Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
1 The Rock Cycle 1 Overview You know that paper, plastic, and aluminum can be recycled. But did you know that the Earth also recycles? And one of the things that Earth recycles is rock. READI NG WARM-U P This section introduces the rock cycle and the processes that shape the surface of the Earth. The processes of weathering, erosion, deposition, uplift, melting, cooling, and metamorphism are explained, in the context of the rock cycle. Objectives • • • Describe two ways rocks have been used by humans. Describe four processes that shape Earth’s features. Describe how each type of rock changes into another type as it moves through the rock cycle. List two characteristics of rock that are used to help classify it. • Bellringer Terms to Learn rock cycle rock erosion Pose the following questions to students: • How can rock be recycled? deposition composition texture READI NG STRATEGY • How long would recycling a rock take? Reading Organizer As you read this section, make a flowchart of the steps of the rock cycle. • What would the rock look like before, during, and after the process of recycling? Scientists define rock as a naturally occurring solid mixture of one or more minerals and organic matter. It may be hard to believe, but rocks are always changing. The continual process by which new rock forms from old rock material is called the rock cycle. The Value of Rock Rock has been an important natural resource as long as humans have existed. Early humans used rocks as hammers to make other tools. They discovered that they could make arrowheads, spear points, knives, and scrapers by carefully shaping rocks such as chert and obsidian. Rock has also been used for centuries to make buildings, monuments, and roads. Figure 1 shows how rock has been used as a construction material by both ancient and modern civilizations. Buildings have been made out of granite, limestone, marble, sandstone, slate, and other rocks. Modern buildings also contain concrete and plaster, in which rock is an important ingredient. ✓Reading Check Name some types of rock that have been used to construct buildings. (See the Appendix for answers to Reading Checks.) Discussion ----------------------------------g Geologic Time Some students may not realize how long changes in the rock cycle take. The processes that shape rock can take millions to tens of millions of years. Discuss the geologic time scale with students to give them a perspective of how long the Earth has existed and how long these processes have been affecting the Earth. Figure 1 The ancient Egyptians used a sedimentary rock called limestone to construct the pyramids at Giza (left). Granite, an igneous rock, was used to construct the Texas state capitol building in Austin (right). l Logical CHAPTER RESOURCES Chapter Resource File CRF • Lesson Plan • Directed Reading A b • Directed Reading B s Technology Transparencies • Bellringer 90 Chapter 4 • Rocks: Mineral Mixtures Answer to Reading Check Types of rock that have been used by humans to construct buildings include granite, limestone, marble, sandstone, and slate. Processes That Shape the Earth Certain geological processes make and destroy rock. These processes shape the features of our planet. These processes also influence the type of rock that is found in a certain area of Earth’s surface. Weathering, Erosion, and Deposition Group rock a naturally occurring solid mixture of one or more minerals or organic matter rock cycle the series of processes The process in which water, wind, ice, and heat break down rock is called weathering. Weathering is important because it breaks down rock into fragments. These rock and mineral fragments are the sediment of which much sedimentary rock is made. The process by which sediment is removed from its source is called erosion. Water, wind, ice, and gravity can erode and move sediments and cause them to collect. Figure 2 shows an example of the way land looks after weathering and erosion. The process in which sediment moved by erosion is dropped and comes to rest is called deposition. Sediment is deposited in bodies of water and other low-lying areas. In those places, sediment may be pressed and cemented together by minerals dissolved in water to form sedimentary rock. in which a rock forms, changes from one type to another, is destroyed, and forms again by geological processes erosion the process by which wind, water, ice, or gravity transports soil and sediment from one location to another deposition the process in which material is laid down Heat and Pressure Sedimentary rock made of sediment can also form when buried sediment is squeezed by the weight of overlying layers of sediment. If the temperature and pressure are high enough at the bottom of the sediment, the rock can change into metamorphic rock. In some cases, the