# Introduction to Igneous Intrusions

#### Summary

Students make Play-Doh models of sills and dikes.

## Context

#### Audience

200-level undergraduate course, "Introduction to Geologic Structures." This course is a pre-requisite for most upper-level undergraduate geoscience courses in the core curriculum, including Structural Geology. It has a pre-requisite of an introductory level Geology course.

#### Skills and concepts that students must have mastered

Familiarity with the classification of common types of igneous rocks is helpful, but we also review those definitions during the activity.

#### How the activity is situated in the course

This activity is embedded in a lecture about igneous rocks. I begin with a "review" of igneous rock classification and associated tectonic settings of formation, which students are expected to have learned about in Physical Geology. I then describe igneous rock fabrics, focusing on foliations and lineations. Then I move into to the geometry of igneous intrusions, for which we use the Play-Doh.

## Goals

#### Content/concepts goals for this activity

Students will be able to recognize and name different types of igneous intrusions on the basis of their 3D geometry.

#### Higher order thinking skills goals for this activity

Students will be able to make reasonable interpretations of the 3D geometries of igneous intrusions from information on geologic maps and cross-sections.

#### Other skills goals for this activity

3D spatial visualization

## Description and Teaching Materials

As I lecture about igneous intrusions, I have students make physical models of various types of intrusions, using Play-Doh. I direct students to examine those models from a variety of perspectives and consider how each one appears in map view and in cross-sections.

I emphasize similarities and differences between the map patterns and cross-sectional views. The Play-Doh models allow students to see these relationships in 3D. See examples below.

## Science of Learning: Why It Works

Three-dimensional models can help to improve students' understanding of geological phenomena. Physical models, such as playdough models, serve as analogies to geological features and geologic maps. Analogies support the development of spatial thinking skills by allowing the student to draw from their knowledge and apply it to new cases (e.g., Gentner 1983). For example, students can reason from what they can experience when carving off pieces of playdough to how erosion will reveal geological structures). Analogical learning also applies to "mapping" -- that is, relating -- the features of models onto real world phenomena (e.g., this layer of playdough corresponds to a layer of sandstone). Physical models provide analogies to real-world phenomena, support cognitive offloading, and promote spatial accommodation.

Practice constructing spatial analogies can help students develop the mental models that allow them to recognize new cases of familiar concepts in the field. When instructors provide accurate physical models of geologic features, students can self-assess their understanding by comparing their mental model -- or their own physical model -- to the instructor's physical model. When students make their own physical models, these models serve as a means of "inscription," in much the same ways that mapping and sketching do: they allow for students to record their conceptual understanding of a natural phenomenon (Mogk and Goodwin, 2012). However, unlike mapping and sketching, a playdough model allows for this record to be three-dimensional, like the phenomenon itself, which can reduce the cognitive demands inherent in the process of inscription by reducing the need to generate a 2D representation of 3D space (Newcombe, 2012). In addition, physical models support spatial accommodation: when a student compares their (mental or physical) model of a phenomenon or region to their instructor's model and recognizes a difference between the models, the student is prompted to revise their mental model (Davatzes et al., 2008). When the change required is spatial the student may use the feedback directly to revise the model. When the change is significant, the student may need to completely discard their old model and construct a new one. Playdough models of geological structures can thus serve as the basis for improved mental models.

## Teaching Notes and Tips

Dental floss slices through Play-Doh with minimal smearing.

## Assessment

Student understanding of the geometry of igneous intrusions is not assessed directly, but is an essential pre-requisite for many of the course activities, including multiple lab exercises.

## References and Resources

Davatzes et al. (2018). Learning to form accurate mental models. Eos, 99, https://doi.org/10.1029/2018EO091643. Published on 07 February 2018.

Gentner, D. (1983). Structure-mapping: A theoretical framework for analogy. Cognitive Science, 7(2), 155–170.

Mogk and Goodwin (2012). Learning in the field: Synthesis of research on thinking and learning in the geosciences, in Earth and Mind II: A synthesis of research on thinking and learning in the geosciences, edited by Kim A. Kastens and Cathryn A. Manduca. GSA Special Paper 486:131-163. DOI: 10.1130/2012.2486(24)

Newcombe, N. S. (2012). Two ways to help students with spatial thinking in geoscience. Geological Society of America Special Papers, 486, 85-86.