Igneous Rocks and Triangle Diagrams

Eileen Herrstrom
University of Illinois at Urbana-Champaign
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This activity takes place in a laboratory setting and requires ~1.5-2 hours to complete. Students observe igneous rock compositions and plot them on triangle diagrams, normalize rock compositions using a spreadsheet, and identify the rocks.

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Undergraduate class on introductory physical geology or quantitative reasoning for non-majors

Skills and concepts that students must have mastered

Must know how to recognize minerals in rocks, have basic knowledge about using Microsoft Excel (enter formulas, fill down), and understand how to read triangle diagrams

How the activity is situated in the course

This is a laboratory activity that follows lectures on igneous rocks and is the eighth laboratory exercise of the course.


Content/concepts goals for this activity

Visually estimate the mineral compositions of rocks in hand specimen and thin section and name the rocks using a triangle diagram

Higher order thinking skills goals for this activity

Apply the method of point counting to identify mineral compositions of rocks more precisely, express point counts as percentages, and compare estimates of composition derived by the visual and point counting methods

Other skills goals for this activity

Normalize point counting data using a spreadsheet, plot the compositions on the correct triangle diagram, and classify several intrusive igneous rocks based on composition

Description of the activity/assignment

Geologists describe rocks as they do minerals – by their physical properties – but rocks have only two properties, namely, composition and texture. Different rock types emphasize different aspects of composition and texture, and there are so many variations that geologists have coined hundreds of different rock names. In this exercise, we focus on one particular set of igneous rocks and learn how geologists approach describing and naming them. For igneous rocks, we begin usually by considering mineral composition, i.e., what minerals are present and at what percentages. All of the rocks used in this exercise are coarse-grained and crystalline.

Student materials for this exercise include an instruction/question file, a Microsoft Excel spreadsheet with with data for various types of igneous rocks, and an image file illustrating rock samples (PDF). The exercise is divided into three parts.

Part I introduces the concept of composition for igneous rocks. Students learn how to estimate mineral percentages visually for two hand speciments and one thin section.

Part II applies the method of point counting to the three samples from Part I. Students compare the precision and results of the rock identification methods in Parts I and II.

In Part III, students practice normalizing compositional data for use on triangle diagrams. They apply this method to a simplified quartz-alkali feldspar-plagioclase (QAP) triangle diagram. Students discover the drawbacks of the QAP diagram and the advantage of using other triangle diagrams for mafic and ultramafic rocks.

Determining whether students have met the goals

In both the traditional face-to-face and online versions of the course, this activity is assessed based on the answers to the questions. It is also possible to have students submit their completed spreadsheets, although this option works best in a small class.

More information about assessment tools and techniques.

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Other Materials

Supporting references/URLs

Fichter, L.S., 2000, Reading a ternary diagram: Online resource – Accessed 16 June 2019 http://csmgeo.csm.jmu.edu/geollab/Fichter/SedRx/readternary.html

King, H.M., 2005, Uses of Granite: Online resource – Accessed 16 June 2019

Earth Science Australia, 2018: Igneous Rock Classification: Online resource – Accessed 16 June 2019

Terry, R.D., and G.V. Chilingar, 1955, Summary of "Concerning Some Additional Aids in Studying Sedimentary Formations" by M.S. Shvetsov: Journal of Sedimentary Petrology, v. 25, n. 3, pp. 229-234. Online resource – Accessed 16 June 2019