rock gets hot enough to melt. This melting creates the magma that eventually cools to form igneous rock. How the Cycle Continues Buried rock is exposed at the Earth’s surface by a combination of uplift and erosion. Uplift is movement within the Earth that causes rocks inside the Earth to be moved to the Earth’s surface. When uplifted rock reaches the Earth’s surface, weathering, erosion, and deposition begin. Figure 2 Bryce Canyon, in Utah, is an excellent example of how the processes of weathering and erosion shape the face of our planet. v -------g Rates of Weathering Tombstones can be used to determine rates of weathering. In the 1960s, professor E. M. Winkler calculated the weathering rate for a marble marker in Indiana to be 1.5 mm in 43 y. In 1916, American geologist D. C. Barton estimated the weathering rate for granite monuments near the Nile River to be only 1 to 2 mm per millennium! With your students, visit a cemetery, and measure weathering rates of grave markers. Quartz veins in marble markers do not erode easily. Students can measure how much the marble has eroded relative to a quartz vein. By dividing the amount that a grave marker has eroded by the number of years the stone has been standing (the date of death), students can calculate the rate of weathering. l Visual/Kinesthetic Cultural Awareness g Stone Cities Between 900 and 1400 CE, the Anasazi Indians of the American Southwest carved small towns in cliff sides. In what is now Cambodia, a vast temple complex called Angkor was carved from clay, sandstone, and laterite in the 12th century. In the 1300s, African traders built the Great Zimbabwe, a walled city guarded by huge monoliths. Have students write a report or build a model of one of these ancient sites. Writing MISCONCEPTION ALERT Weathering, Erosion, and Deposition Some students may be confused by weathering, erosion, and deposition. Explain that weathering is the process that actually breaks down rock, whereas erosion is the removal and transport of the sediment that is formed by weathering. Deposition is the process by which sediments are dropped in a new location. The processes of weathering and erosion are not easy to separate because they happen simultaneously. l Logical/Kinesthetic Section 1 • The Rock Cycle 91 Illustrating the Rock Cycle You have learned about various geological processes, such as weathering, erosion, heat, and pressure, that create and destroy rock. The diagram on these two pages illustrates one way that sand grains can change as different geological processes act on them. In the following steps, you will see how these processes change the original sand grains into sedimentary rock, metamorphic rock, and igneous rock. Using the Figure -----g Diagramming the Rock Cycle Ask students to use the information in the rock-cycle illustration to draw a diagram of the rock cycle in their science journal. The first step in the illustration is the formation of sedimentary rock; ask students to begin their rock cycle with a different step. Encourage them to write a descriptive caption for every stage of the rock cycle. l Visual PORTFOLIO ion Dep osi tio n Eros MISCONCEPTION ALERT Rock Cycle Misconceptions Rocks rarely undergo the complete process shown in the rock-cycle diagram. Sedimentary rocks can become igneous rocks, and metamorphic rocks can become sedimentary rocks. Also, some students may not realize the length of time it takes for changes to occur in the rock cycle. The process shown in the diagram can take millions to tens of millions of years. 1 Sedimentary Rock Grains of sand and other sediment are eroded from hills and mountains and wash down a river to the ocean. Over time, the sediment forms thick layers on the ocean floor. Eventually, the grains of sediment are compacted and cemented together to form sedimentary rock. Compaction and cementation 2 Me ta m or phi sm Metamorphic Rock When large pieces of the Earth’s crust collide, some of the rock is forced downward. At great depths, intense heat and pressure heat and squeeze the sedimentary rock to change it into metamorphic rock. INCLUSION Strategies • Learning Disabled • Hearing Impaired • Attention Deficit Disorder Groups will use different foods to model sedimentary, metamorphic, and igneous rocks and minerals. Have real rock samples available (enough for each group). Organize students into groups of three or four students. Give students a chocolate-chip cookie to model granite (igneous), a sugar cube to model marble (metamorphic), a brownie to model shale (sedimentary), a 92 Chapter 4 • Rocks: Mineral Mixtures magnifying glass, a large sheet of paper, and magic markers. Ask students to describe and sketch their three samples. To describe each sample, students should note the sample’s texture, color, and composition. On the board, compile descriptions for each “rock” from each group. Pass