Mountains, like those shown in Figure 1, provide some of the most

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1 Section Rock Deformation 1 FOCUS Section Objectives 11.1 Identify the factors that determine the strength of a rock and how it will deform Explain how rocks permanently deform Distinguish among the types of stresses that affect rocks List the three main types of folds and identify the main types of faults. Key Concepts What determines the strength of a rock? What are the types of stresses that affect rocks? What are the three main types of folds? What are the main types of faults? Vocabulary deformation stress strain anticline syncline monocline normal fault reverse fault thrust fault strike-slip fault Reading Strategy Comparing and Contrasting After you read the section, compare types of faults by completing the table below. Types of Fault Description Normal fault a.? b.? c.? d.? e.? f.? g.? Reading Focus Build Vocabulary Word Parts The terms anticline and syncline have the same Greek root word, klinein, meaning to lean. Anti- means opposite or against. In an anticline, the layers bend downward in opposite directions from the crest. Syn- is a Greek prefix meaning together with, so a syncline has layers that dip toward each other. Reading Strategy a. hanging wall block moves down relative to footwall block; high angle fault b. Reverse fault c. hanging wall block moves up relative to footwall block; high angle fault d. Thrust fault e. hanging wall block moves up and over the footwall block; low angle fault f. Strike-slip fault g. movement is horizontal and parallel to the trend of the fault surface; usually consists of a zone of roughly parallel fractures 2 INSTRUCT Factors Affecting Deformation Integrate Physics Force Remind students that force is defined as a push or pull exerted on an object. A force has magnitude; for example, you can push hard or gently on an object. Force also has direction; you can push on an object to the left, to the right, up, or down. Logical Figure 1 Mountain Ranges This peak is part of the Karakoram Range in Pakistan. 308 Chapter 11 Mountains, like those shown in Figure 1, provide some of the most spectacular scenery on our planet. It is theorized that all continents were once mountainous masses and grow by the addition of mountains to their edges. As geologists unravel the secrets of mountain formation, they also gain a deeper understanding of the evolution of Earth s continents. However, if continents do grow by adding mountains to their edges, then how do mountains exist in the interior of continents? Factors Affecting Deformation Every body of rock, no matter how strong, has a point at which it will bend or break. Deformation is a general term that refers to all changes in the original shape and/or size of a rock body. Most crustal deformation occurs along plate margins. Plate motions and interactions at plate boundaries create forces that cause rock to deform. Stress is the force per unit area acting on a solid. When rocks are under stresses greater than their own strength, they begin to deform, usually by folding, flowing, or fracturing. The change in shape or volume of a body of rock as a result of stress is called strain. How can rock masses be bent into folds without being broken? When stress is gradually applied, rocks first respond by deforming elastically. Changes that result from elastic deformation are recoverable. Like a rubber band, the rock will return to almost its original size and shape once the force is removed. Once the elastic limit or strength of a rock is surpassed, it either flows or fractures. The factors that influence the strength of a rock and how it will deform include temperature, confining pressure, rock type, and time. 308 Chapter 11

2 Temperature and Pressure Rocks deform permanently in two ways: brittle deformation and ductile deformation. Rocks near the surface, where temperatures and confining pressures are low, usually behave like brittle solids and fracture once their strength is exceeded. This type of deformation is called brittle failure or brittle deformation. You know that glass objects, wooden pencils, china plates, and even our bones show brittle failure once their strength is exceeded. At depth, where temperatures and confining pressures are high, rocks show ductile behavior. Ductile deformation is a type of solidstate flow that produces a change in the size and shape of an object without fracturing the object. Objects that display ductile behavior include modeling clay, bee s wax, caramel candy, and most metals. For example, a copper penny placed on a railroad track will be flattened and deformed without breaking by the force applied by a passing train. Ductile deformation of a rock that is strongly aided by high temperature and high confining pressure is somewhat similar to the deformation of a penny flattened by a train. Rock Type The mineral composition and texture of a rock also greatly