The idea for the tectonic sandbox was germinated over twenty-five years ago when I was a student and was trying to understand the physical processes that shape the earth's surface. At that time, the Plate Tectonic hypothesis was entering the realm of becoming a theory, a theory that could pull together and explain a multitude of observations into a framework that verifies we live on the surface of a dynamic planet. Indeed, the earth's lithospheric plates slide around and interact over geologic time causing earthquakes, uplifting mountains, and rifting and separating continents. It had become evident that earthquakes are the frequent reminders of plate interactions, and my curiosity of the mechanics of the earthquake processes began.
While researching what causes earthquakes, I came across an interesting paper entitled "Mechanical Basis For Certain Familiar Geologic Structures" written in 1951 by M. King Hubbert [1]. In this paper, Hubbert showed pictures of a device with which he could generate displacements along "faults" in loose sand that look very similar to the geometry of faults one can find in the field. The device is elegantly simple: a paddle is mounted on a Acme screw and moves in a box of sand. When layers of sand are placed in the box and the paddle has not been moved, the weight of the sand determines the initial stress tensor values, as shown in Figure 1.
Figure 1. Initial condition where 
1 = 3 = 
gh
Only the maximum and minimum values of the initial stresses are shown. As the paddle is moved to the right as is shown in Figure 2,
Figure 2. The change in stress when paddle is moved.
the stress tensor is changed on either side of the paddle. On the right hand side of
the paddle, the sand is compressed and the value for 1 is now greater than 3, the weight of the sand. This situation is similar to the
stress tensor alignment in orogenic belts (mountain building) or subduction environments,
where work is accomplished by overcoming the weight of rock. On the left hand side of the
paddle, a "tensional" environment is created, similar
to the alignment of the stress tensor in landslide or rifting environments, where the
maximum force is supplied by the weight of the rock as shown if Figure 2.
Hubbert worked out the theory that connects the physics within the sandbox to the faulting or brittle failure that we geologists observe on the surface of the earth. Students will be able to measure the geometry of the faulting which occurs on either side of the paddle. This can lead to an understanding of how different styles of deformation are related to different orientations of the stress tensor. For younger students, just watching what happens through the glass front of the sandbox can be instructive.
It was when I began teaching that the ideas in this classical paper inspired me to act. Whenever I have started to talk about tectonic processes, normal faults, reverse faults, brittle failure, stress tensors, etc., I have thought how much easier my task would be if I had a device to show my class like the sandbox that Hubbert invented. So, I thought why not build one using the talents of both faculty and students here at CR?
The first step was to see if the metal shop could tackle the fabrication of such a device, so with pictures of what I wanted, I approached Larry Doyle in the machine tool shop. He said it would be no problem for his students to build such a device but they would need exact drawings to go by. I next went to Steve Brown of drafting technology to see if he could help with preparing the required drawings. After a brief discussion, Steve thought this might be a good project for his DT63 class to undertake. Our plan was to create a situation similar to what the students might experience in a consulting environment out in the business community. We determined that I was to come to Steve's class as a client with a concept. I would explain what I wanted to the class, show them some rough sketches, and it would be their role to come up with designs, drawings, and part list that would enable us to build a prototype of the Tectonic Sandbox. During that first class we spent about an hour making sure that everyone understood what I wanted. I gave them as few constraints as possible, the idea for this being that they could let their imaginations range freely in order that the end product be as innovative as possible.
After the first meeting, momentum and enthusiasm increased steadily. The students in DT63 began to research suppliers and engineers around the state. After a week or two of research and discussion, two designs emerged and were presented to me. The first was similar to the device that Hubbert built in 1951, which used an Acme screw to move the paddle in the box. The second design was suggested by a student to use a gear assembly on a rack to move the paddle through the sand as shown in Figure 3.
Figure 3. The Tectonic Sandbox.
The second design was exciting for me because it is a novel idea which originated in this drafting class here at CR. Since I knew that the Hubbert model did work, I thought that going with the gear- assembly/rack design as pictured in Figure 3 would reward the class for ingenuity, and more importantly, let them try out a design that to my knowledge had not been used before and was therefore "home grown" here at College of the Redwoods. We were going where no instructional-aid design engineers had gone before, and it looked like this new idea could work well.
In selecting the gear assembly design, the class had to overcome many structural and gearing problems. Steve and I sensed the students' excitement in the week by week progress and how by teamwork, they solved each dilemma as one arose. One of the main problems to solve was to construct a gear assembly that had the proper ratio, about 1:10, to enable the paddle to move through the sand. So, gear reduction combinations were tossed around. Another challenge was to design the paddle assembly to be strong enough so as not to twist and torque as the sand is compressed. Again, as a team many solutions initiated by the students were presented to solve each new predicament. Steve and I let the students discuss the merits of each solution to a particular problem, with me (the client) choosing what I thought was best for the Tectonic Sandbox.
As of November 1, all materials have been ordered and we are in the final drawing stage, the prelude to beginning the actual assembly. I think this has been a challenging project for the students. They had to come up with solutions for each obstacle, they seemed to enjoy creating a device that will be used for teaching, and they have taken pride in seeing the project from an idea to the final product!
Acknowledgments. The author is indebted to Susan McPherson for her editing which greatly improved this article. The funding to buy the materials for this project was provided by The PARITY Project, an Eisenhower Grant that supports teaching teachers science. Also special thanks to members of Steve Brown's Drafting Technology 63 class: Steve Anderson, David Berriman, Terry Bisconer, Steve Burroughs, Justin Elstad, Paul Knight, Scott Langlet, Allison Sousa, Alberto Tucay.
Bob McPherson has been a part-time instructor teaching Geology and Physical Science at College of the Redwoods for the past three years. Prior to teaching here, he was the Project Seismologist for the Humboldt Bay Seismic Network from 1974-1986 with TERA Corporation. Also, since 1993, Bob continues to be a Research Associate in the Geology Department at Humboldt State University.
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