When the elastic limit is reached (point X), if stress continues to accumulate as strain, the rocks will deform plastically, and will not return to their original shape if the stress is released. As stress and strain increase, rocks first experience elastic deformation, and will return to their original shape if the stress is released. Deformation that results in breaking is called brittle deformation.įigure 8.2 | A stress and strain diagram. Deformation that does not involve a rock breaking is called ductile deformation. It may lead to the rocks bending into folds, or if too much strain accumulates, the rocks may fracture. Plastic deformation means that the deformation does not go away when stress is removed. Deformation is elastic until the rocks reach their elastic limit (point X on Figure 8.2), at which point the rock will begin to deform plastically. Initially, as rocks are subjected to increased stress, they behave in an elastic manner, meaning that once the stress is removed, they will return to their original shape (the first part of the curve in Figure 8.2). Source: Randa Harris (2015) CC BY-SA 3.0 view sourceĪpplying stress to a rock can create deformation in that rock, known as strain. This beam is experiencing tensional stress, and rocks have very little strength when exposed to such stress. Why did the Romans use so many vertical columns to hold up the one horizontal beam? If the horizontal beam spanned a long distance without support, it would buckle under its own weight. Rocks can withstand much more compressional stress than tensional stress, as is apparent in some aspects of classical architecture (Figure 8.1).įigure 8.1 | The Roman Forum. Simple shear force is created when rocks move horizontally past each other in opposite directions. Tensional forces operate when rocks pull away from each other. When compressional forces are at work, rocks are pushed together. There are three main types of stress: compression, tension, and shear. If stress is not concentrated at one point in a rock, the rock is less likely to break or bend because of that stress. The stress is more spread out in an athletic shoe. In the high heeled shoe heel, the area is very small, so much stress is concentrated at that point. For example, imagine the stress that is created at the tip of the heel of a high heeled shoe and compare it to the bottom of an athletic shoe. Because stress is a function of area, changing the area over which a force is applied will change the resulting stress. Video showing motion in a strike-slip fault.īends along strike-slip faults create areas of compression or tension between the sliding blocks (see Chapter 2).Rocks change as they experience stress, defined as a force applied to a given area. If the opposing block moves right, it is dextral motion. If the block on the opposing side of the fault moves left relative to the observer’s block, this is called sinistral motion. The direction of the strike-slip movement is determined by an observer standing on a block on one side of the fault. In pure strike-slip motion, fault blocks on either side of the fault do not move up or down relative to each other, rather move laterally, side to side. Strike-slip faults are most commonly associated with transform plate boundaries and are prevalent in transform fracture zones along mid-ocean ridges. Strike-slip faults have side-to-side motion. \): Ketobe Knob in the San Rafael Swell of Utah displays an example of a thrust fault.
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