real rock samples to each group, and have the groups match their samples to e the real rocks. l Visual v------------------------------------------b hering Weat 5 Sediment Uplift and erosion expose the igneous rock at the Earth’s surface. The igneous rock then weathers and wears away into grains of sand and clay. These grains of sediment are then transported and deposited elsewhere, and the cycle begins again. Solidification Rock Dictionary Have students prepare a Rock Dictionary. Ask them to list the three types of rock and the processes that occur in the rock cycle. Ask students to record the dictionary definition for each rock type or rock-cycle process and then define it in their own words. Encourage students to make up mnemonic devices, such as jokes or rhymes, to help them remember the meaning of each term. l Logical 4 Igneous Rock The sand grains from step 1 have changed a lot, but they will change more! Magma is usually less dense than the surrounding rock, so magma tends to rise to higher levels of the Earth’s crust. Once there, the magma cools and solidifies to become igneous rock. Cooling CONNECTION v Language Arts -----------------------------b Passing Through Ask students to imagine being an ancient grain of sand on a beach. Have them write a letter describing their lifetime in the rock cycle. The grain of sand should pass through several processes in the rock cycle during its lifetime. Students can share their letters with the class. Writing l Intrapersonal 3 Mel ting Magma The hot liquid that forms when rock partially or completely melts is called magma. Where the metamorphic rock comes into contact with magma, the rock tends to melt. The material that began as a collection of sand grains now becomes part of the magma. Is That a Fact! Geologists can use the Earth’s magnetic field to determine the approximate age of rocks. They examine both igneous and sedimentary rocks to determine the pattern of magnetic reversals. When these rocks form, the magnetic minerals they contain orient with the direction of the Earth’s magnetic field. The pattern of magnetic reversals allows scientists to date the rock layers. CHAPTER RESOURCES Technology e e h -----------------------------g Illustration Have students make a poster that illustrates the rock cycle. Encourage them to cut out pictures from magazines of the different types of rock and processes in the rock cycle. For example, marble is a metamorphic rock that could be represented by a picture of a marble statue. l Visual e Transparencies • The Rock Cycle Section 1 • The Rock Cycle 93 Figure 3 Awareness Sedimentary rock g g in n s io W an ea d th e Igneous rock Co ol in g e r io n s ro Avicenna The Persian scholar Avicenna (980–1037) contributed immensely to our knowledge of medicine, astronomy, mathematics, and geology. In the Book of Minerals, he described how rivers and seas laid down sediment that eventually became rock. Avicenna’s theories contributed to the foundations of Western geology. Many of his controversial ideas did not gain acceptance in Europe until the 1600s. Encourage students to learn more about Avicenna. Have them come to class as Avicenna and act out his life story. l Logical He pr e at a ssu nd re W a n d e at h er er o Cultural The Rock Cycle in g Metamorphic rock Me ltin g Magma Round and Round It Goes You have seen how different geological processes can change rock. Each rock type can change into one of the three types of rock. For example, igneous rock can change into sedimentary rock, metamorphic rock, or even back into igneous rock. This cycle, in which rock is changed by geological processes into different types of rock, is known as the rock cycle. Rocks may follow various pathways in the rock cycle. As one rock type is changed to another type, several variables, including time, heat, pressure, weathering, and erosion may alter a rock’s identity. The location of a rock determines which natural forces will have the biggest impact on the process of change. For example, rock at the Earth’s surface is primarily affected by forces of weathering and erosion, whereas deep inside the Earth, rocks change because of extreme heat and pressure. Figure 3 shows the different ways rock may change when it goes through the rock cycle and the different forces that affect rock during the cycle. CONNECTION to Environmental Science -----------------------------------------------a Cycles in Nature Many important substances on Earth follow cycles. Examples include water, carbon, and rock. Use the teaching transparency entitled “The Water Cycle” to lead a discussion about the ways that the water cycle and the rock cycle interact with each other. l Visual ✓Reading Check What processes change rock deep within the Earth? CHAPTER RESOURCES Technology Transparencies • LINK TO LIFE SCIENCE The Water Cycle Workbooks Math Skills for Science • Percentages g 94 Chapter 4 • Rocks: Mineral Mixtures Answer to Reading Check Rock within the Earth is affected by temperature and pressure. Answer to Math Practice Rock Classification 100% of rock (30% quartz ! 