affect how it will deform. Rocks like granite and basalt that are composed of minerals with strong internal molecular bonds usually fail by brittle fracture. Sedimentary rocks that are weakly cemented or metamorphic rocks that contain zones of weakness such as foliation are more likely to deform by ductile flow. Rocks that are weak and most likely behave in a ductile manner when under force include rock salt, gypsum, and shale. Limestone, schist, and marble are of intermediate strength and may also behave in a ductile manner. Time In nature small stresses applied over long time spans play an important role in the deformation of rock.you can see the effects of time on deformation in everyday settings. For example, marble benches have been known to sag under their own weight over a span of a hundred years or so. Forces that are unable to deform rock when first applied may cause rock to flow if the force is maintained over a long period of time. What is brittle deformation? Types of Stress Rocks are exposed to many different forces due to plate motions. The three types of stresses that rocks commonly undergo are tensional stress, compressional stress, and shear stress. Look at Figure 2. When rocks are squeezed or shortened the stress is compressional. Tensional stress is caused by rocks being pulled in opposite directions. Shear stress causes a body of rock to be distorted. Customize for Inclusion Students Figure 2 Undeformed material is changed as it undergoes different types of stress. The arrows show the direction of maximum stress. A Compressional stress causes a material to shorten. B Tensional stress causes a material to be stretched or to undergo extension. C Shear stress causes a material to be distorted with no change in volume. Types of Stress B Unstressed Compressional stress D A Tensional stress C Shear stress Mountain Building 309 Build Reading Literacy Refer to p. 306D, which provides the guidelines for KWL (Know-Want to Know-Learned). KWL Have students make a KWL chart containing three columns entitled What I Know, What I Want to Know, and What I Learned. Before reading this section, have students fill in the first column with what they know about the factors affecting deformation of rocks. They should have prior knowledge about rocks from Chapters 2 and 3. The second column should be filled out as students read pp. 308 and 309. Finally, students should fill in the third column after they have finished reading this section. The material in this column can take the form of an outline of the material under the head Factors Affecting Deformation. Verbal, Logical Types of Stress Figure 2 Have students examine the diagrams. Ask: What happens to the shape of a rock that undergoes compressional stress? (It shortens.) What happens to the shape of a rock that undergoes tensional stress? (It stretches or extends.) What happens to the shape of a rock that undergoes shear stress? (The rock becomes distorted.) Visual Build Science Skills Using Models Have students model the three kinds of stress by pulling or pushing on a marshmallow or piece of foam rubber. The material can be cut in half to model shear stress. Students will be able to observe how shape changes with tensional and compressional stress and how the shape becomes distorted with shear stress. Visual, Kinesthetic Visually Impaired Many of the concepts in this section can be modeled for visually impaired students. Faults can be represented by using blocks of wood. Folds can be modeled using construction paper or sheets of flexible foam rubber. Types of stress can be modeled using marshmallows or foam rubber, as in the Build Science Skills activity on this page. Rock deformation can be illustrated by using a stick of chewing gum. When a stick of gum is cold, an applied stress causes it to crack and break. When the gum is warm, it will bend easily. Answer to... Brittle deformation is deformation where the strength of a material is exceeded, and the material breaks or fractures. Mountain Building 309

3 Section 11.1 (continued) Folds Figure 3 Have students examine the diagram of the types of folds. Ask: Where would you expect to find the oldest rock layer in an anticline? (in the center of the fold) Where would you expect to find the oldest rock layer in a syncline? (on the outside of the fold) How would you describe a symmetrical fold? (a fold in which the two sides are mirror images) How would you describe an overturned fold? (a fold in which one limb is tilted beyond the vertical) Folds During mountain building, flat-lying sedimentary and volcanic rocks are often bent into a series of wavelike ripples called folds. Folds in sedimentary strata are much like those that would form if you were to hold the ends of a sheet of paper and then push them together. In nature, folds come in a wide variety of sizes and shapes. The three main types of folds are anticlines, synclines, and monoclines. Anticlines The two most common types of folds