Igneous Rocks Model
- 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
- Four rock samples, including pieces of
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.
- 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.