55% feldspar) " 15% biotite mica You have already learned that scientists divide all rock into three main classes based on how the rock formed: igneous, sedimentary, and metamorphic. But did you know that each class of rock can be divided further? These divisions are also based on differences in the way rocks form. For example, all igneous rock forms when magma cools and solidifies. But some igneous rocks form when magma cools on the Earth’s surface, and others form when magma cools deep beneath the surface. Therefore, igneous rock can be divided again based on how and where it forms. Sedimentary and metamorphic rocks are also divided into groups. How do scientists know how to classify rocks? They study rocks in detail using two important criteria—composition and texture. What’s in It? Assume that a granite sample you are studying is made of 30% quartz and 55% feldspar by volume. The rest is made of biotite mica. What percentage of the sample is biotite mica? Composition The minerals a rock contains determine the composition of that rock, as shown in Figure 4. For example, a rock made of mostly the mineral quartz will have a composition very similar to that of quartz. But a rock made of 50% quartz and 50% feldspar will have a very different composition than quartz does. ✓Reading Check Figure 4 composition the chemical makeup of a rock; describes either the minerals or other materials in the rock Group v -----------b Describing Rocks Organize the class into small groups. Give each group samples of sandstone, limestone, and conglomerate. Number the samples. Provide a magnifying lens, a mineral identification key, a small dental pick, and paper towels (to capture any pieces of rock that break off during the activity). Write the following instructions on the board: 1. Describe the color and texture of each rock. What determines a rock’s composition? 2. Using your unaided eye, examine the grains that make up each rock. Describe what you see. Two Examples of Rock Composition The composition of a rock depends on the minerals the rock contains. 3. Using the magnifying lens, Granite Limestone try to identify the mineral composition of each rock. 99 A 010 10% Biotite mica 4. Use the dental pick to test how well each rock is cemented, and record what you discover. 5. Classify each rock by grain size as fine grained, medium grained, or coarse grained. 95% Calcite 5% Aragonite 35% Quartz 55% Feldspar After groups have analyzed the rocks, have group members discuss their findings. l Logical/Kinesthetic CONNECTION to Math ---------------------------------------------------------------------------g Percentages A percentage is a ratio that is expressed in terms of hundredths. When analyzing pure substances, percentage composition remains the same at any mass. For example, in terms of atomic mass, the percentage of oxygen by weight in water is 88.8%, whether you are describing a single raindrop or an entire ocean. l Logical e Answer to Reading Check The minerals that a rock contains determine a rock’s composition. Section 1 • The Rock Cycle 95 Figure 5 Fine-grained Reteaching -------------------------------------b Diagramming the Rock Cycle Have students create a diagram of the processes that shape Earth’s surface. The diagram should be a simplified version of the rock cycle. The diagram should include the processes of weathering, erosion, deposition, and uplift. l Visual Quiz ---------------------------------------------------------------------g 1. List four processes that change rock from one type to another. (weathering, changes in pressure, melting, and cooling) Three Examples of Sedimentary Rock Texture Medium-grained Siltstone Sandstone Conglomerate Texture texture the quality of a rock that is based on the sizes, shapes, and positions of the rock’s grains 2. What are the three main classes of rock? (igneous, sedimentary, and metamorphic) 3. How is a brick similar to a metamorphic rock? (Bricks are made from clay and then baked to become strong and resistant to weathering. Bricks are “metamorphosed” by high temperatures.) The size, shape, and positions of the grains that make up a rock determine a rock’s texture. Sedimentary rock can have a finegrained, medium-grained, or coarse-grained texture, depending on the size of the grains that make up the rock. Three samples of textures are shown in Figure 5. The texture of igneous rock can be fine-grained or coarse-grained, depending on how much time magma has to cool. Based on the degree of temperature and pressure a rock is exposed to, metamorphic rock can also have a fine-grained or coarse-grained texture. The texture of a rock can provide clues as