are anticlines and synclines. An anticline is most commonly formed by the upfolding, or arching, of rock layers, as shown in Figure 3. Synclines Often found in association with anticlines are downfolds, or troughs, called synclines. Notice in Figure 3 that the limb of an anticline is also a limb of the adjacent syncline. Folds do not continue forever. Instead their ends die out much like the wrinkles in cloth. Making an Anticline Purpose Students observe how an anticline is produced. Materials stack of construction paper sheets in several colors Procedure Make a stack containing several sheets of colored construction paper. Each colored sheet will represent a rock layer. Lay the stack on a table. Place your two hands on the stack, one at each end of the stack. Press down on the paper with your hands and slowly push them together. Expected Outcomes The entire stack of paper will form an anticline. Rock layers will be visible when viewed from the side. Kinesthetic, Visual Anticline Syncline Anticline Syncline Normal limb Symmetrical fold Asymmetrical fold Overturned fold Figure 3 Anticlines and Synclines The upfolded or arched structures are anticlines. The downfolds or troughs are synclines. Notice that the limb of an anticline is also the limb of the adjacent syncline. Monoclines Although we will discuss folds and faults separately, in the real world folds are generally closely associated with faults. Examples of this close association are broad, regional features called monoclines. Monoclines are large, step-like folds in otherwise horizontal sedimentary strata. Monoclines seem to occur as sedimentary layers have been folded over a large faulted block of underlying rock. Monoclines are prominent features of the Colorado Plateau area in Colorado, New Mexico, Utah, and Arizona, as shown in Figure 4 on the next page. What is a syncline? Overturned limb Students may have trouble remembering the direction in which an anticline and a syncline fold. The terms were named according to the direction of the limbs in relation to the axis of the fold. Remind students that anti- means opposite or against. In an anticline, the layers bend downward in opposite directions from the crest. The prefix synmeans together with, so a syncline has layers that dip toward each other. Students can remember the direction of these folds if they think of an ant climbing up a hill. The word anticline contains the word ant. 310 Chapter 11 Facts and Figures The Colorado Plateau is a high, sparsely vegetated region of deep canyons, mesas, and plateaus. It encompasses almost 363,000 sq km at the four corners the area where Utah, Colorado, Arizona, and New Mexico come together. The Colorado Plateau includes the Colorado River and its tributaries. The high Sierra Nevada mountain range located to the west of the Plateau prevents moisture-laden air masses from reaching the region. This rain shadow effect causes the region to be very dry; the average precipitation is about 25 cm per year. Because plant cover is so sparse, the area has been eroded by fast-moving streams, which has exposed the bare rocks that contribute to the area s beauty. 310 Chapter 11

4 A Recall that faults are fractures in the crust along which movement has taken place. Small faults can be recognized in road cuts where sedimentary beds have been offset a few meters, as shown in Figure 5. Faults of this size usually occur as single breaks. By contrast, large faults, like the San Andreas fault in California, have displacements of hundreds of kilometers and consist of many interconnecting fault surfaces. These fault zones can be many kilometers wide and are often easier to identify from high-altitude photographs than at ground level. The rock surface that is immediately above the fault is commonly called the hanging wall, and the rock surface below the fault is called the footwall. The major types of faults are normal faults, reverse faults, thrust faults, and strike-slip faults. Normal Faults A normal fault occurs when the hanging wall block moves down relative to the footwall block. Most normal faults have steep dips of about 60, as shown in Figure 6A on the next page. These dips often flatten out with depth. The movement in normal faults is mainly in a vertical direction, with some horizontal movement. Because of the downward motion of the hanging wall block, normal faults result in the lengthening, or extension, of the crust. Figure 4 Monocline A Monocline located near Mexican Hat, Utah. B This monocline consists of bent sedimentary beds that were deformed by faulting in the bedrock below. Figure 5 Normal Fault Faulting caused the vertical displacement of these beds located near Kanab, Utah. Arrows show the relative motion of rock units. Observing Which side of the fault is the hanging wall? B Build Science Skills Using Models Have students model the formation of a monocline by holding two wooden blocks together so the fault between them runs at 60 