendations of the International Union of Geological Sciences, Subcommission of the Systematics of Igneous Rocks. Cambridge University Press, 2002. ISBN 0-521-66215-X

Worked examples

Example 1 — a first encounter with TAS classification

Start with the simplest possible case. Write down what TAS classification claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to TAS classification before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about TAS classification ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of TAS classification

In research: TAS classification appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses TAS classification in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: TAS classification is common in secondary-school and first-year university syllabi. It links to neighbouring topics Igneous petrology, Igneous rocks, so understanding it makes those chapters shorter.
In everyday life: Look for TAS classification outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “TAS classification” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study TAS classification in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what TAS classification means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain TAS classification out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is TAS classification in simple terms?

Why does TAS classification matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study TAS classification?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on TAS classification.