4 Deforming the Earth s Crust

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1 CHAPTER 7 4 Deforming the Earth s Crust SECTION Plate Tectonics BEFORE YOU READ After you read this section, you should be able to answer these questions: What happens when rock is placed under stress? What are three kinds of faults? How do mountains form? National Science Education Standards ES 1b, 2a What Is Deformation? In the left-hand figure below, the girl is bending the spaghetti slowly and gently. The spaghetti bends, but it doesn t break. In the right-hand figure, the girl is bending the spaghetti quickly and with a lot of force. Some of the pieces of spaghetti have broken. STUDY TIP Learn New Words As you read, underline words that you don t understand. When you learn what they mean, write the words and their definitions in your notebook. How can the same material bend in one situation but break in another? The answer is that the stress on the material is different in each case. Stress is the amount of force per unit area that is placed on an object. DEFORMATION Like the spaghetti, rocks can bend or break under stress. When a rock is placed under stress, it deforms, or changes shape. When a small amount of stress is put on a rock slowly, the rock can bend. However, if the stress is very large or is applied quickly, the rock can break. 1. Describe Circle the picture in which the girl is putting the most force on the spaghetti. READING CHECK 2. Define What is stress? Interactive Textbook 127 Plate Tectonics

2 READING CHECK 3. Explain How do folds indicate that deformation has happened? What Happens When Rock Layers Bend? Folding happens when rock layers bend under stress. Folding causes rock layers to look bent or buckled. The bends are called folds. Most rock layers start out as horizontal layers. Therefore, when scientists see a fold, they know that deformation has happened. TYPES OF FOLDS Three of the most common types of folds are synclines, anticlines, and monoclines. In a syncline, the oldest rocks are found on the outside of the fold. Most synclines are U-shaped. In an anticline, the youngest rocks are found on the outside of the fold. Most anticlines are -shaped. In a monocline, rock layers are folded so that both ends of the fold are horizontal. The figure below shows these kinds of folds. In a syncline, the youngest rocks are on the inside of the fold. In an anticline, the oldest rocks are on the inside of the fold. 4. Identify Color the oldest rock layers in the figure blue. Color the youngest rock layers red. In a monocline, both sides of the fold are horizontal. What Happens When Rock Layers Break? When rock is put under so much stress that it can no longer bend, it may break. The crack that forms when rocks break and move past each other is called a fault. The blocks of rock that are on either side of the fault are called fault blocks. When fault blocks move suddenly, they can cause earthquakes. Interactive Textbook 128 Plate Tectonics

3 HANGING WALL AND FOOTWALL When a fault forms at an angle, one fault block is called the hanging wall and the other is called the footwall. The figure below shows the difference between the hanging wall and the footwall. Fault Hanging wall Footwall 5. Compare How is the hanging wall different from the footwall? The footwall is the fault block that is below the fault. The hanging wall is the fault block that is above the fault. Scientists classify faults by how the fault blocks have moved along the fault. There are three main kinds of faults: normal faults, reverse faults, and strike-slip faults. NORMAL FAULTS In a normal fault, the hanging wall moves down, or the footwall moves up, or both. Normal faults form when rock is under tension. Tension is stress that pulls rock apart. Therefore, normal faults are common along divergent boundaries, where Earth s crust stretches. READING CHECK 6. Explain Why are normal faults common along divergent boundaries? Normal faults form when rocks are pulled apart. Interactive Textbook 129 Plate Tectonics

