Exploring relationships between map patterns and stereonets of cylindrically folded layers
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Summary
This activity utilizes the Geology Explorer and Stereonet modules of Visible Geology to help students understand how map patterns of folded stratigraphy are related to their corresponding stereograms. Students begin with exploration of tilted layers before moving onto upright horizontal folds, and plunging folds of multiple wavelengths.
The exploration of these models will help students to see:
- How map patterns of folded layers relate to stereonets
- How fold axes are represented on stereonets
- What aspects of fold geometry are lost when represented on stereonets
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Learning Goals
- Strengthen visualization skills needed to investigate sections through geologic structures
- Determine how elements of fold geometry (namely, fold axis orientation) are represented on a stereonet
- Recognize and describe the aspects of fold geometry that cannot be communicated using a stereographic projection
Context for Use
This activity is written for an undergraduate structural geology course as either an in-class lab or a take-home problem set, taking students 1–2 hours to complete. The activity is well-suited in part of a structural geology course covering descriptive analysis, or perhaps as preparation for a field trip to a site exhibiting folds.
To be successful, students should have already been briefly introduced to the anatomy of a stereonet and how planar and linear features are represented. They should already know basic features of folds: wavelength, amplitude, fold hinges and limbs, vergence, cylindrical and noncylindrical folds, etc. Students should be able to recognize and interpret strike/dip map symbology.
Description and Teaching Materials
An introduction to this exercise—either before assigning it as a take-home problem set or at the start of a lab period—would consist of a brief review or lesson on fold geometry (e.g., plunge, vergence, interlimb angle) and anatomy of a stereonet (e.g., primitive, great and small circles, representation of line and planes, poles to planes).
From there, students can ideally work through the assignment with minimal instructor intervention as long as they have access to a computer/tablet with which to access the Visible Geology models.
Visible Geology teaching activity_map patterns and stereonets.docx (Microsoft Word 2007 (.docx) 5.4MB May15 26)
Visible Geology Teaching Activity - Map Patterns and Stereonets.pdf (Acrobat (PDF) 589kB May15 26)
Teaching Notes and Tips
- Students will benefit from reminders of fold terminology when they are asked to describe fold geometry.
- I would assign this as a paper copy for students to sketch in the sides of the block model templates, but an electronic version would also benefit the students so they can access links to each block model hosted by Visible Geology.
- To save and share their work in Visible Geology, students are encouraged to log in with their Google Account.
Assessment
Students are assessed based on the accuracy and completeness of their responses to each prompt. Answers to the final set of questions asking students to consider the sum of what they have observed in their models is the best gauge of student understanding.
References and Resources
- Clear illustrations of fold anatomy can be found within chapter 8 of Jean-Pierre Burg's Script to Structural Geology. These figures are useful resources for students as they describe the geometry of structures in Visible Geology block models.
- Additional well-illustrated fold diagrams are contained within Haakon Fossen's web-hosted material associated with the text: Fossen, H., 2016. Structural Geology, second ed. Cambridge University Press, Cambridge (ISBN 978-1-107-05764-7).
- For tips, tricks and how-to videos for using Visible Geology, check out the Visible Geology Help & Resources.