to how and where the rock formed. Look at the rocks shown in Figure 6. The rocks look different because they formed in very different ways. The texture of a rock can reveal the process that formed it. ✓Reading Check Give three examples of sedimentary rock textures. Figure 6 Alternative Assessment ---------------------------g Texture and Rock Formation Basalt, a fine-grained igneous rock, forms when lava that erupts onto Earth’s surface cools rapidly. Rock Cycle Skit Have students write a skit portraying the rock cycle. Roles can include the minerals that make up rock and the forces that affect them. To represent the forces—heat, pressure, erosion, and weathering— suggest that students create special costumes. l Kinesthetic Answer to Reading Check Fine-grained rocks are made of small grains, such as silt or clay particles. Medium-grained rocks are made of medium-sized grains, such as sand. Coarse-grained rocks are made of large grains, such as pebbles. 96 Coarse-grained Chapter 4 • Rocks: Mineral Mixtures Sandstone, a medium-grained sedimentary rock, forms when sand grains deposited in dunes, on beaches, or on the ocean floor are buried and cemented. Answers to Section Review Review Summary • Rock has been an important natural • resource for as long as humans have existed. Early humans used rock to make tools. Ancient and modern civilizations have used rock as a construction material. Weathering, erosion, deposition, and uplift are all processes that shape the surface features of the Earth. • The rock cycle is the continual process by which new rock forms from old rock material. • The sequence of events in the rock cycle depends on processes, such as weathering, erosion, deposition, pressure, and heat, that change the rock material. Composition and texture are two characteristics that scientists use to classify rocks. • • The composition of a rock is determined by the minerals that make up the rock. • The texture of a rock is determined by the size, shape, and positions of the grains that make up the rock. Using Key Terms Complete each of the following sentences by choosing the correct term from the word bank. rock composition rock cycle texture 1. The minerals that a rock is made of determine of that rock. the 2. is a naturally occurring, solid mixture of crystals of one or more minerals. Understanding Key Ideas 3. Sediments are transported or moved from their original source by a process called a. deposition. b. erosion. c. uplift. d. weathering. 4. Describe two ways that rocks have been used by humans. Critical Thinking 8. Making Comparisons Explain the difference between texture and composition. 9. Analyzing Processes Explain how rock is continually recycled in the rock cycle. Interpreting Graphics 10. Look at the table below. Sandstone is a type of sedimentary rock. If you had a sample of sandstone that had an average particle size of 2 mm, what texture would your sandstone have? Classification of Clastic Sedimentary Rocks Texture Particle size coarse grained > 2 mm medium grained 0.06 to 2 mm fine grained < 0.06 mm 5. Name four processes that change rock inside the Earth. 6. Describe four processes that shape Earth’s surface. 7. Give an example of how texture can provide clues as to how and where a rock formed. For a variety of links related to this chapter, go to www.scilinks.org Topic: Composition of Rock SciLinks code: HSM0327 CHAPTER RESOURCES Chapter Resource File CRF • Section Quiz g • Section Review g • Vocabulary and Section Summary g 1. composition 2. Rock 3. b 4. Rocks have been used by humans to make tools and weapons and to construct buildings. 5. Four processes that change rock inside the Earth are compaction and cementation, metamorphism, melting, and cooling. 6. Weathering is the process by which water, wind, ice, and heat break down rock. Erosion is the process by which sediment is removed from its source. Deposition is the process by which sediment moved by erosion is laid down. Uplift is the process by which rock within the Earth moves to Earth’s surface. 7. Answers may vary. Sample answer: Fine grains in an igneous rock indicate that the rock cooled quickly, which means it was likely to have formed at Earth’s surface. 8. Composition is the percent of elements that make up a rock. Texture is a quality of a rock that is based on the size, shape, and position of its grains. 9. Answers may vary. Sample answer: Rock is continually recycled by different processes in the rock cycle. Melting of sedimentary, metamorphic, or igneous rock creates new igneous rock. The weathering, erosion, deposition, burial, compression, and cementation of igneous, metamorphic, or sedimentary rock creates new sedimentary rock. Igneous, sedimentary, or metamorphic rock that is subjected to increased heat and pressure can be metamorphosed. 10. a medium-grained texture Section 1 • The Rock Cycle 97