4 REVERSE FAULTS In a reverse fault, the hanging wall moves up, or the footwall moves down, or both. Reverse faults form when rock is under compression. Compression is stress that pushes rock together. Therefore, reverse faults are common at convergent boundaries, where plates collide. 7. Identify Label the hanging walls and the footwalls on the normal and reverse faults. Reverse faults form when rocks are pushed together. STRIKE-SLIP FAULTS In a strike-slip fault, the fault blocks move past each other horizontally. Strike-slip faults form when rock is under shear stress. Shear stress is stress that pushes different parts of the rock in different directions. Therefore, strike-slip faults are common along transform boundaries, where tectonic plates slide past each other. Strike-slip faults form when rocks slide past each other horizontally. 8. Describe How do strike-slip faults form? It can be easy to tell the difference between faults in a diagram. However, faults in real rocks can be harder to tell apart. The figure on the top of the next page shows an example of a fault. You can probably see where the fault is. How can you figure out what kind of fault it is? One way is to look at the rock layers around the fault. The dark rock layer in the hanging wall is lower than the same layer in the footwall. Therefore, this is a normal fault. Interactive Textbook 130 Plate Tectonics

5 Hanging wall Footwall 9. Explain How can you tell that this is a normal fault? In these rocks, the hanging wall has moved down compared to the footwall. Therefore, this is a normal fault. How Do Mountains Form? As tectonic plates move over Earth s surface, the edges of the plates grind against each other. This produces a lot of stress in Earth s lithosphere. Over very long periods of time, the movements of the plates can form mountains. Mountains can form in three main ways: through folding, faulting, or volcanism. FOLDED MOUNTAINS Folded mountains form when rock layers are squeezed together and pushed upward. Folded mountains usually form at convergent boundaries, where continents collide. For example, the Appalachian Mountains formed hundreds of millions of years ago when North America collided with Europe and Africa. Critical Thinking 10. Apply Concepts Why does it take a very long time for most mountains to form? FAULT-BLOCK MOUNTAINS Fault-block mountains form when tension makes the lithosphere break into many normal faults. Along these faults, pieces of the lithosphere drop down compared with other pieces. This produces fault-block mountains. READING CHECK 11. Identify What kind of stress forms fault-block mountains? Fault-block mountains form when tension causes the crust to break into normal faults. Interactive Textbook 131 Plate Tectonics

6 Say It Investigate Find out more about a volcanic mountain chain, such as the Andes, the Hawaiian islands, or Japan. Share what you learn with a small group. VOLCANIC MOUNTAINS Volcanic mountains form when melted rock erupts onto Earth s surface. Most major volcanic mountains are found at convergent boundaries. 12. Identify What kind of convergent boundary have the Andes mountains formed on? The Andes mountains are examples of volcanic mountains. The Andes have formed where the Nazca plate is subducting beneath the South American plate. Volcanic mountains can form on land or on the ocean floor. Volcanoes on the ocean floor can grow so tall that they rise above the surface of the ocean. These volcanoes form islands, such as the Hawaiian Islands. Most of Earth s active volcanoes are concentrated around the edge of the Pacific Ocean. This area is known as the Ring of Fire. 13. Describe Fill in the table with the features of each kind of mountain. Include where the mountains form and what they are made of. Type of Mountain Folded Fault-block Volcanic Description How Can Rocks Move Vertically? There are two types of vertical movements in the crust: uplift and subsidence. Uplift happens when parts of Earth s crust rise to higher elevations. Rocks that are uplifted may or may not be deformed. Subsidence happens when parts of the crust sink to lower elevations. Unlike some uplifted rocks, rocks that subside do not deform. Interactive Textbook 132 Plate Tectonics

7 CAUSES OF SUBSIDENCE AND UPLIFT Temperature changes can cause uplift and subsidence. Hot rocks are less dense than cold rocks with the same composition. Therefore, as hot rocks cool, they may sink. If cold rocks are heated, they may rise. For example, the crust at mid-ocean ridges is hot. As it moves away from the ridge, it cools and subsides. Old, cold crust far from a ridge has a lower elevation than young, hot crust at the ridge. 14. Explain Why does ocean crust far from a mid-ocean ridge subside? Changes in the weight on the crust can also cause uplift or subsidence. For example, glaciers are huge, heavy bodies of ice. When they form on the crust, they can push the crust down and cause subsidence. If the glaciers melt, the weight on the crust decreases. The crust slowly rises back to its original elevation in a process called rebound. 15. Identify What force caused the crust to subside in the left-hand figure? Fault-block mountains are an example of a third way subsidence can happen. When the crust is under tension, rocks are stretched. They can break and form normal faults. The crust can sink along these faults, causing subsidence. This kind of subsidence is common in rift zones. A rift zone is a set of deep cracks that forms at a divergent boundary. Interactive Textbook 133 Plate Tectonics