angle. Have another student carefully drape the towel or fabric over the wooden blocks. The student holding the blocks should slide them along the fault until the towel forms a fold. Ask: What does the towel represent? (sedimentary rock layers) What do the wooden blocks represent? (rock blocks) Kinesthetic, Logical Figure 5 Have students examine the photograph of a normal fault. Ask: Which side of the fault is the footwall? (the right side) How can you tell which side is the footwall? (A normal fault occurs when the hanging wall block moves down relative to the footwall. In the picture, the right side is higher than the left side, so it must be the footwall.) Can you tell from the photo which side of the fault has moved? (No, there is no way to tell. The two sides of the fault have moved relative to each other but it is impossible to know whether one side has moved up or the other side has moved down.) Mountain Building 311 Facts and Figures The terms hanging wall and footwall were coined by prospectors and miners who excavated shafts and tunnels along fault zones because these are frequent sites of ore deposits. In these tunnels, the miners would walk on the rocks below the mineralized fault zone (the footwall) and hang their lanterns on the rocks above (the hanging wall). Answer to... Figure 5 The hanging wall is on the left side. A syncline is a down fold or trough. Mountain Building 311

5 Section 11.1 (continued) Figure 6 Have students examine the diagrams. Tell them to imagine two houses located side by side. Ask: Will the houses be closer together or farther apart if a normal fault forms between them? Why? (They will be farther apart because normal faults are tensional.) Will the houses be closer together or farther apart if a reverse fault forms between them? Why? (They will be closer together because reverse faults are compressional.) Will the houses be closer together or farther apart if a thrust fault forms between them? Why? (They ll be closer together because thrust faults are compressional.) What will be the relative location of the houses if a strike-slip fault forms between them? (The houses will move farther apart, but they will no longer be side by side. One house will be somewhat behind the other.) Build Reading Literacy Refer to p. 246D in Chapter 9, which provides the guidelines for relating cause and effect. Relate Cause and Effect Ask: What type of faulting would you expect to find in an area where continental plates are diverging? (a normal fault) Why would you find this type of fault? (When continental plates diverge, the crust is pulled apart. This creates tensional forces that stretch or expand the crust.) What type of faulting would you expect to find in an area where plates are subducting or colliding? Why? (a reverse fault or a thrust fault because plate subduction and collision cause compressional forces, which cause the crust to shorten) Logical Four Types of Faults A C Footwall Normal fault (tensional) Hanging wall Q How do you determine which side of a fault has moved? A For the fault shown in Figure 5, did the left side move down, or did the right side move up? Since the surface at the top of the photo has been eroded flat, either side could have moved, or both sides could have moved, with one side moving more than the other. That s why geologists talk about relative motion across faults. In this case, the left side moved down relative to the right side, and the right side moved up relative to the left side. 312 Chapter 11 Footwall Hanging wall Thrust fault (compressional) Figure 6 A Normal fault B Reverse fault C Thrust fault D Strike-slip fault Interpreting Diagrams Which type of fault would cause extension in an area? B D Hanging wall Footwall Reverse fault (compressional) Strike-slip fault (shear) Reverse Faults and Thrust Faults A reverse fault is a fault in which the hanging wall block moves up relative to the footwall block. Reverse faults are high-angle faults with dips greater than 45. Thrust faults are reverse faults with dips of less than 45. Because the hanging wall block moves up and over the footwall block, reverse and thrust faults result in a shortening of the crust, as shown in Figure 6B and 6C. Most high-angle reverse faults are small. They cause only local displacements in regions dominated by other types of faulting. Thrust faults, on the other hand, exist at all scales. In mountainous regions such as the Alps, northern Rockies, Himalayas, and Appalachians, thrust faults have displaced rock layers as far as 50 kilometers over adjacent rocks. The result of this large-scale movement is that older rocks end up on top of younger rocks. Normal faults occur due to tensional stresses, and reverse and thrust faults result from compressional stresses. Compressional forces generally produce folds as well as faults. These compressional forces result in a thickening and shortening of the rocks. What are the major types of faults? 312 Chapter 11

