Going to the freezer for a midnight snack, I pulled out a pizza whose form is portrayed in the geologic map (Fig. 1a). During mapping, I arbitrarily oriented the pizza with respect to north. The pizza, as shown, is arbitrarily oriented the pizza with respect to north. In most respects this was a normal pizza. A thin stratum of cheese rested atop tomato sauce and crust. And pepperoni, lightly dusted with cheese, was distributed across the face of the pizza. The diameter of the pizza was 23 cm. Topographic relief of the pizza, as revealed in cross-sectional profile was merely 13 mm (Fig. 1b). My delight as a structural geologist came upon observing that two pepperoni-sized circular depressions existed near the edge of the pizza in the northeast and northwest quadrants. There was not even a trace of cheese in these depressions, let alone pepperoni. Furthermore, opposite these depressions, in the southeast and southwest quadrants, two pieces of pepperoni overlapped in low-angle (overthrust) fault contact. The cross-section of Figure 1.9B clarifies this relationship.
|Fig. 1 - Geological map (a) and structure profile of a medium-sized pepperoni pizza (b). Kinematic model of the translation and rotation of the pepperoni (c).|
The physical and geometric properties of the pizza demanded the following interpretation of the deformation (Fig. 1c): somehow the pepperoni that had once occupied the circular depressions now devoid of cheese had moved 12.9 cm and 9.4 cm, respectively, to their present locations. The path of movement for each pepperoni was determined by measuring the orientation of a line connecting the exact center of the circular depression and the exact center of the closest faulted pepperoni. Orientations for pepperoni 1 and 2 were S5°E and S4°W, respectively. Careful matching of outlines of each of the faulted pepperoni with each of the outlines of the circular depressions revealed that pepperoni 1 underwent about 15° of counterclockwise rotation during translation and pepperoni 2 underwent about 5° of clockwise rotation during its translation. No microscopic study of the frozen sauce or cheese was undertaken to evaluate internal distortion due to the translation.
What can be concluded about the dynamics of origin of this structural system? By far, this is the most speculative part of analysis, but the most enjoyable. (You will note that the language used in interpretation is always cautious). First, it would seem that the force(s) that triggered movement of the pepperoni did not violate the general integrity of the pizza. The box was not crushed in any way, nor was the crust distorted beyond primary kneading and shaping. The probable force causing deformation was gravity. Gravitational
forces would not have triggered movement of the two pepperoni, however, unless the pizza had been tilted at some stage in its history.
My working model is that the manufacturer, after preparing the pizzas, chose not to stack the boxed pizzas horizontally in freezer compartments. The manufacturer may have concluded, perhaps on the basis of experimentation, that tall stacking of pizza-filled boxes might have the adverse affect of flattening cardboard to cheese and tomato sauce before freezing set in. Instead, the pizzas may have been filed vertically. If stacked vertically while cheese and tomato sauce were yet warm and/or moist, the pepperoni, under the influence of gravitational forces, might have been vulnerable to translation along the low-viscosity tomato sauce discontinuity. Each of the pepperoni rounds would have ceased moving when it encountered the frictional resistance of another one. What was not clear to me then, nor is it now, is the rate at which the pepperoni moved: was it rapid or sluggish? The magnitude of the stresses required to initiate movement is also a puzzle. In fact, interpreting the strength of the various materials as a function of temperature would constitute a major study in itself.
My working model is, of course, only one interpretation. Maybe the structural event that dislodged the pepperoni was of an entirely different nature. Maybe my interpretation is correct except for timing: after all, the pizza, stacked vertically, may have been undeformed and solidly frozen… until the power failure. Interpreting the timing of structural events is often very difficult.
From: Davis & Reynolds, 1984 – Structural Geology of rocks and regions. 2nd edition, John Wiley & Sons, Inc. Eds.