Modeling Folds: Block Diagrams and Structure Contours
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
Students need to be familiar with the terminology describing fold geometry and spatial orientation. We review these terms during the activity. They also need to understand what structure contour lines are.
How the activity is situated in the course
This exercise follows, and builds on, both an introduction to folds and an introduction to structure contours. It is the first time that I ask students to think about structure contours of a non-planar (i.e., curved) surface.
Content/concepts goals for this activity
Students will be able to sketch block diagrams of upright, vertical, and plunging folds.
Students will be able to sketch structure contour maps of upright and plunging folds, and will be able to describe what a structure contour map of a vertical fold would look like.
Higher order thinking skills goals for this activity
Students will be able to visualize structure contour lines on a curved surface, such as a folded stratigraphic contact.
Other skills goals for this activity
3D spatial visualization
Description and Teaching Materials
Working in groups of 3, students assemble 3-4 layers of Play-Doh "stratigraphy." Every student has their own Play-Doh, which they purchase as part of their course materials. I have each group do the following:
1. Upright fold:
- Fold layers into an upright, vertical anticline
- Slice the top to make it flat topography; slice through the sides with vertical planes to create a block model
- Orient your model so that the hinge line trends E-W; vertical axial plane strikes E-W
- Sketch the block diagram
- Add strike and dip symbols to your sketch; estimate dips
- Sketch a structure contour map, in cm, of one of the contacts
2. Vertical fold:
- Rotate your play-doh model so that both the hinge and the axial plane are vertical
- Orient your model with the axial plane striking 345 (there are two possible correct map views for this)
- Discuss what the side faces look like
- Sketch the block diagram
- Discuss what a structure contour map would look like
3. Plunging syncline
- Make a new fold with a vertical axial plane that strikes N-S
- Orient it with the hinge plunging 45 toward 360
- Slice through the top horizontally and the sides vertically, to approximate a block shape
- Put a pencil in the hinge; look down-plunge
- What do you see? What is the "apparent" thickness of the beds in the fold?
- Sketch a geologic map (top surface of your block)
- Add strike and dip symbols; start near the hinge – what is the dip there?
- What is the relationship between the strike of the limbs and the map contacts?
- Is 45 the maximum or minimum dip?
- Sketch a structure contour map of one of the contacts
- Discuss it in your group before you begin; start by sketching just one contour line
- Fill in other structure contour lines, at regular intervals – remember the 45 degree dip
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.
Student understanding is not formally assessed in this activity, but is an essential pre-requisite for many of the subsequent course activities, including homework and lab exercises.
References and Resources
This exercise builds on previous activities in the course, including:
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.