6 Strike-Slip Faults Faults in which the movement is horizontal and parallel to the trend, or strike, of the fault surface are called strike-slip faults, as shown in Figure 6D. Because of their large size and linear nature, many strike-slip faults produce a trace that is visible over a great distance. Rather than a single fracture, large strike-slip faults usually consist of a zone of roughly parallel fractures. The zone may be up to several kilometers wide. The most recent movement, however, is often along a section only a few meters wide, which may offset features such as stream channels. Crushed and broken rocks produced during faulting are more easily eroded, often producing linear valleys or troughs that mark the locations of strikeslip faults. Scientific records of strike-slip faulting were made following surface ruptures that produced large earthquakes. Strike-slip faults are commonly caused by shear stress. The San Andreas fault in California and the Great Glen fault in Scotland are well-known examples of strike-slip faults. Joints Among the most common rock structures are fractures called joints. Unlike faults, joints are fractures along which no appreciable movement has occurred. Although some joints have a random orientation, most occur in roughly parallel groups, as shown in Figure 7. Joints usually form as the result of large-scale regional stresses. Section 11.1 Assessment Reviewing Concepts 1. What factors determine the strength of a rock? 2. In what ways do rocks deform? Explain the differences in these deformations. 3. Describe the different types of stress. 4. List the three types of folds. 5. Explain the direction of movement in the four types of faults. Critical Thinking 6. Inferring What type of deformation would a rock in the lower part of the mantle be more likely to undergo? Explain. Figure 7 Joints These joints are found in Arches National Park, near Moab, Utah. The joints in the sandstone stand out because chemical weathering is enhanced along them. 7. Comparing and Contrasting How is an anticline different from a syncline? 8. Applying Concepts What type of faults should be most common at a spreading ocean ridge? Explain. Compressional Stress Review the types of plate boundaries in Chapter 9. At which type of boundary would compressional stresses be the dominant force? It is commonly believed that California is in danger of sliding into the Pacific Ocean due to the movement of the San Andreas Fault. Have students examine the map on p. 325 and note the general direction of the fault. (northwest) Remind them that the San Andreas Fault is a strike-slip fault. Ask: Is it possible that the San Andreas Fault will cause California to slide into the ocean? Explain. (No, the movement in strike-slip faults is horizontal and parallel to the trend, so the southern California coast is moving north, not west.) 3 ASSESS Evaluate Understanding To assess students knowledge of the section content, ask: How does a joint differ from a fault? (A fault is a fracture along which movement has occurred. A joint is a fracture along which no movement has occurred.) How does a reverse fault differ from a thrust fault? (A thrust fault is a reverse fault with a dip of less than 45. A reverse fault usually has a dip greater than 45.) Reteach Have students make a table listing the four factors that influence the strength of a rock. For each factor, students should write a brief description of how that factor influences rock deformation. Compressional forces are dominant at convergent plate boundaries. Section 11.1 Assessment 1. temperature, confining pressure, rock type, and time 2. brittle deformation which causes an object to fracture, and ductile deformation, which changes the shape and size of the object without fracturing it 3. Compressional stress is a force that compresses, shortens, or squeezes a rock. Tensional stress is a force that pulls a rock apart, causing extension or lengthening of Mountain Building 313 the rock. Shear stress is a force that causes a rock body to be distorted. 4. anticlines, synclines, and monoclines 5. In normal faults, the hanging wall moves down relative to the footwall and the movement is mainly vertical. In reverse and thrust faults, the hanging wall moves up relative to the footwall. In reverse faults the movement is mainly vertical; in thrust faults, the movement is mainly horizontal. Strike-slip faults move parallel to the trend of the fault, and the movement is mainly horizontal. Answer to... Figure 6 A normal fault causes extension or stretching in an area. normal, reverse, thrust, and strike-slip faults 6. ductile deformation because of high temperatures and pressures in the lower mantle 7. Anticline: Rock layers are folded upwards to form an arch. Syncline: Rock layers are folded downwards to form a trough. 8. Normal faults; the plates at an ocean ridge are being pulled apart or are undergoing extension, which results in normal faults. Mountain Building 313