Pedagogy in Action > Library > Interactive Lecture Demonstrations > Examples of Teaching with Demonstrations > Igneous Rocks Model

Igneous Rocks Model

David Steer (steer@uakron.edu) and Kyle Gray (krg10@uakron.edu), University of Akron
This material is based upon work supported by the National Science Foundation under Grant No. GEO-0506518.

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This page first made public: Aug 25, 2008

Summary

While working in groups to facilitate peer tutoring, students use samples of four igneous rocks (gabbro, basalt, granite, and rhyolite) to observe differences in texture, color and grain size and make inferences about the relative cooling histories and silica content associated with each magma type.

Learning Goals

Students will:
  • Be able to identify the differences between volcanic and plutonic igneous rocks
  • Be able to identify samples of "high silica" rocks from "low silica" igneous rocks
  • Relate the properties of an igneous rock to the rock's cooling history

Context for Use

This model works well during units on the properties and formation of igneous rocks, as well as surveys of the different rock types or factors that influence the explosivity of a volcanic eruption. Students work in groups of 2-4 individuals as they make observations and predictions and the students are allowed to use the rock identification chart in their texts.

Description and Teaching Materials

Igneous Rocks
Each student model consists of the following material:
Four rock samples, including pieces of
  • Gabbro
  • Basalt
  • Granite
  • Rhyolite

Samples should be uniform both within and between specimens. The rhyolite and basalt samples should contain few phenocrysts. All rocks should be clearly identified (e.g. samples 1-4), and all sample sets should have the same identification scheme (i.e. all granites are #1). Samples used in this exercise were purchased from a scientific supply company.

Teaching Notes and Tips

Students are provided the four samples and instructed to find similarities and differences between the samples. Students typically note mineral sizes and color differences. Processes associated with plutonic and volcanic rocks are discussed. Students identify the texture (fine-grained vs coarse-grained) of each rock sample and use their observations to identify the volcanic and plutonic rocks and discuss cooling histories of the rocks.

Students sort rocks with similar texture rocks in rows and similar color rocks in columns. Connections between silica content and bulk rock color are discussed. Students sort the rocks based on their silica content (gabbro and basalt vs granite and rhyolite) and label each rock. The instructor assesses student understanding by asking which rocks belong to the high (or low) silica category. Higher-order processing skills can be enhanced by asking which rocks represent magma that would produce a more (or less) explosive eruption if magma viscosity has been discussed.

Students name each rock type by using the texture and color of each rock sample.

Caution: It may be difficult to find samples of gabbro that do not look like the basalt. Also, the actual samples may vary in color and texture from the photos shown in class or in the textbook.

Assessment

Several different assessment techniques can be used depending on time and the needs of your class.
  • If you have a personal response system, you can ask ConcepTest questions that focus on using textural information to infer cooling or eruptive histories. (See References and Resources for sample ConcepTest questions.)
  • By walking around the class, the instructor can observe how well the groups comprehend the underlying concepts and ask individual students to explain their conceptual understanding
  • Students could complete a worksheet containing questions about inferring the cooling histories of igneous rocks, or the students can fill in a Venn diagram showing the relationship between the different types of igneous rocks.
  • Students could write a 'minute paper' explaining the cooling history of a rock with a given texture and color.

References and Resources