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Name Class CHAPTER 11 Date Deformation of the Crust 1 How Rock Deforms SECTION KEY IDEAS As you read this section, keep these questions in mind: • What is isostasy? • What are the three main types of stress? • How are folds and faults alike and different? What Is Isostasy? Mountain ranges are proof that the shape of Earth’s surface is constantly changing. These changes are caused by deformation. Deformation is the bending and breaking of Earth’s crust. Deformation can happen when the weight of some part of Earth’s crust changes. If the lithosphere becomes thicker and heavier, it sinks down into the asthenosphere. If the lithosphere becomes thinner and lighter, it rises in the asthenosphere. The vertical movement of the lithosphere depends on two opposing forces. One is the force of gravity, which pulls the lithosphere down. The other is the buoyant force of the asthenosphere, which pushes up on the lithosphere. When these two forces are balanced, the lithosphere and asthenosphere are in a state of isostasy. When the weight of the lithosphere changes, the lithosphere sinks or rises until the forces balance again. The movement of the lithosphere puts forces on the rock in it. These forces can cause deformation. A Gravitational force B Original elevation READING TOOLBOX Summarize As you read this section, underline sentences that relate to the Key Idea questions. When you finish reading, write a short answer to each Key Idea question using the underlined information. READING CHECK 1. Explain What are the two forces that must balance out to maintain a state of isostasy? C New elevation Final elevation Continental lithosphere LOOKING CLOSER 2. Explain Why are the mountains shrinking from image to image? Buoyant force As erosion wears away the crust, the lithosphere becomes lighter and rises. Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 161 Deformation of the Crust Name SECTION 1 Class Date How Rock Deforms continued TYPES OF ISOSTATIC ADJUSTMENTS READING CHECK 3. Describe What happens in the process of uplift? READING CHECK 4. Explain Why does the ocean floor rise when large glaciers form? An isostatic adjustment happens when Earth’s lithosphere rises or sinks to maintain isostasy. Isostatic adjustments are happening all the time in Earth’s crust. For example, a mountain goes through isostatic adjustments as it erodes. Over millions of years, wind, water, and ice wear away the rock. The mountain becomes shorter and lighter. As the mountain shrinks, it rises in a process called uplift. The opposite of uplift is subsidence. During subsidence, the lithosphere becomes heavier. It sinks into the asthenosphere. Subsidence is common in places where large rivers flow into oceans. Large rivers generally carry large amounts of sediment, including mud, sand, and gravel. When the river flows into the ocean, the sediment drops onto the ocean floor. The extra weight of the sediment makes the ocean floor sink. Isostatic adjustments can also happen when glaciers grow or shrink. A glacier is a huge river of ice. Glaciers can hold huge amounts of water. Therefore, they are very heavy. The weight of the ice makes the lithosphere beneath the glacier sink. At the same time, the ocean floor rises because the weight of the ocean water is less. The water has moved onto land and has been frozen in the glacier. When the glacier melts, the water returns to the ocean. The extra weight of the water causes the ocean floor to sink. At the same time, the land that was covered with ice rises because the weight of the crust has decreased. Isostatic Adjustments Cause Effect Wind, water, and ice carry away rock from mountains. The mountains become lighter, and uplift occurs. Rivers deposit The ocean floor becomes heavier, and subsidence sediment on the ocean occurs. floor. Glaciers form as ice and snow build up. The weight of the glacier causes subsidence under the glacier. The decreased weight of ocean water causes uplift in the oceans. Glaciers melt as the climate gets warmer. The decreased weight of the glacier causes uplift on land. The increased weight of ocean water causes subsidence in the oceans. Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 162 Deformation of the Crust Name SECTION 1 Class Date How Rock Deforms continued What Kinds of Stress Can Act on Rocks? As Earth’s lithosphere moves, the rock in the crust is squeezed, stretched, and twisted. These actions put force on the rock. Stress is the amount of force applied to a given area of rock. Stress occurs when the lithosphere sinks and is squeezed by the weight of rock above it. It also occurs when the rock in the crust rises and is stretched out. The movement of tectonic plates past one another can also produce stress. There are three main types of stress: compression, tension, and shear stress. Compression is a type of stress that squeezes rock. Compression can change the shape of a rock or reduce the amount of space the rock takes up. Compression is a common kind of stress in places where tectonic plates are moving together. Tension is the opposite of compression. Tension is stress that stretches rock. Tension can make rock longer and thinner. Tension is a common kind of stress in places where tectonic plates are moving apart. Shear stress deforms rock by pushing its different parts in opposite directions. Sheared rock can bend, twist, or break apart as it moves past other rocks. Shear stress is a common kind of stress in places where tectonic plates slide past each other. Critical Thinking 5. Apply Concepts Which type of stress is probably most common in places where new mountains are forming? READING CHECK 6. Identify Which kind of stress causes rock to twist? The pictures on the left show how different kinds of stress can affect rock layers. The pictures on the right show how the kinds of stress can affect a square block. The dotted lines show the original shape of each block. LOOKING CLOSER 7. Identify Label the pictures to indicate which pictures show compression, which show tension, and which show shear stress. Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 163 Deformation of the Crust Name SECTION 1 Class Date How Rock Deforms continued THE EFFECT OF STRESS ON ROCKS READING CHECK 8. Identify Which type of strain can cause a rock to stretch? Stress can cause deformation. Deformation that is caused by stress is called strain. Changes in shape and size are examples of strain. Strain is not always permanent. If the stress is applied slowly, the rock might go back to its original shape when the stress is removed. However, if too much stress is put on the rock, the strain may become permanent. There are two main types of permanent strain: brittle and ductile. Rocks that break or fracture under stress are brittle. Cracks and breaks in rock are types of brittle strain. In contrast, ductile rocks respond to stress by bending without breaking. Folds and bends in rock are types of ductile strain. The type of strain a rock shows depends on several factors. These factors include temperature, pressure, the composition of the rock, and how fast the stress is applied. Near Earth’s surface, where temperature and pressure are low, brittle strain is most common. Brittle strain is also more common when a lot of stress is applied quickly. At higher temperature and pressure, ductile strain is more common. Small amounts of stress applied over long periods of time can also cause ductile strain. LOOKING CLOSER 9. Describe Name two sets of conditions that might have produced the strain in the picture. Cracks are examples of brittle strain. What Features Can Strain Produce? Different kinds of strain produce different features in rock. Two main kinds of strain are folds and faults. A fold occurs when a rock responds to stress in a ductile way. A fault occurs when a rock responds to stress in a brittle way. Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 164 Deformation of the Crust Name SECTION 1 Class Date How Rock Deforms continued CHARACTERISTICS OF FOLDS A fold is a bend in a rock layer. Most folds form when rocks are compressed. As compression acts on the rock, the rock layers wrinkle and fold over themselves. Some folds also form because of shear stress. Folds have different parts. They have sloping sides called limbs. The limbs meet at the bend, or hinge, of the rock layers. Folds can be symmetrical or asymmetrical. If a fold can be sliced in two identical halves, it is symmetrical. The dividing line is called the axial plane. However, most folds are not symmetrical. Axial plane LOOKING CLOSER 10. Describe Is this fold symmetrical or asymmetrical? How do you know? Hinge Limb Folds can have many different shapes. Many folds are bent vertically. However, some are overturned and seem to be lying on their sides. Folds can be open or very tight. The limbs can be even, or one can be steeper than the other. The hinge can be a smooth bend or a sharp point. Each fold is unique because it formed under a unique combination of conditions. Folds can vary in size as well as shape. Some are smaller than your hand. Others cover thousands of square kilometers. Some folds are so large that they form ridges and valleys. TYPES OF FOLDS Geologists classify folds based on their characteristics. There are three main kinds of folds: anticlines, synclines, and monoclines. In an anticline, the oldest rocks are in the middle of the fold. In a syncline, the oldest rocks are on the outside of the fold. In a monocline, the two limbs are horizontal or almost horizontal. The pictures on the next page show examples of these kinds of folds. READING CHECK 11. Identify What are the three main kinds of folds? Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 165 Deformation of the Crust Name SECTION 1 Class Date How Rock Deforms continued Anticline The oldest rocks are on the inside of an anticline. Most anticlines are arch shaped. Syncline LOOKING CLOSER 12. Analyze Processes Why would a syncline be more likely to have a bowl shape? The oldest rocks are on the outside of a syncline. Most synclines are bowl shaped. Monocline Monoclines form when one part of the crust is lifted relative to another part. CHARACTERISTICS OF FAULTS READING CHECK Faults form when rock deforms in a brittle way. A fault is a break in the body of a rock along which the surrounding rock moves. The fault plane is the plane along which the rock moves. If the fault plane is not vertical, the rock above the fault plane is known as the hanging wall. The rock below the fault plane is called the footwall. 13. Define What is the hanging wall? Footwall Hanging wall Fault plane Like folds, faults can vary greatly in size. Small faults may affect only a few layers of rock found in a small region. Other faults are thousands of kilometers long. These large faults are often composed of smaller, connected faults, rather than a single fault. Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 166 Deformation of the Crust Name Class SECTION 1 Date How Rock Deforms continued TYPES OF FAULTS There are two main types of faults: normal faults and reverse faults. In a normal fault, the hanging wall moves down relative to the footwall. In a reverse fault, the hanging wall moves up relative to the footwall. A thrust fault is a type of reverse fault in which the fault plane is almost horizontal. Footwall Hanging wall Most normal faults form when rock is under tension. Normal fault Most reverse faults form when rock is under compression. Reverse fault LOOKING CLOSER A thrust fault is a type of reverse fault. Thrust fault In some faults, the rock does not move up and down. Instead, it moves horizontally. The two pieces of rock slide past one another. This type of fault is called a strike-slip fault. The rock in a strike-slip fault moves parallel to the direction of the fault’s length. The fault plane in a strike-slip fault may be vertical or tilted. Strike-slip faults are most commonly found at transform boundaries, where tectonic plates grind past each other. They may also occur at fracture zones between segments of mid-ocean ridges. 14. Describe Label the hanging wall and the footwall in the pictures of the reverse fault and the thrust fault. LOOKING CLOSER 15. Compare How is a strike-slip fault different from a normal fault? Strike-slip faults are common in areas that have shear stress acting on them. Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 167 Deformation of the Crust Name Class Date Section 1 Review SECTION VOCABULARY deformation the bending, tilting, and breaking of Earth’s crust; the change in the shape of rock in response to stress fault a break in a body of rock along which one block slides relative to another; a form of brittle strain fold a form of ductile strain in which rock layers bend, usually as a result of compression isostasy a condition of gravitational and buoyant equilibrium between Earth’s lithosphere and asthenosphere stress the amount of force per unit area that acts on a rock strain any change in a rock’s shape or volume caused by stress 1. Compare Complete the Venn diagram below to compare folds and faults. Folds Faults 2. Apply Concepts What would you expect a rock that has experienced tension to look like? 3. Infer About 20,000 years ago, many areas in northern North America were cov- ered with glaciers. Since then, the glaciers have melted. The lithosphere in many of these areas is now experiencing uplift. Why is this? 4. Describe How can a geologist tell if a fold is a syncline or an anticline? Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 168 Deformation of the Crust