Igneous Rocks and Triangle Diagrams
Skills and concepts that students must have mastered
How the activity is situated in the course
Content/concepts goals for this activity
Higher order thinking skills goals for this activity
Other skills goals for this activity
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
Teaching materials and tips
- Activity Description/Assignment:Student Instructions for Igneous Rocks (Microsoft Word 2007 (.docx) 4.1MB Jun16 19)
- Instructors Notes:Instructors Notes for Igneous Rocks (Acrobat (PDF) 4.9MB Jun16 19)
- Samples for Igneous Rocks (Acrobat (PDF) 35.8MB Jun16 19)
- Student Workbook for Igneous Rocks (Excel 2007 (.xlsx) 12kB Jun16 19)
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