Unit 9 Lab Crustal Deformation Fall 23 R1 sh.pdf

UNIT NINE: CRUSTAL DEFORMATION Randa Harris INTRODUCTION The Earth is an active planet shaped by dynamic forces. Such forces can build mountains and crumple and fold rocks. As rocks respond to these forces, they undergo deformation, which results in changes in shape and/or volume of the rocks. The resulting features are termed geologic structures. This deformation can produce dramatic and beautiful scenery, as evidenced in Figure 1, which shows the deformation of originally flat (horizontal) rock layers. Figure 1. Rocks that have been deformed along the coast of Italy. Why is it important to study deformation within the crust? Such studies can provide us with a record of the past and the forces that operated then. The correct interpretation of features created during deformation is critical in the petroleum and mining industry. It is also essential for engineering. Understanding the behavior of deformed rocks is necessary to create and maintain safe engineering structures. When proper geological planning is not considered in engineering, disasters can strike. For example, the Vajont Dam was constructed at Monte Toc,

Italy in the early 1960’s. The place was a poor choice for a dam, as the valley was narrow, thorough geological tests were not performed, and the area surrounding the dam was prone to large landslides. In 1963, a massive landslide in the area displaced much of the water in the dam, causing it to override the top of the dam and flood the many villages downstream, resulting in the deaths of almost 2,000 people (Figure 2). Figure 2. An image of the Vajont reservoir shortly after the massive landslide (you can see the scar from the landslide on the right, and the dam is located in the foreground on the left). Learning Outcomes After completing this chapter, you should be able to: • Understand the different types of stress that rocks undergo, and their responses to stress • Demonstrate an understanding of the concepts of strike and dip • Recognize the different types of folds and faults, and the forces that create them • Use block diagrams to display geologic features • Create a geologic cross-section

Key Terms: Stress Strain Compressional forces Tensional forces Shear forces Contact Strike Dip Monocline Anticline Syncline Dome Basin Normal fault Reverse fault Strike-slip fault Horst & graben STRESS AND STRAIN Rocks change as they undergo stress , which is just a force applied to a given area. Since stress is a function of area, changing the area to which stress is applied makes a difference. For example, imagine the stress that is created both at the tip of high heeled shoes and the bottom of athletic shoes. In the high heeled shoe, the area is very small, so that stress is concentrated at that point, while the stress is more spread out in an athletic shoe. Rocks are better able to handle stress that is not concentrated in one point. There are three main types of stress: compression, tension, and shear. When compressional forces are at work, rocks are pushed together. Tensional forces operate when rocks pull away from each other. Shear forces are created when rocks move horizontally past each other in opposite directions. Rocks can withstand compressional stress more than tensional stress (see Figure 3).

Figure 3. This is a picture of the Roman Forum. 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. This beam is under tensional stress, so it is not as strong. Applying stress creates a deformation of the rock, also known as strain . As rocks are subjected to increased stress and strain, they at first behave in an elastic manner, which means they return to their original shape after deformation (Figure 4). This elastic behavior continues until rocks reach their elastic limit (point X on Figure 4), at which point plastic deformation commences. The rocks may bend into folds, or behave in a brittle manner by fracturing (brittle behavior can be easily envisioned if you think of a hammer hitting glass), but regardless they do not return to their original shape when the stress is removed in plastic deformation. The resulting deformation from applied stress depends on many factors, including the type of stress, the type of rock, the depth of the rock and pressure and temperature conditions, and the length of time the rock endures the stress. Rocks behave very differently at depth than at the surface. Rocks tend to deform in a more plastic manner at depth, and in a more brittle manner near the Earth’s surface.

Figure 4. A stress and strain diagram. As stress and strain increase, rocks first experience elastic deformation (and can return to their original shape) until the elastic limit is reached, depicted at point X. After this point, rocks will fracture or experience plastic deformation, so that their original shape is destroyed. STRIKE AND DIP To learn many of the concepts associated with structural geology, it is useful to use block diagrams. As you examine these blocks, note the different ways that you can view them. If you look at a block from along the side, you are seeing the cross-section view (like what you see along roads that have been cut through the mountains). If you look at the block from directly above it, you are looking at map view (Figure 5).

Figure 5. You are viewing the top block in this image in map view, viewed from directly above the block. The lower block is from the same rock layers, and you are viewing it in cross-section (or from the side). Note that in cross-section, you can see how the rock layers are tilted. As you think about how rocks have changed through the process of deformation, it is useful to remember how they deposited in the first place. We will briefly review some of the geologic laws that you learned earlier in the course. Sedimentary rocks, under the influence of gravity, will deposit in horizontal layers (Law of Original Horizontality). The oldest rocks will be on the bottom (because they had to be there first for the others to deposit on top of them), and are numbered with the oldest being #1 (Law of Superposition). The wooden block in Figure 6 displays how sedimentary rocks originally deposit. Figure 6. In the image above, different rock types are given different colors. The oldest rock, on the bottom, is given a #1. The youngest rock in this image is #4.

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