This hands-on, in-class exercise takes place about midway through my structural geology course.

We already have discussed general fault geometries and rock fracture and are beginning discussion of normal, thrust, and strike-slip fault systems.

This activity functions as a stand-alone exercise.

The main goal of this activity is to free students from visualizing faulting as typically viewed in photographs or outcrops—often a single surface that establishes a somewhat misleading idea of the faulting process due to the small, static snapshot of a larger, more complex system in time and space.

A secondary goal is to encourage detailed observation of geologic features and help students arrive at hypotheses to explain the phenomena they are viewing.

Finally, the students must explain what they have observed in writing in a concise and informative manner, which often is difficult for many students at this level. The students also learn how to place text and draw lines on a digital image using Adobe Illustrator.

Many papers by Ken McClay and colleagues have established the value of scale-model experiments in understanding the evolution and geometries of extensional fault systems (e.g. T. Dooley and K.R. McClay, 1997, Analogue modeling of pull-apart basins: American Association of Petroleum Geologists Bulletin, v. 81, no.11, p. 1804—1826).

There are few resources available that help students visualize the dynamic nature of faulting, especially the complex interplay of faults during growth and evolution of a fault system. Such an understanding is critical, however, if students are to think meaningfully about fault geometries and what they imply.

Conducting scale-model experiments in a class setting is useful, but very time-consuming, difficult for all students to see well, and very temporary, except for the end product. Accordingly, taking a cue from a movie produced by Ken McClay, I constructed a deformation apparatus, conducted and filmed several experiments conducted by McClay, and then produced QuickTime movies of the experiments. This approach makes it possible for students to observe an experiment in a minute or two that took 30-45 minutes to produce and to view the experiment repeatedly, so as to become very familiar with all that is taking place.

Individual frames from the movie provide a template on which students can identify the sequence of fault development, rotation of features, and cessation of motion on some faults as they become inactive. Requiring students to document their observations, establish a chronological sequence of events, and explain in writing what happens during the experiment results in an increased awareness of the faulting process.

Students hand in an image of the final fault system which the chronological sequence of fault development indicated by numbers. Letters may be place on the image for reference purposes to support material in their writing assignment. Both the image and writing assignment are graded. We also discuss during a subsequent class period what students have observed and their explanations for what is taking place.

Included for download and use in class are a large and small format movie for each of two experiments. Screen captures from each movie are provided for students to use in tracing fault evolution. Also included are my quick interpretations of these screen captures for instructor use (or for student use).

The sand used in these experiments was purchased from Activa Products, P.O. Box 472, Westford MA 01886, 508-692-9300. It is just listed as craft sand (e.g. Pre-dyed craft sand, 50 lb. bag, dark green). The cost used to be $25/bag.

I have a sketch of the apparatus used in the films (exploded drawing) and a couple of digital photos I am willing to send to anyone who might like to construct a similar device.