8 Section 4 Review NSES ES 1b, 2a SECTION VOCABULARY compression stress that occurs when forces act to squeeze an object fault a break in a body of rock along which one block slides relative to another folding the bending of rock layers due to stress subsidence the sinking of regions of the Earth s crust to lower elevations tension stress that occurs when forces act to stretch an object uplift the rising of regions of the Earth s crust to higher elevations 1. Compare How are folding and faulting similar? How are they different? 2. Describe Fill in the spaces in the table to describe the three main kinds of faults. Kind of fault Description Kind of stress that produces it Normal Hanging wall moves up; footwall moves down. shear stress 3. Explain Why are strike-slip faults common at transform boundaries? 4. Infer Why are fault-block mountains probably uncommon at transform boundaries? 5. Define What is the Ring of Fire? Interactive Textbook 134 Plate Tectonics

9 Earth Science Answer Key continued 2. Magma rises toward the surface at mid-ocean ridges. As the tectonic plates move away from each other, the sea floor spreads apart, and magma fills the gap. Then the magma solidifies. 3. younger 4. When oceanic lithosphere forms, magnetic minerals in the magma align with Earth s magnetic field. They are frozen in place when the magma cools and hardens. As sea-floor spreading continues, that part of the lithosphere moves away from the ridge. If Earth s magnetic field reverses, then minerals will align in the opposite direction in the new lithosphere forming at the ridge. Since lithosphere is produced on both sides of the ridge, the normal and reversed magnetic stripes form parallel patterns. 5. The Earth is not getting bigger, new sea floor is being created, and the oldest sea floor is only 180 million years old. If those three things are true, then oceanic crust must be being destroyed somewhere on Earth at the same rate that it is being produced. SECTION 3 THE THEORY OF PLATE TECTONICS 1. using GPS equipment 2. mid-ocean ridges 3. The lithosphere on each plate is very thick. When the plates collide, the lithosphere is piled up to form tall mountains. 4. mountains, volcanoes 5. when matter carries heat from place to place 6. In slab pull, the driving force comes from subducting slabs. In ridge push, the driving force comes from the formation of new sea floor. Review 1. a theory that describes how tectonic plates move and change shape as part of Earth s outermost layer 2. convergent, divergent, transform 3. slab pull, ridge push, convection 4. a few centimeters per year 5. Mid-ocean ridges and ocean trenches are found only at plate boundaries. Earthquakes and volcanoes are found mainly at plate boundaries. 6. Oceanic lithosphere is colder and denser than continental lithosphere. DEFORMING THE EARTH S CRUST 1. The image on the right should be circled. 2. the amount of force per unit area on an object 3. Most rock layers are horizontal when they form. They can show folded shapes only if they have been deformed. 4. The oldest rock layers should be blue, and the youngest should be red. 5. The hanging wall is found above the fault. 6. A lot of tension is produced at divergent boundaries. 7. The wall above the fault plane should be labeled hanging wall ; the wall below the fault plane should be labeled footwall. 8. when fault blocks slide past each other horizontally 9. The hanging wall has moved down relative to the footwall. 10. Most mountains are the result of plate movements. Since plates move very slowly, mountains form very slowly. 11. tension 12. continent-ocean 13. Type of mountain Folded Fault-block Volcanic Description formed at convergent boundaries; made of folded and crumpled rock formed where the crust is under tension; made of faulted rock formed where volcanoes erupt above Earth s surface; made of igneous rock 14. As it cools, it becomes denser and sinks. 15. the weight of the glacier Review 1. Folding and faulting are both responses to stress. During folding, rocks bend under stress. During faulting, rocks break under stress. 2. Kind of fault Description Kind of stress that produces it Normal Hanging wall moves down; footwall moves up. tension Reverse Strike-slip Hanging wall moves up; footwall moves down. Fault blocks move past each other horizontally. compression shear stress Interactive Textbook Answer Key 42 Earth Science

