Modeling Unconformities

Basil Tikoff, University of Wisconsin-Madison
Naomi Barshi, University of Wisconsin-Madison
Carol Ormand, SERC, Carleton College

Author Profile

Summary

Students make models of various kinds of unconformity: disconformity, angular unconformity, and buttress unconformity. They examine those models from a variety of perspectives and consider how each one appears in map view and in cross-sections (parallel and perpendicular to strike).

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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 definition of unconformity and the different types of unconformity is helpful, but we also review those definitions during the activity.

How the activity is situated in the course

This activity is a "review" of unconformities, which students are expected to have learned about in Physical Geology. It builds on this review to introduce students to the idea that unconformities exist in three dimensions, and that their appearance differs depending on our visual perspective. It also eases students into building Play-Doh models, as this is the first of many such activities in the course.

After this activity, I show a series of photographs of outcrops and ask students to identify the unconformities in each of the images. Shortly thereafter, we have a field trip to the Black Hills, where students will see several unconformities.

Goals

Content/concepts goals for this activity

Students will be able to recognize and name different types of unconformities, on the basis of their 3D geometry and the lithologies above and below the unconformity.

Higher order thinking skills goals for this activity

Students will understand that 2D slices through different kinds of unconformities can look the same. For example, looking at an angular unconformity perpendicular to the strike of the layers, it may appear visually identical to a disconformity.

Other skills goals for this activity

3D spatial visualization

Description and Teaching Materials

As I lecture about the different types of unconformity, I have students make physical models of each type, using Play-Doh. I demonstrate and talk through the construction process for each type of unconformity, and direct students to examine those models from a variety of perspectives and consider how each one appears in map view and in cross-sections (parallel and perpendicular to strike). I emphasize similarities and differences between the types of unconformity as seen in map view and cross-sectional view. The Play-Doh models allow students to see these relationships in 3D. See examples below.

Angular unconformity

Map view (with the top layer partially eroded) and two cross-sectional views (parallel and perpendicular to strike)


Disconformity

With and without a "paleosol"


Buttress unconformity

The central (yellow) unit represents paleotopography, around and on top of which younger sedimentary layers have been deposited.


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

Every student has their own Play-Doh, which they purchase as part of their course materials (along with colored pencils, protractor, etc.) and bring to class nearly every day.

Assessment

Student understanding of unconformities is not formally assessed in this activity, but is an essential pre-requisite for many of the subsequent activities, including field trips and 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.