Its not your fault; its not my fault! Transform faults are part of our planet's geology. We need to understand them for our own survival, as they are the source of innumerable, destructive earthquakes. Transform faults are long and relatively continuous where they cut across continents, but tend to appear as short discontinuous segments offsetting sections of spreading ridges on the ocean floor. The transform fault is simply a fault connecting two other kinds of active plate boundaries, but that is a deceptively simple definition. Follow the link about paper models of transform faults. Do that now, please, and print out the model. (The web address is http://web.mala.bc.ca/earle/transform-model/ if you have trouble with the link.) Directions for printing the model are contained in that website. Then, return here:
Welcome back. Now, assemble your model from the directions given. Your model is very similar to small portions of the Mid-Atlantic Ridge between South America and Africa.
Begin with the model in its closed position. The two points marked A and B should be juxtaposed (what does "juxtaposed" mean?), and the gaps at the two spreading ridges should be closed tightly. Now open the model. Note how new plate material is "created" at each spreading ridge and an equal area is added to each of plate A (the shaded plate) and plate B (unshaded). Also, notice that after spreading has occurred, the two ridges are no farther apart than before spreading. Verify this by measuring distance DD' with the model closed and distance EE' with the model open.
The model illustrates the fact that the shape of an oceanic ridge does not change with time.
Now turn your attention to the fault itself. With the model closed, imagine yourself standing on point A, looking across the fault to the juxtaposed point A'. Slowly open the model, watching how point A' moves as seen from point A. If you are standing at point A, you will see point A' move to your right.
Turn the model around and repeat the process. If you are standing at point A', you will see point A moving to your right. Because of this independence of where you happen to stand, this particular transform fault is said to display right-lateral motion. That is, the motion is side-to-side and the other side of the fault always moves to your right.
Transform faults may also be left-lateral, if the ridges are offset in the opposite sense. To see this, turn the model page over to the other side and trace the positions of points A and A' on the reverse side of the paper. Holding the closed model so that you are looking at its reverse side, open it and note the motions of points A and A'. If you are standing at point A, you will see point A' move to your left.
The combination of ridge segments and transform faults forms a rectilinear zigzag pattern for oceanic plate boundaries that may be seen clearly in Figure 3-3 (on the website). It is still not clear just how this zigzag pattern is formed initially, but because the pattern does not generally change shape with time, it must have come into existence at about the same time as the ridges themselves. The process by which this happens is still not fully understood.
Where transform faults cut across continents, however, they tend to be long and relatively continuous, with few, if any, spreading segments. The best known and most studied example of a continental transform fault is California's San Andreas.
Now look at the satellite photo below:
It shows land and ocean floor to the west of the San Andreas Fault. This area is part of the Pacific Plate and is moving to the northwest, parallel to the fault. To the east of the fault is the North American Plate. Where the San Andreas Fault crosses the North American continent it is long and unbroken, but where it goes out to sea, it is cut into shorter segments separated by spreading ridges. Some of the ridge segments themselves are quite short, as in the Gulf of California.
Note that the ridge and fault geometry is similar to that of your paper model, for which right-lateral motion is expected along the fault. This is in fact what is observed along the length of the San Andreas. It is a classic transform fault, where the Pacific Plate is sliding past the North American Plate, carrying Los Angeles and Baja California along with it.
Along the fault, the rocky edges of the plates grind against one another. In a few places, the slippage occurs smoothly. Here, any structure such as a fence or road that crosses the fault is offset at a rate of up to six centimeters (2-1/2 inches) per year. But in other places, the fault is jammed and does not move steadily. As the plates continue their inexorable motion, the forces exerted on the pinned fault build up with each passing year. Finally the rock can stand no more and it breaks, unleashing the pent-up energy as strong vibrations of the ground: an earthquake.
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