10 Earth Science Answer Key continued 3. At transform boundaries, tectonic plates move past each other and produce shear stress. 4. Fault-block mountains form because of tensional stress. There is little tensional stress at transform boundaries. 5. the edge of the Pacific Ocean, where many volcanoes are found Chapter 8 Earthquakes SECTION 1 WHAT ARE EARTHQUAKES? 1. A fault is a break in the crust that rocks slide along. 2. The plate boundaries should be drawn along the areas with large numbers of earthquakes. 3. The rocks are folded. 4. The movement of the plates causes pressure to build up. When the pressure is released, an earthquake occurs. 5. a place where two tectonic plates are moving apart 6. The fault labels should be located at the places where the blocks slide past each other. Star should be somewhere along one of the faults. 7. A lot of pressure builds up before the rock breaks. 8. Arrows should be perpendicular to the plate boundary, pointing toward each other. 9. Arrows should be parallel to the fault, pointing in opposite directions; arrows should show the direction of displacement of each fault block. 10. Most earthquakes happen at plate boundaries. 11. body waves and surface waves 12. surface waves 13. P waves move particles back and forth, and S waves move them side to side. 14. Possible answer: All of their energy is released at the surface. Review 1. Possible answers: An earthquake is a shaking or movement of the Earth, and a fault is a crack in the crust that rocks can slide on; earthquakes happen along faults. 2. Most earthquakes happen at tectonic plate boundaries. 3. Possible answers: Earthquakes happen when pressure builds up on a rock and it breaks; elastic rebound of rocks causes earthquakes. 4. Body waves travel through the Earth s interior, but surface waves travel only on its surface. 5. The strength of an earthquake is directly related to the amount of pressure that builds up on the rock before it breaks. Some rocks are stronger than others, so more pressure builds up before the rock breaks. When a lot of pressure builds up, larger earthquakes happen. 6. Almost all earthquakes happen when rock breaks. The rock in the crust is brittle and breaks in response to stress. Rock in the mantle flows in response to stress. SECTION 2 EARTHQUAKE MEASUREMENT 1. The focus is the place where the rock breaks, and the epicenter is the point on the surface directly above the focus. 2. The epicenter should be marked directly above the focus, on the surface. 3. about 15 s 4. about 8,500 km 5. There should be a star at the place where all three circles intersect. 6. Possible answer: Seismologists look at the relative sizes of the different waves. 7. A magnitude 5.0 earthquake is 10 times stronger than a magnitude 4.0. A magnitude 6.0 is 10 times stronger than a 5.0. So, 10 times 10 makes it 100 times stronger. Review 1. A seismograph is an instrument that measures ground movements. A seismogram is a tracing of ground movements, which is produced by a seismograph. 2. San Francisco is more likely to be an epicenter because it is near a plate boundary. 3. A time vs. distance graph can be used to measure how far away the epicenter is from different seismograph stations. Those distances can be used to make circles that overlap at the epicenter. 4. One seismogram tells you how far away the epicenter was and how strong the earthquake was. It doesn t tell you which direction the waves came from. 5. Magnitude is the strength of an earthquake, but intensity is how much damage it causes. A moderate earthquake can cause a lot of damage if it happens in a place with weak soil or non-earthquake-resistant buildings. Interactive Textbook Answer Key 43 Earth Science