Introduction to Structure Contours
Students construct structure contour lines for a "dipping bed" in our classroom and on a geologic map. In my class, this is a multi-day activity.
In part 1 of this exercise, students use "topographic contours" and "outcrop locations" marked on the side walls of the classroom (mimicking a slot canyon) to develop their conceptual understanding of structure contours lines. They sketch a structure contour map of the "outcropping unit" and use this map to calculate the dip of the unit. The unit in question is a planar dipping bed.
In part 3, students construct structure contours for one unit on a geologic map. It is a map of several planar dipping beds, with some modest topography. In preparation for this, we spend significant time talking through the process of analyzing the geologic map to make sure we understand it correctly. After this discussion, I show them a Play-Doh model of the map area.
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
Reading a topographic map; knowing the definition of a topographic contour
How the activity is situated in the course
This multi-day series of activities follows several class periods focusing on topographic maps and geomorphology
Content/concepts goals for this activity
Students will be able to define what a structure contour line is, in their own words.
Higher order thinking skills goals for this activity
Students will be able to visualize structure contours on a planar surface, whether the surface is still present, has been eroded, or is located in the subsurface.
Students will be able to draw and label structure contour lines on a geologic map of planar rock layers, such as parallel dipping beds.
Other skills goals for this activity
3D spatial visualization
Description and Teaching Materials
- Have students connect spot outcrops of a lithologic contact (we use blue painters' tape) on each side wall of the classroom / slot canyon; topographic lines are marked on the walls using measuring tapes taped to the walls. See photo.
- Sketch the geologic map of the contact on a "topo map" of the classroom. We use a blue marker for the contact (matching the color of tape on the wall).
- Use thick string / thin cord to represent structure contour lines connecting the two sides of the classroom, at multiple elevations. We use an orange cord; it is easy to see.
- Add structure contour lines to the map, in the same color as the string/cord.
- Calculate the slope of the contact (surface of interest) = rise / run.
- Study the Play-Doh models of a dipping contact with topographic contours etched in, structure contours indicated with toothpicks where the contact has been eroded. See photos below.
- Lecture / discussion of structure contour lines on a geologic map of planar dipping beds. For this exercise, I simultaneously describe and demonstrate my protocol for constructing structure contour lines:
- Do nothing! In particular, make no assumptions. Misconceptions are very difficult to dislodge.
- Find the scale of the map, the north arrow, and identify the type of map. Proceed only if it is a geologic map.
- Figure out the topography.
- Simplify the task: focus on just one surface. For example, choose one contact.
- Figure out the strike: connect two points of equal elevation along that surface.
- Figure out the dip direction. One way to do this is to compare the elevations of multiple strike lines.
- Figure out whether the surface is the top of the unit or the bottom of the unit.
- I follow up with a whole class Q&A about this process.
- Then I show the class a Play-Doh model of the map area. See photos 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.
Students complete a homework exercise on structure contour lines, with a different but geologically similar map.
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.