![]() FaultingĪ fault is a boundary between two bodies of rock along which there has been relative motion (e.g., Figure 13.23). Figure 13.22 Joints developing to accommodate the larger horizontal component of compression (large red arrows). The joints accommodate the larger compression stress (larger red arrows) by allowing the rock to stretch in the up-down direction (along the green arrows). Joints can also develop in a rock a rock under compression as a way to accommodate the change in shape (Figure 13.22). Figure 13.21 Joints developed in the hinge zone of folded rocks. Joints can develop where rocks are being folded, because the hinge zone of the fold is under tension as it stretches to accommodate the bending (Figure 13.21). Nevertheless, it is possible for joints to develop where the overall regime is one of compression. Figure 13.20 Half Dome at Yosemite National Park is an exposed granite batholith that displays exfoliation joints, causing sheets of rock to break off. Exfoliation joints, which make the rock appear to be flaking off in sheets (Figure 13.20), occur when a body of rock expands in response to reduced pressure, such as when overlying rocks have been removed by erosion. It can also be from a body of rock expanding. The tension can be from a rock contracting, such as during the cooling of volcanic rock (Figure 13.9, upper left). Most joints form when the overall stress regime is one of tension (pulling apart) rather than compression. Figure 13.19 Joint sets have broken these siltstone and shale beds into long rectangular planks. ![]() Joints with a common orientation make up a joint set (Figure 13.19). If there is no movement of one side relative to the other, and if there are many other fractures with the same orientation, then the fractures are called joints. If rocks on one side of the break shift relative to rocks on the other side, then the fracture is a fault. When rocks break in response to stress, the resulting break is called a fracture. ![]()
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