A Simple Paper Model of a Transform Fault at a Spreading-Ridge

Journal of Geoscience Education, Sep 2004 by Earle, Steven

ABSTRACT

The process of spreading at a divergent boundary, the formation of sea-floor magnetic patterns, and the mechanism of transform faulting can be demonstrated using a very simple paper model. The use of this model in an introductory geology course allows for active and experiential learning of concepts that can be difficult to understand through passive learning techniques.

INTRODUCTION

Many students in introductory geology courses have difficulty understanding the various processes that take place on spreading ridges and along the fracture zones that cut across ridges. One of the more elusive concepts is the existence of transform faults along the sections of fracture zones between offset ridge segments. I have found that a simple but elegant paper model of the process of spreading and transform faulting, originally used by J. Tuzo Wilson, but first described by Stewart (1990), is very useful for teaching about transform faults and about some of the other features of the ridge region.

THE MODEL

The concept of transform faulting was originally proposed by Wilson (1965). As described by Glen (1982), Wilson was trying to understand relationships among several major features off the western coast of North America, including the Queen Charlotte Fault, the Juan de Fuca ridge, the Mendocino fracture zone and the San Andreas fault. In an interview with Glen in 1980, Wilson said:

"I remember that I started, not with a folded piece of paper, but had two L-shaped pieces, which I played with and recognized ... that you get a shear with opening and spreading. You get a shear (in two places) when you move the L-shaped pieces apart and together: this is a spreading ridge, with a shear at either end. [That] led to the idea of the transform fault. I wrote the paper on transforms about three weeks before it was published in Nature." (Glen, 1982, p. 374).

In his description of the history of the plate-tectonics revolution, Stewart (1990) notes that Wilson continued to use a paper model "to communicate his idea to others" (Stewart, 1990, p. 82). An adaptation of the paper model, described by Stewart as being similar to one used by Wilson, is shown in Figure 1. Stewart (1990, p. 81) urges his readers to try the model out, because "it illustrates how working models provide tacit knowledge not communicated easily by words."

The model is illustrated in Figure 1. To prepare the model, cut along the fracture zone from point a to point b with a razor blade. Fold the paper on either side of the fracture zone as indicated. Make 90° downward folds at the four locations marked Fold down, and 180° upward folds at the two locations marked Pinch together. The resulting model will now be about one-half as long as the original, and will have two folds of paper (the soon to be created sea floor) projecting down from the surface (Figure 2). Students must work in pairs to operate the model. One holds the ridge segments together by gently pinching each of the downward folds just below the surface. The other holds the paper at either end (adjacent to points a and b) and slowly pulls the new sea floor out of the ridge crests (Figure 3).

The magnetic anomaly patterns shown on the model (Fiure 1) accurately represent the earth's magnetic reversal chronology for approximately the last 2.4 million years.

Ready-to-print copies of the model shown here, plus a similar model with four ridge segments, can be downloaded from: http://web.mala.bc.ca/earle/transform-model/

DISCUSSION

Two important concepts can be discovered using this model. Firstly, one can observe how sea-floor magnetic patterns are created, why they are symmetrical with respect to the ridge, and why they look different on

opposite sides of a fracture zone. secondly, one can observe the nature of motion along the fracture zone; that while both sides move in the same direction outside of the ridge segments, they move in opposite directions between the ridge segments (the transform fault zone), and that earthquakes are largely restricted to the transform fault.

The value of this exercise - as stated by Stewart (1990) - is that some relatively difficult spatial concepts can be readily understood using a working model, rather than relying on explanations in words or pictures. Although the action of pulling paper out from between someone's fingers is not a good analogy for the complex physical and chemical processes occurring within the mantle and crust beneath a spreading ridge, the model clearly illustrates the magnetic anomaly patterns and how they evolve, and it clearly shows the nature of the relative plate motions on either side of a fracture zone, how that motion produces transform faults, and why it is related to earthquakes.

Some other strong points of the exercise are that students can actually operate the model themselves, rather than having to watch someone else do it, they have to cooperate with their peers to make the model work, and they can take the model away and demonstrate the concept to others.